Sorptive sample preparation review

Abstract Most sample-enrichment procedures currently
available rely on adsorption of the analytes of interest by
a suitable adsorbent material. Although good performance
can be obtained for many practical problems, in some
cases the applicability of adsorptive sample preparation
falls short, particularly for the enrichment of polar and/or
high-molecular-weight compounds, especially in combi-
nation with thermal desorption. Because of the very strong
retention of adsorbent materials, undesired effects such
as incomplete desorption and artifact formation are ob-
served. Polar solutes are easily adsorbed but readily un-
dergo surface-catalyzed reactions and on desorption yield
compounds different than those originally sampled. High-
molecular-weight compounds cannot be desorbed because
of extremely strong interactions with the adsorbent and
their low volatility.

To overcome some of these problems sample-prepara-

tion techniques based on polydimethylsiloxane sorption
have been developed over the past 15 years. In contrast with
adsorptive trapping, sorption is based on dissolution of
the analytes in a liquid polymeric material. This is a much
more inert means of solute retention which overcomes
some of the limitations encountered when working with
adsorbents. In this contribution, the basic principles of
sorption, the different instrumentation used, and applica-
tions of the technique will be reviewed. The review cov-
ers the sorptive sample-preparation techniques, open-tubu-
lar trapping (OTT), solid-phase microextraction (SPME),
gum-phase extraction (GPE), equilibrium gum-phase ex-

traction (EGPE), and stir-bar-sorptive extraction (SBSE).
Because of the nature of sorptive sample-preparation tech-
niques, which perform particularly well in combination
with thermal desorption, this review focuses strongly on
gas chromatography as the means of chemical analysis.

Keywords Sorptive sample preparation · Gas
chromatography · Thermal desorption ·
Polydimethylsiloxane

Introduction

Most of the sample-preparation techniques currently avail-
able rely on trapping of the analytes of interest from the
sample (gas, liquid, or solid) by an adsorbent material; this
is followed by desorption and (chromatographic) analysis.
Adsorbents are porous materials with a high internal sur-
face area (typically 5–1000 m

–1

) and the analytes are

temporarily stored on the adsorbent surface. After analyte
trapping and matrix removal the trapped analytes can be
released by extraction with a small amount (typically mil-
liliters) of an organic solvent. An aliquot (typically mi-
crolitres) of this extract is subsequently injected into the
analytical instrument. Although this approach works quite
successfully, it is likely to result in poor sensitivity be-
cause only a fraction of the sample is used. Overcoming
this sensitivity limitation is the topic of much research
into sample-preparation. Possible solutions include on-
line combination of extraction with liquid chromatogra-
phy and injection of large volumes into the analytical sys-
tem (i.e. large-volume injection in gas chromatography).

As an alternative to liquid desorption, thermal desorp-

tion under an inert gas stream is increasingly being used.
Thermal desorption can be coupled rather conveniently to
a gas chromatograph and the (heated) carrier gas is used
for thermal desorption. When cryogenic focusing is em-
ployed, quantitative transfer of the analytes trapped on the
adsorbent material to the chromatographic column is pos-
sible; this results in a considerable increase in sensitivity
compared with liquid desorption. Consequently, thermal

E. Baltussen · C. A. Cramers · P. J. F. Sandra

Sorptive sample preparation – a review

Anal Bioanal Chem (2002) 373 : 3–22
DOI 10.1007/s00216-002-1266-2

Received: 21 November 2001 / Revised: 15 February 2002 / Accepted: 20 February 2002 / Published online: 9 April 2002

S P E C I A L I S S U E PA P E R

E. Baltussen (✉)
Notox B.V., Hambakenwetering 7,
5231DD, ‘s Hertogenbosch, The Netherlands
e-mail: erik.baltussen@notox.nl

C.A. Cramers
Eindhoven University of Technology,
Laboratory of Instrumental Analysis, P.O. Box 513,
5600 MB Eindhoven, The Netherlands

P.J.F. Sandra
Research Institute for Chromatography,
Kennedypark 20, B-8500, Kortrijk, Belgium

desorption can be a very attractive alternative to classical
procedures involving liquid desorption. It must be noted,
however, that if analysis is to be successful the analytes
subjected to thermal desorption must be thermally stable,
otherwise decomposition will occur. Because volatile and
thermally stable analytes are amenable to GC analysis,
thermal desorption is, in practice, only used in combina-
tion with gas chromatography.

Many standardized methods are available for enrich-

ment of analytes from gaseous and liquid samples on ad-
sorbent cartridges followed by liquid desorption and in-
jection into a chromatographic system. Unfortunately, rel-
atively few standardized analytical methods take advan-
tage of the high sensitivity of thermal desorption, because
although a wide variety of adsorbent materials is available,
none is universally suitable for thermal desorption. Many
adsorbents (i.e. most inorganic adsorbents) interact too
strongly with the trapped analytes, requiring very high
desorption temperatures, which lead to degradation reac-
tions. This limits the use of inorganic adsorbents to highly
volatile apolar analytes only. Organic adsorbents, on the
other hand, often lead to poor blanks because of thermal
decomposition of the material itself. Most adsorbents also
have significant catalytic activity, even at low tempera-
tures; this prevents their use for the enrichment of chemi-
cally labile compounds. This is, of course, another highly
undesirable effect that often prevents the application of
organic adsorbents.

These problems with the thermal sorption of adsor-

bents have prompted several research groups to focus on
another class of material – sorption materials. In contrast
with adsorbents, sorption (dissolution or partitioning) ma-
terials are a group of polymeric materials that are above
their glass transition point (T

) at all the temperatures em-

ployed. In this temperature range sorbents are in a gum-
like or liquid-like state and behave similarly to organic sol-
vents. Sorbents are, in principle, homogeneous, non-po-
rous materials in which the analytes can dissolve. The an-
alytes do not, therefore, undergo real (temporary) bonding
with the material but are retained by dissolution.

In this contribution the state-of-the-art in sorptive sam-

pling and thermal desorption will be reviewed. First some
basic concepts, illustrating the different approaches used for
sorptive sampling, are presented. This is followed by a de-
tailed overview of the different techniques described in the
literature – open-tubular trapping (OTT), solid-phase mi-
croextraction (SPME), gum-phase extraction (GPE), equi-
librium gum-phase extraction (EGPE), and stir-bar-sorp-
tive extraction (SBSE). In this contribution, emphasis will
be on the high-sensitivity sorptive techniques GPE, EGPE,
and SBSE; the older (and less sensitive) techniques OTT
and SPME are described in much less detail and the reader
is referred to the literature for more detailed information.

Basic concepts

The primary aim of all sample-preparation methods is
transfer of the analytes of interest from their original sur-

roundings (sample matrix) into a form more suitable for
introduction into the analytical instrument. This can be
achieved by many different techniques all of which have
their strengths for specific analytes and analyte–matrix
combinations. Usually the sample is placed in direct con-
tact with the extraction phase (extractant) to accomplish
transfer of the analytes into the extractant. Subsequently
the extractant can be processed further or, occasionally, it
can be introduced directly into the analytical device. Sev-
eral basic sample preparation concepts are described in
forthcoming sections.

Static sampling

In static sampling techniques all the extractant is in con-
tact with all the sample during extraction, i.e. neither the
sample nor the extractant is renewed. Static techniques
rely on diffusion of the sample analytes into the extractant
with the ultimate goal of reaching equilibrium between
both phases. Selection of the extractant phase is usually
based on the so-called ‘like–like’ principle – a substance
will always have the highest affinity for a phase with
properties similar to those of the substance itself. This
means that if an apolar compound is to be extracted from
a polar matrix, an apolar extractant should be used. It
should be noted that mixing procedures such as stirring,
shaking, or sonication are often used to promote diffusion
of analytes from the sample into the extractant. This, how-
ever, only affects the time required for equilibration; it
does not affect the equilibrium itself or other properties of
the static process.

The most important factor governing static extraction

is the distribution constant (K) which is defined as:

× β

(1)

where C

is the concentration of analyte in the sample, in

g L

–1

; C

is the concentration of analyte in the extractant,

in g L

–1

; m

is the mass of analyte remaining in the sam-

ple, in g; m

is the mass of analyte in the extractant phase,

in g; V

is the volume of the extractant, in L; and V

is the

volume of sample, in L.

is the phase ratio of the static

extraction system and is defined as V

. In subsequent

equations, the total mass of analyte in the system is de-
fined as m

tot

). Rewriting Eq. (1) leads to a more

useful expression, that of the extraction efficiency (

tot

η =

+ 1

(2)

The extraction efficiency is usually expressed as a per-
centage and as such is generally known as the recovery.
From Eq. (2) it is important to note that the only two
terms affecting the recovery of an analyte are

and K. For

very large partitioning constants the numerator becomes
unity, leading to 100% recovery. Very large phase ratios
(small volume of extractant relative to the sample vol-
ume) lead to a large numerator and, consequently, to low

recovery. In practice, K is often a more or less fixed con-
stant which depends mainly on properties of the analyte
and the characteristics of the sample and extractant phases.

is chosen by selecting the phase volumes applied, usu-

ally to ensure high recovery with a minimum amount of
extractant. It should be noted, however, that under static
conditions extraction is never complete and that some of
the analytes always remains in the sample no matter how
large K is or how small

is.

Although static sampling can be an easy, reliable, and

straightforward technique, because it relies on the equilib-
rium distribution of compounds rather than on exhaustive
extraction care should be taken to ensure the distribution
constant (K) is equal in all experiments, including calibra-
tion and sample extraction. Although this seems to be a
simple requirement, in practice it is often not so. In chem-
ical equilibria, temperature has a dominant effect on equi-
librium and distribution constants, so careful control of
the temperature, often within 1–2 °C is necessary. In the
laboratory this requirement is easily met; it can be prob-
lematic in field-sampling applications.

As has been pointed out above, a distinction between

adsorption and sorption extraction phases must be made.
Sorption phases (including all organic solvents, water,
ideal gases, and polymeric materials at a temperature above
their glass transition point) retain solutes purely by disso-
lution (at sufficiently low concentrations). The analytes
partition into the bulk of these phases where they can dif-
fuse freely throughout the sorbent. They therefore experi-
ence the bulk properties of the sorbent and, as long as the
total amount of sorbed compounds is less than 1%, these
bulk properties do not change significantly with concen-
tration. High concentration levels of this kind are seldom
found in practice and static sorptive extraction is, there-
fore, usually a very reliable approach.

Static extraction performed with an adsorbing phase

is, unfortunately, much more complicated. Here, the ana-
lytes are retained on an active surface containing a fixed
number of adsorptive sites. The equilibrium reached is
that between analytes present in the sample and those ad-
sorbed on the adsorbent surface. This means that if the
sample concentration is high, all the adsorptive sites are
occupied and increasing the concentration of the sample
will no longer lead to an increase in the amount of com-
pound adsorbed; this is, of course, a highly undesirable ef-
fect. Analyte concentration levels are, however, usually
low enough to circumvent this effect, so if a single com-
pound is adsorbed from an otherwise clean sample this
is not a real problem. When several analytes are adsorbed
simultaneously this might, however, become problematic.
Not only do the different analytes compete for the same
adsorptive sites but matrix compounds present at rela-
tively high concentrations that are of no interest in a par-
ticular analysis (e.g. salts, humic acids, proteins) can
block adsorptive sites leading to unpredictable and irre-
producible results. The application of adsorbents in static
sampling is, therefore, limited to clean and dilute samples.
In special circumstances, particularly if the sample is a
solid, the sample itself might also have adsorbing proper-

ties, preventing the reliable use of static sampling tech-
niques.

Dynamic sampling

Whereas in static sampling mixing, stirring, and other dy-
namic processes are solely a means of promoting faster
equilibration, dynamic sampling procedures essentially
require that these basic dynamic processes ensure com-
plete extraction. In dynamic sampling all the extractant is
not immediately brought into contact with all the sample.
Many dynamic sampling techniques resemble chromatog-
raphy in that they also are based on the use of a stationary
phase (often the extractant) and a moving, mobile phase
(often the sample), see Fig. 1.

Gaseous or liquid samples are usually pumped through

the extractant that can, for example, be a packed bed. The
analytes will be retained in the packed bed and, consequent-
ly, the concentration of analyte in the sample will decrease
through the bed. Initially the concentration of analyte in the
outgoing sample phase will be zero and sampling is usu-
ally stopped when the first analyte of interest starts to elute
from the trap. This is called “breakthrough sampling” and
will be discussed in this section. It is, however, also pos-
sible to continue sampling beyond the breakthrough point
until all analytes are in equilibrium with the extractant.
This is a relatively new technique, called “equilibrium
sampling”; it will be illustrated by discussion of EGPE.

The most important property in breakthrough sampling

is the breakthrough volume, which determines the maxi-
mum volume of sample that can be passed through the
trapping device before analytes are no longer sufficiently
retained. It is important to state that there is no single def-

Fig. 1 Principle of dynamic breakthrough sampling. Top, the sam-
ple is pumped through the extraction trap; middle, the concentra-
tion profiles in the sample and in the extractant; bottom, analyte
concentration in the outgoing (extracted) sample as a function of
the volume sampled

inition of breakthrough volume; rather the breakthrough
volume depends on the acceptable loss of analyte. The ac-
ceptable loss of analyte is usually taken as 5 to 10%. A gen-
eral equation for calculation of the breakthrough volume is:

= V

× (1 + k) × f (N, b)

(3)

where V

is the breakthrough volume, in L; V

is the void vol-

ume of the trap, in L; k is the retention factor; and f(N,b) is a
function taking into account the number of theoretical plates
in the trap (N) and the acceptable breakthrough loss (b). The
retention factor is the same as that defined in chromatogra-
phy and corresponds to K/

, with

defined here as V

For strongly retained compounds (k>>1), Eq. (3) reduces to:

= V

× K × f (N, b)

(4)

This equation represents the “conventional concept” of
breakthrough – the amount of stationary phase multiplied
by the capacity factor, corrected for the speed of sam-
pling. Both V

and K are usually readily determined from

simple experiments, so all that must be found is an ex-
pression for f(N,b). In the literature there has been some
controversy about the definition of the breakthrough fac-
tor, as will be illustrated below.

Definition of the breakthrough factor (b)

In the literature two definitions of the breakthrough loss or
breakthrough factor (b) are used side by side. The first, and
most commonly applied, breakthrough factor is based on
the momentary loss of analyte; this will be referred to as the
differential breakthrough factor (b

). The second definition

of breakthrough is based on the total loss of analyte and will
be referred to as the integral breakthrough factor (b

). Mathe-

matically, the differential breakthrough factor is defined as:

(5)

where C

O,V

is the concentration of analyte in the outgoing

sample at a certain sampled volume (V) and C

is the con-

centration of analyte in the original sample. This is pre-
sented graphically in Fig. 2 where the 10% breakthrough
factor is shown on the curve.

Although the differential definition can easily be cal-

culated from chromatographic theory, for real-life sam-
pling this is not often very useful, because the value of b

only represents the momentary loss at the point sampling
is stopped, the breakthrough volume. The momentary an-
alyte loss increases with increasing volume sampled,
however, as is apparent from Fig. 2; this means that al-
though at the predicted breakthrough volume the momen-
tary analyte loss will be b

, the overall amount of analyte

lost will be less than b

. If, therefore, a sample loss of

10% is acceptable (a b

of 0.1), at the calculated break-

through volume the actual overall analyte loss will always
be less than 10%. Not only does this lead to reduced sen-
sitivity, because a larger volume could have been sam-
pled, it is also impossible to correct for sample losses, be-
cause the exact analyte loss it not known.

The arguments presented above have led to the adop-

tion of the integral breakthrough factor, b

, which is de-

fined as:

(6)

As is apparent from Fig. 3, the integral definition of the
breakthrough volume is based on the amount of analyte
lost from the trap relative to the total amount of analyte
sampled. At the breakthrough volume under integral break-
through conditions the amount of analyte lost is exactly
equal to b

. The predicted breakthrough volume under in-

tegral conditions thus represents the true maximum vol-
ume that can be sampled before a predetermined portion
of analyte (b

) is lost.

In practice care should be taken when comparing dif-

ferent means of calculation of f(N,b), because b can be ei-
ther a differential- or integral-based term. If, moreover,
breakthrough factors are determined experimentally, inte-
gral rather than differential breakthrough factors are de-
termined. The use of b

rather than b

is, therefore, usually

recommended.

Differential breakthrough factors

This model for analyte breakthrough was initially devel-
oped by Werkhoven-Goewie [1] and assumes that ana-

Fig. 2 Illustration of the definition of the differential breakthrough
factor (b

). Shown here is the 10% breakthrough volume under

differential conditions

Fig. 3 Illustration of the definition of integral breakthrough. Shown
here is the 10% breakthrough volume under integral conditions

lytes elute from the extraction column as Gaussian-shaped
bands; this is valid for sufficiently large plate numbers.
The expression then found for f(N,b), from Eq. (4), is:

(N, b) = 1 −

)

√

(7)

)

0.1%

3.090

2.326

2.5%

1.960

1.645

10%

1.280

where a

is a constant which depends on the accepted

breakthrough. Values for a

are shown in Eq. (7). Figure 4

shows a graph of the dependence of f(N,b) on N for dif-
ferent values of b

. It is clear that at any b

a non-negative

breakthrough volume is obtained only at a certain, critical
plate number (N

crit

). In practice this implies that at

very low plate numbers immediate breakthrough is expect-
ed. A sample loss of 10% before sampling is even started
is, however, very unrealistic and is, in fact, an artifact of
Gaussian theory, which is valid at relatively high plate
numbers only. Care should, therefore, be taken when us-
ing values predicted by Eq. (7), which should, preferably,
be used only if f(N,b) is predicted to be in excess of 0.5
(50%).

With the theory presented above it is possible to cal-

culate breakthrough volumes – all that is required is an
expression for calculation of plate numbers. For capillary
(open tubular) traps this can be the Golay equation,
which gives an exact theoretical description of the plate
number; for packed columns semi-empirical equations
such as those proposed by van Deemter and Knox can be
used.

Integral breakthrough factors

The problems associated with the differential break-
through theory described above have led researchers to
develop alternative, more realistic, expressions for f(N,b).
Lövkvist and Jönsson [2] compared several dedicated
equations for breakthrough curves at low plate numbers

and suggested the following expression, which they found
to be valid for strongly retained compounds.

(N, b) =

1
2

(8)

where a

L,0

, a

L,1

, and a

L,2

are constants which depend on the

breakthrough factor (b

). Values for a

L,0

through a

L,2

are

listed in Table 1. Breakthrough curves predicted by use of
Eq. (8) are shown in Fig. 5. It is clear that non-negative
breakthrough volumes are predicted at any plate number.
For high plate numbers f(N,b) can become larger than
unity; this is indicative of a breakthrough volume in ex-
cess of the retention volume. This is not an artifact of Eq. (8)
but is a result of the definition of the integral break-
through factor. For a trap with an infinite plate number
(N=

∞

) no analyte is lost before the retention volume is

reached. At V

, therefore, the breakthrough factor is still

zero. For sample volumes in excess of the retention vol-
ume the amount of analyte sampled at the trap inlet is
equal to the amount lost at the trap outlet, hence the break-
through factor increases from this point on.

The authors of Eq. (8) recommend the use of their break-

through expression with a modified Knox equation [3, 4]
for prediction of plate numbers:

= 3υ

+ 0.05υ

(9)

where h

is the reduced plate height and

is the reduced

velocity in the trapping column. These reduced properties
are defined as:

×d

υ =

×d

(10)

Fig. 4 f(N,b) predicted from the differential Gaussian breakthrough
model (Eq. 7). Five curves are drawn for different breakthrough
fractions (b

)

Fig. 5 f(N,b) predicted from the integrated Lövkvist breakthrough
model (Eq. 8). Five curves are drawn for breakthrough fractions
(b

) of 0.5 to 10%

Table 1 Terms of Eq. (8) as a function of the breakthrough level
(b

)

L,0

=(1–b

)

L,1

L,2

0.5%

0.990025

17.92

26.74

0.9801

13.59

17.6

0.9604

9.686

10.69

0.9025

5.360

4.603

10%

0.81

2.787

1.941

where H is the plate height, in m; d

is the diameter of

packing particles, in m; L is the length of the trapping
column, in m; u is the superficial linear velocity in the
trap, in m s

–1

and D

is the diffusion constant in the mo-

bile phase, in m

–1

. Eqs. (9) and (10) are valid only

when the pressure drop over the packed bed can be ne-
glected.

Adsorptive sample preparation

Adsorbents for thermal desorption can be divided into
three categories. The first includes inorganic carbon-based
materials such as carbon blacks, carbon molecular sieves,
and activated carbon. These materials usually have very
high affinity for organic compounds and are most often
used for gaseous samples. Carbon molecular sieves can be
used to retain C

–C

hydrocarbons, and even methane,

whereas activated carbon and carbon blacks are more
suited to less volatile analytes. Because of their inorganic
nature, these materials can be heated to high temperatures
(400–450 °C) without degradation.

The second category includes inorganic materials based

on silica and alumina. Silica-type materials can be used
unaltered to trap analytes from gaseous and liquid sam-
ples and are generally more suited to larger molecules
than are inorganic carbon-based adsorbents. These materi-
als are thermally stable up to 400–600 °C. The surfaces of
these materials can also be covered with organic groups,
resulting in materials such as octadecylsilica (ODS),
which enable very successful enrichment of liquid sam-
ples. The thermal stability of organic-coated silicas is, how-
ever, poor, because at elevated temperatures (>100 °C) the
organic groups tend to be expelled from the surface.

The third category is the polymeric adsorbents. This is

the largest and most diverse group and comprises many
commonly used materials, e.g. Tenax and Chromosorb.
Most are synthetic and consist of polymers of building
blocks such as styrene. One of the most important draw-
backs of this type of material is that (especially on heat-
ing) depolymerization occurs, releasing monomeric units
and reaction products thereof. These, unfortunately, include
many of the target analytes, e.g. styrene and benzene. At
moderate temperatures, at which adsorbent degradation is
not very pronounced, small quantities of emitted com-
pounds can easily lead to false-positive results. Tenax, the
most commonly used organic adsorbent, is particularly
notorious for its background of benzaldehyde, acetophe-
none, benzophenone, and other aldehydes and ketones.
In practice maximum temperatures range from 150 °C
for some Chromosorbs to 350 °C for Tenax, although
traces of water or oxygen strongly promote degradation
and can lead to significant deterioration of the back-
ground.

The adsorbent surface always contains active groups

(adsorptive sites) that can interact with the analytes and
bond them to the surface. Depending on the nature of ad-
sorbent and analyte the interaction can range from very
weak van der Waals-type bonding to very strong ionic in-

teraction. The strength of the interaction also determines
the desorption process required. Desorption with liquid
can break strong adsorbent–analyte interactions whereas
thermal desorption can overcome relatively weak van der
Waals- type interactions only. Adsorptive sampling is,
therefore, most often combined with liquid desorption.
Thermal desorption is prone to poor recoveries, even at
very high temperatures. These high temperatures might
result in degradation of the adsorbent materials or promote
catalytic breakdown of the trapped analytes; this is most
pronounced for polar analytes. Adsorptive sampling is,
therefore, not often used in combination with high-sensi-
tivity thermal desorption. Applications of adsorptive sam-
pling–thermal desorption are almost completely limited to
very apolar analytes, e.g. hydrocarbons such as alkanes,
alkenes, and aromatics. There is clearly an urgent need for
alternative techniques that enable extension of the range
of applicability of thermal desorption to more polar com-
pounds. The rest of this review evaluates sorptive sample
preparation as an ideal means of overcoming the limita-
tions of adsorptive sampling.

Sorptive sample preparation

Sorptive materials (or sorbents) are a group of polymeric
materials with a glass transition temperature (T

) below the

temperature at which the material is used during the sam-
pling–storage–desorption process. Although, initially, this
might seem a trivial requirement, the consequences are
enormous. At temperatures above their T

polymeric ma-

terials no longer behave as solid materials but assume a
gum-like, or even liquid-like, state with properties, e.g.
diffusion and distribution constants, similar to those of or-
ganic solvents.

Sorbents are, in principle, homogeneous, non-porous

materials in which analytes can actually dissolve. The an-
alytes do not, therefore, undergo real (temporary) bonding
with the material but are retained by dissolution. It should
be noted that all sorptive materials work only in the sorp-
tion regime above their glass transition point. This means
that on cooling any sorbent loses the sorptive mechanism
below its glass transition point and is then turned into an
adsorbent with a low specific surface area.

The most commonly used sorbent is the apolar polydi-

methylsiloxane (PDMS), a 100% methyl-substituted silox-
ane polymer commonly used as a stationary phase in gas
chromatography. Its structure is shown in Fig. 6. This ma-
terial is so popular because it is very inert, reducing the
risk of losses of unstable and/or polar analytes by irre-
versible adsorption or by catalytic (surface) reaction. Re-
tention data for many compounds can be found in the lit-
erature. In addition, PDMS synthesis is relatively simple
and leads to very reproducible properties, and consistency
between manufacturers. Its degradation products are, more-
over, very well known and can easily be identified by mass
spectrometry. These advantages and the lack of availabil-
ity of other materials as stable, reproducible, and inert as
PDMS account for its widespread use in sorptive sample-

preparation techniques. Alternative materials, e.g. the po-
lar poly(butyl)acrylates, are used for more polar analytes
that have a low affinity for PDMS and consequently do
not partition very well into this material. The mechanical
stability of sorbents is usually provided by means of
crosslinking, which ensures that the extraction phase will
retain its shape, even at elevated temperatures. It is essen-
tial to realize that when sorbents are used preconcentra-
tion of analytes occurs by sorption of the analytes into the
polymeric liquid phase instead of adsorption on to a solid
adsorbent surface. Different sorptive enrichment proce-
dures are based on the different approaches and geome-
tries in which the sorbent is used for sample extraction.

Four techniques, all of which have been well described

in the literature, can be distinguished. The first, open-
tubular trapping, is the oldest technique and employs a
(thick film) capillary GC column for sampling. The sec-
ond technique, solid-phase microextraction (SPME) is
based on use of a PDMS-coated fiber which, when not in
use, is protected by being withdrawn within the needle of
a syringe-like device. The third technique, gum-phase ex-
traction (GPE), is based on a bed packed with sorbent ma-
terial. The applicability of this technique has been illus-
trated in the breakthrough mode and by use of the novel
equilibrium sampling approach (EGPE). The fourth, and
latest, sorptive technique is stir-bar-sorptive extraction
(SBSE) which is based on static extraction of liquid sam-
ples with a sorbent-coated stir-bar.

Open-tubular trapping

Although open-tubular traps coated with adsorbent parti-
cles have been used [6, 7], focus has been on sorbent-
coated capillaries, because of their favorable characteris-
tics (which are similar to those of capillary GC columns).
Occasionally short capillary traps, which can be desorbed
in the injector (e.g. PTV or split/splitless) of a gas chro-
matograph, have been used [8]. More commonly, coated
lengths of fused silica columns with an inner diameter of
0.3–0.5 mm are employed; the typical film thickness is
10–15

m [9] but occasionally high-capacity open-tubular

traps with extremely thick films of 100

m [10] or even

165

m [11] are preferred. Although films up to 15

can be prepared in short columns by procedures com-
monly employed for the preparation of thick film capillary
columns [12], the use of a low-molecular-weight methyl-
siloxane polymer was suggested by Bicchi et al. [13], be-

cause it enables the preparation of 15-

m films in capil-

lary traps up to 5 m long. Immobilization of the stationary
phase is performed by addition of dicumyl peroxide to the
coating solution. Film stability is further enhanced by
treatment with azo-t-butane. Both agents effect crosslink-
ing upon heating. It has been found that conditioning for
one week at 250 °C is required if a perfectly immobilized
and stable film is to be obtained. Classical coating tech-
niques are not suitable for thicker films, because the de-
posited film will quickly rearrange into droplets owing to
drainage and Rayleigh instability [14].

An approach that enables preparation of traps contain-

ing very thick films and circumvents the problems associ-
ated with dynamic coating has been described by Roeraade
et al. [10, 15, 16]. The film was fixed by heat-acceler-
ated crosslinking of a suitable pre-polymer in a process in
which the column is pulled through an oven at the same
speed as evaporation of the coating liquid. In this way sta-
ble films up to 100

m thick could be obtained. Traps

with even thicker films can be produced by means of an
innovative process described by Burger et al. [11, 17]. In-
stead of using coating solutions or pre-polymers these
authors used (crosslinked) polydimethylsiloxane tubing
(0.65 mm outer diameter, o.d. and 0.3 mm inner diameter,
i.d.), which was transferred into a fused-silica capillary by
use of liquid nitrogen. This procedure facilitates the pro-
duction of very thick films. The advantage of this ap-
proach is that the capillary obtained is very stable with a
low background profile and favorable sorption character-
istics.

Open-tubular trapping – gaseous samples

Open-tubular traps have been applied to the analysis of
gaseous samples by many groups [8, 9, 10, 11, 18]. Most
commonly traps up to 1–3 m long are used for the reten-
tion of gaseous analytes. Sampling is usually performed
by means of a vacuum pump attached to the outlet of the
open-tubular trap. The reverse, pushing the sample through
a trap by means of a pump is not a very good approach,
because this can easily lead to contamination of the sam-
ple by compounds released by the pump or to alteration of
the composition of the sample as a result of (ad)sorption
of analytes inside the pump. This limitation to sampling
by suction implies that restriction of flow by the open-
tubular trap should not be excessive, otherwise a very low
flow rate will result, leading to excessively long sampling
times. For high capacity (expressed as K

, Eq. 8), on

the other hand, a long open-tubular trap is required, espe-
cially for volatile compounds. In practice, a compromise
must be found between sample capacity and sampling
speed.

Open-tubular traps have been successfully employed

for a range of gaseous samples including wine headspace
[19], plant volatiles [9, 13, 20], pheromones [21], and en-
vironmental air samples [22, 23]. For a wide variety of
compounds, e.g. alkanes, aromatics, esters, and alcohols,
good performance was obtained at trace concentration lev-

Fig. 6 Structures of sorbents commonly used for sample enrich-
ment. Glass transition temperatures [5]: polydimethylsiloxane
(–125 °C) and polybutylacrylate (–54 °C)

els, illustrating the favorable properties of sorbents. For
gaseous samples, in particular, the use of very-thick-film
traps seems essential, because sufficient trapping capacity
from samples up to ca. one liter is possible with these
traps, even for the most volatile analytes. The disadvan-
tage of using films up to 200

m is that desorption is slow

and cryofocusing of the thermally released analytes be-
fore injection on to the analytical column becomes essen-
tial. Cryotrapping can be performed either in a separate
device or on-column [24] with liquid carbon dioxide or
nitrogen; this results in full utilization of the efficiency of
the analytical column.

By use of open-tubular traps coated with 80-

m PDMS

films Blomberg and Roeraade [25] demonstrated the via-
bility of OTT for collection of fractions of compounds
eluting from a capillary GC column. Over extended time
periods and from multiple GC runs compounds were
trapped quantitatively on the OTT. Recovery of the collect-
ed volatile compounds was accomplished either by ther-
mal desorption or by extraction of the OTT with pentane.
Complete recoveries could be obtained by either method.

The use of open-tubular traps for enrichment of sam-

ples for high-speed narrow-bore capillary gas chromatog-
raphy was described by Pham-Tuan et al. [26] In contrast
with the normal sampling procedure, in which sampling is
stopped before breakthrough of the analytes, sampling
was continued until the trap was saturated with analyte
and had reached equilibrium with the sample. In equilib-
rium sampling the open-tubular trap was fully saturated
with sample before thermal desorption. The trap was sub-
sequently heated for desorption and only a small part
(time-sliced injection) was transferred to the analytical
column. This enabled the use of open-tubular enrichment
without the need for cryotrapping or other focusing tech-
niques. This can be a substantial advantage in field appli-
cation of (micro) GC, when cryofocusing is very imprac-
tical; the use of sorptive sampling preparation can, there-
fore, be extended to this field also. Unfortunately, heart-
cutting injection results in comparatively low sensitivity,
particularly in combination with the thermal conductivity
detector (TCD) of the micro-GC; this results in a very
limited range of application.

Open-tubular trapping – liquid samples

Open-tubular trapping can be an attractive alternative to
classical techniques for the enrichment of aqueous sam-
ples. The main advantage of OTT over alternative tech-
niques is that complete removal of water from the trap can
be achieved by purging the capillary with a short plug of
gas. Long drying times, often needed for solid-phase ex-
traction, are not required. The main disadvantage of OTT
is, however, is its low retentive power for the trapping of
compounds from aqueous samples, particularly very polar
compounds that do not partition strongly into the station-
ary phase. Also, because of the low diffusion coefficient
of compounds in the liquid phase, the flow rate during
sampling is rather critical and only very low flow rates

can be tolerated [27]. An HPLC pump can be used for ef-
fective delivery of the sample, free from contamination, to
the capillary trap so that pressure limitations are not im-
portant, as was observed for gaseous samples.

Although open-tubular traps with films <15

m can be

used successfully, low breakthrough volumes often result,
because of the small amount of stationary phase present.
The use of thick-film traps [28], with films up to 165

seems more promising, because they enable the retention
of analytes from larger sample volumes. Thin-film traps
can be used for sample volumes up to 2.5 mL if the sta-
tionary phase is swollen by absorption of chloroform. This
has been demonstrated by Mol et al. for analytes ranging
from the apolar toluene to the more polar dimethylphenol
and chloroaniline [29]. Kaiser and Rieder [30] described
an OTT technique which used the same capillary for both
analyte enrichment and chromatographic separation. This
was achieved by backflushing the capillary between these
two steps and cryotrapping the analytes at the head of the
column.

Mol et al. [31] described the use of an open-tubular

trapping column as a means of phase switching in on-line
reversed phase LC–GC. By use of the breakthrough the-
ory described above (Eqs. 3, 4, 5, 6, 7, 8) in combination
with the Golay equation for determination of the number
of theoretical plates in the open tubular column, the con-
ditions for OTT were calculated and optimized. As an ap-
plication, the 16 EPA priority PAH were trapped from an
LC effluent on an open-tubular trap. After brief drying the
analytes were desorbed with a small amount (80

L) of

hexane which was subsequently injected in to a large-vol-
ume-injection (LVI)–PTV–GC system.

Equilibrium enrichment for aqueous samples analo-

gous with that used for gaseous samples has been described
by Aguilar et al. [32]. Again the sample was passed through
the trap until equilibrium was reached between the sample
and coated stationary phase. The applicability of the sys-
tem was illustrated for very polar and small analytes
which cannot easily be determined quantitatively by any
other technique. Analysis of amines at low

g L

–1

levels

in aqueous samples is illustrated in Fig. 7.

The past four years has seen renewed interest in open-

tubular trapping as a technique for enrichment of aqueous

Fig. 7 Equilibrium enrichment of a sample of river water contain-
ing ethylamine (1), pentylamine (2), and hexylamine (3), each at a
concentration of 2

g L

–1

. The sample volume was 40 mL and the

desorption temperature 275 °C. Published in Journal of Chro-
matography A [32]

samples. Under the name “in-tube solid-phase microex-
traction” (ITSPME) several groups have published sev-
eral interesting applications. It should be noted, however,
that this technique is not the same as solid-phase microex-
traction, which is described below. Surprisingly, all publi-
cations on ITSPME employ adsorptive stationary phases
for sample extraction, probably because the small amount
of stationary phase that can be coated inside an OTT re-
quires the use of highly adsorptive materials. When com-
bined with the equilibrium sampling approach chosen in
all ITSPME applications, particular attention must be de-
voted to matrix effects and competitive adsorption prob-
lems. Although ITSPME does not qualify as a sorptive
enrichment technique it is still briefly described below,
because of its strong analogy with OTT.

In the original publication of Eisert and Pawliszyn

[33], Omegawax was used to extract several phenylurea
pesticides from aqueous samples. Although this Carbo-
wax-type stationary phase is often used as a sorptive sta-
tionary phase in gas chromatography, it has true sorptive
properties only above its glass transition point (50–60 °C).
At the temperature of sampling, which was not reported
but was probably room temperature, this material is es-
sentially an adsorbent material. The adsorptive nature of
Omegawax might also explain the relatively slow rate at
which equilibrium was reached. This is yet another illus-
tration of the limitations of (non-porous) adsorptive mate-
rials in sample preparation. Analysis of phenylurea herbi-
cides by ITSPME–HPLC–UV was demonstrated. Detec-
tion limits were approximately 10

g L

–1

. By use of more

or less the same system, combined with mass spectromet-
ric detection,

-blockers and metabolites were success-

fully monitored at low ng mL

–1

levels in urine and serum

[34].

Use of a polypyrrole extraction column resulted in im-

proved performance for these compounds, with detection
limits below 0.1 ng mL

–1

[35]. Analysis of the drug ranit-

idine at the low- to sub-ng L

–1

level has been achieved by

ITSPME on an Omegawax column in combination with
liquid desorption–LC–MS [36]. Reliable quantitation of
ranitidine in serum over the range 5–1000 ng mL

–1

, with

within- and between-day variation of 2.5 and 6.2%, re-
spectively, was demonstrated. Tan et al. [37] have de-
scribed the use of ITSPME with a poly(ethylene glycol)
extraction capillary combined with liquid desorption–GC
analysis for the analysis of phenols and BTX compounds
(benzene, toluene, xylene and related compounds) from
aqueous samples. The performance of the technique under
practical sampling conditions cannot, unfortunately, be as-
sessed, because the concentration levels investigated were
unrealistically high (between 10 and 40 mg L

–1

In a recent paper Mullett et al. [38] reported the use of

a molecularly imprinted polymeric material for the selec-
tive determination of propranolol in biological samples.
Although recovery was good, because of the high affinity
of the adsorbent for the target analytes, improved selectiv-
ity of the material for the target compound was not dem-
onstrated.

Solid-phase microextraction

Solid-phase microextraction (SPME) is a powerful and in-
novative extraction procedure introduced by Arthur and
Pawliszyn in 1990 [39]. SPME employs a fused-silica
fiber with an outer diameter of, typically, 150

m which is

coated with an (ad)sorbent layer 5 to 100

m thick. This

fiber can simply be inserted into a gaseous or aqueous
sample for analyte extraction and into the heated zone of
a gas chromatograph injector for desorption [40]. The
small size of the SPME fiber and its cylindrical shape en-
able it to fit inside the needle of a syringe-like device. The
SPME fiber is attached to the syringe plunger and this
arrangement can be used to expose the fiber for extraction
or desorption and to retract the fiber for storage and pierc-
ing of injector and sample vial septa. The latter is neces-
sary because the coated fused-silica fiber has a very low
mechanical strength and cannot, as such, be inserted di-
rectly through septa. Here the SPME literature is re-
viewed briefly to identify the strengths and weaknesses of
the technique. For a more detailed and complete overview
the reader is referred to the literature [41].

An interesting feature of sorptive extraction on PDMS,

that partition constants between sample and fiber can be
estimated from literature data, has been well illustrated
for SPME of both gaseous [42] and aqueous [43] samples.
Linear temperature-programmed retention indices [44]
have been used to estimate equilibrium constants between
the PDMS fiber and a gaseous sample, on the basis of the
assumption that the behavior of solutes in a gas chromato-
graphic column is similar to that of the same solutes in the
gas phase–SPME coating equilibrium. By comparison of
retention indices determined on a capillary GC column and
with the SPME extraction device the validity of this con-
cept was confirmed. A combination of the gas–PDMS equi-
librium constant and Henry’s law (water–gas equilibrium)
was used to yield a value for the PDMS–water distribu-
tion coefficient [45]. This approach can be used for many
volatile solutes, for which a Henry’s law constant is read-
ily available. For semi-volatiles compounds the PDMS–
water distribution constant can be approximated by the
octanol–water partition coefficient [46, 47, 48].

Numerous applications to the determination of pesti-

cides and other priority pollutants in aqueous samples
have been described in the literature; a brief selection is
listed in Table 2. Reported detection limits cover a very
wide concentration range, from as low as 0.01 ng L

–1

(ppt)

to as high as 9 mg L

–1

(ppm). This is partly because of the

different analytical systems used – low-sensitivity FID,
high-sensitivity ECD, and ion-trap detection (ITD) and
other forms of mass spectrometry. More important, the
polarity of the target compounds also differs widely from
very apolar (PAH, PCB) to polar (some pesticides); this has
a substantial effect on the extent to which analytes parti-
tion from the polar water matrix into the SPME fiber. Most
compounds can be monitored below the desired 1

g L

–1

(ppb) level for surface water. The limit for drinking water
analysis (0.1

g L

–1

) cannot, unfortunately, be reached for

many analytes, not even if high-sensitivity GC–ECD or

GC–MS is applied. This lack in sensitivity is the most im-
portant disadvantage of SPME and is partly because the
sorbents have significantly lower analyte capacity than
typical adsorbents. This, combined with the extremely
small amount of sorbent coated on to the SPME fiber, up
to 0.5

L, makes overcoming the sensitivity problem hard

to achieve. As a result sorption SPME is today, almost
10 years after its introduction, still not as widely accepted
as deserved, despite its clear instrumental advantages,
simplicity, and low cost.

To improve the capacity of SPME fibers, several “new”

SPME coatings have been introduced [58, 60]. These in-
clude materials such as copolymers of PDMS with divi-
nylbenzene (PDMS–DVB) and Carbowax (PDMS–WAX)
and physical mixtures of PDMS with adsorbents such as
Carboxen. Although these materials do indeed, have sig-
nificantly increased trapping capacity, an important draw-
back is that the true sorption mechanism is lost, because
these materials are no longer pure polymeric sorbents.
Carbowax, for example, is used below its glass transition
temperature (ca. 70 °C) and Carboxen is an inorganic ad-
sorbent. Application of these materials in static sampling
is likely to lead to irreproducible results, because adsorp-
tion of matrix compounds (salts, humic acids, proteins,
etc.) will compete with the target analytes for available
adsorbent sites. This complicates, even prohibits, reliable
quantitation in SPME.

In a study on the extraction of benzodiazepines from bi-

ological fluids [61] several SPME fibers, PDMS, PDMS–
DVB, acrylate and WAX–DVB, were compared. It was

found that the performance of the PDMS fiber in the ex-
traction of the polar benzodiazepines was very poor, be-
cause these analytes do not partition strongly into the apo-
lar PDMS bulk. The other fibers, which are more polar in
nature, were able to extract a significantly larger amount
of analyte. The highest recoveries were observed for the
WAX–DVB fiber, closely followed by the acrylate fiber,
which yielded recoveries at least half those for the WAX–
DVB fiber for all compounds. The authors preferred the
WAX–DVB fiber, because of its greater recoveries, and
obtained detection limits of the order of 0.02–0.1 mg L

–1

(ppm) from 1–2 mL samples by use of ion-trap mass spec-
trometry. Although addition of salt was found to have a
positive effect on recovery (salting-out effect), the effect
of (high concentrations of) other matrix compounds (e.g.
proteins) was not investigated, even though this also is
likely to have a pronounced effect. If the acrylate sorbent
phase had been used these effects would have played a
much less important role and, therefore, the acrylate fiber
might probably have been preferred, because of its favor-
able sorption, rather than adsorption, characteristics.

An alternative to SPME sampling directly in the aque-

ous phase is SPME extraction of compounds present in
the headspace of the sample – headspace-SPME. This was
described theoretically by Zhang and Pawliszyn [62] and
by Ai [63]. In headspace-SPME volatilized analytes are
extracted and concentrated in the SPME coating; this can
have several advantages over direct SPME extraction in
the liquid phase. For analytes that partition strongly into
the SPME fiber equilibration times can be reduced sub-

Table 2 Overview of the per-
formance of typical SPME pro-
cedures for extraction of pesti-
cides and other priority pollu-
tants from aqueous samples

VOC, volatile organics; VHOC,
volatile halogenated organic
compounds; PAH, polyaromat-
ic hydrocarbons, PCB, poly-
chlorinated biphenyls, ITD,
ion-trap detector; FID, flame-
ionization detector; NPD, ni-
trogen–phosphorus detector;
ECD, electron-capture detector

no film thickness mentioned

phenols were derivatized with

acetic anhydride before analy-
sis

Compounds

SPME fiber

Sample

Technique

Detection limit

volume

VOC, VHOC [49]

PDMS, 100

50 mL

GC–ITD

0.001–1

g L

–1

PAH, PCB [50]

PDMS, 15

40 mL

GC–ITD

low ng L

–1

N-herbicides [51]

Acrylate, 95

4 mL

GC–FID

0.2–20

g L

–1

GC–NPD

0.01–6

g L

–1

GC–ITD

0.01–15 ng L

–1

N,P-Pesticides [52]

PDMS, 100

4 mL

GC–NPD

0.02–37

g L

–1

GC–ITD

0.01–8

g L

–1

Cl-pesticides [53]

PDMS, 100

35 mL

GC–FID

2–9000

g L

–1

GC–ECD

0.05–9

g L

–1

GC–MS

0.02–800

g L

–1

P-insecticides [54]

Acrylate, 85

4 mL

GC–FID

0.2–5

g L

–1

GC–NPD

0.01–0.5

g L

–1

GC–ITD

0.002–0.1

g L

–1

P-pesticides [55]

PDMS, 100

3 mL

GC–NPD

0.002–0.1

g L

–1

Acrylate, 85

0.001–0.1

g L

–1

Triazine herbicides [55]

Acrylate, 85

3 mL

GC–NPD

0.01–0.09

g L

–1

2,6-Dinitroaniline herbicides [55]

Acrylate, 85

3 mL

GC–NPD

0.008–0.06

g L

–1

P-Pesticides [56]

PDMS, 100

3 mL

GC–NPD

0.02–0.5

g L

–1

Acrylate, 85

0.006–0.1

g L

–1

Cl-Pesticides [57]

PDMS, 100

110 mL

GC–ECD

0.3–11 ng L

–1

Anilines [58]

PDMS–DVB

5 mL

GC–FID

0.18–3.17

g L

–1

Phenolic compounds

[59]

Acrylate, 95

40 mL

GC–FID

0.6–30

g L

–1

GC–MS

0.01–1.6

g L

–1

stantially, because diffusion coefficients are higher in the
gas than in the liquid phase [62]. By use of headspace-
SPME samples containing high-molecular-weight or par-
ticulate material can be analyzed with greater accuracy.
Fiber lifetime is also extended, because these unwanted
compounds do not come into contact with the fiber. This
was demonstrated for the analysis of serum samples by
Namera et al. [64] and for urine samples by Fustinoni et
al. [65]. In both applications rapid equilibration was ob-
served, as were high reproducibility and detection limits
at the low

g L

–1

to ng L

–1

level, depending on analyte po-

larity and the detector used.

Llompart et al. [66] compared headspace-SPME with

conventional headspace analysis (at 95 °C) and liquid–liq-
uid extraction for a range of (semi-)volatile compounds. It
was found that quantitative results from headspace-SPME
were similar to those from both conventional techniques,
although for most compounds a large increase in sensitiv-
ity was obtained.

In a study of wine-bouquet components by headspace-

SPME, de la Calle-Garcia et al. [67] used an acrylate fiber
to extract of terpenoids and related compounds. Although
good performance was usually observed it was noted that
the recovery was highly dependent on the number of ex-
tractions performed with the same fiber. During the first
35 headspace-SPME extractions peak areas decreased by
as much as a factor of four, which complicated quantita-
tion. This problem is still not well explained but can be
overcome by use of internal standards.

Headspace-SPME is not only a successful approach to

the analysis of gaseous and liquid samples but can also be
used for analysis for solid samples or even for the direct
analysis of air samples. Fromberg et al. [68] used head-
space-SPME to determine a range of chlorinated and ni-
trated aromatic compounds in soil. It was found that ana-
lyte recoveries depended on many factors, including soil
humidity and the type of soil, and that matrix effects, es-
pecially those which depended on the organic carbon con-
tent, were so large that quantitation of unknown samples
was impossible. It was also found that equilibration times
were very large for high-MW analytes, which partition
strongly into the SPME fiber but diffuse only slowly from
the sample, through the gas phase, to the fiber. Equilibra-
tion times up to 10 h were observed for some analyte–ma-
trix combinations. Only low-MW, apolar analytes could
be quantified in some soils of unknown origin. Other ap-
plications of headspace-SPME include the analysis of
monoterpenes from conifer needles [69], organic acids in
tobacco [70], volatile compounds in apple fruit [71], ter-
penoids in herb-based formulations (including drops)
[72], and methylmercury in fish tissue [73]. Long equili-
bration times were usually required for solid samples, ex-
cept for volatile compounds, and quantitation was diffi-
cult for many analytes. SPME can, therefore, be regarded
as a tool for “rapid” sample screening but the generation
of accurate quantitative data is often difficult. A means of
circumventing long equilibration times in the headspace
SPME extraction of solids was proposed by Moens et al.
[74]. These authors developed a method for the simulta-

neous determination of organomercury, -lead, and -tin
compounds in sediment samples. The approach is based
on liberation of the target analytes from the solid matrix
by ultrasonic liquid extraction then liquid-phase derivati-
zation. The analytes were finally sorbed into a PDMS-
coated SPME fiber and analysis was performed by
CGC–ICP–MS. Detection limits at the ng L

–1

level were

reported and the total analysis time, including gas chro-
matographic determination, was approximately 75 min.

SPME with PDMS fibers can also be used for direct air

sampling [40], using equilibrium constants derived di-
rectly from GC retention data. In an application of SPME
to the analysis of volatile compounds in human breath
[75] several compounds including acetaldehyde, acetone,
ethanol, and isoprene were monitored. Most compounds
reached equilibrium within 30 s on the variety of SPME
fibers, and sampling times could be as short as 10 s under
optimum conditions; this enabled rapid sampling and
monitoring.

So far this discussion of SPME has been limited to gas

chromatography. Coupling of solid-phase microextraction
to liquid chromatography [76] or even LC–MS [77] has
been shown to be a viable alternative to GC, however,
particularly for polar compounds, e.g. polar pesticides [78]
or inorganic and/or charged substances such as metal ions
[79], that cannot be analyzed by GC. The latter were con-
verted into crown ether complexes before extraction. The
application of SPME to the analysis of polar pesticides
seems rather difficult, because the partitioning constants
for these analytes into the SPME fiber coatings will be
low. Low recoveries are observed, resulting in detection
limits above the 1

g L

–1

level for most compounds on a

conventional 4.6 mm i.d. column and above 0.5

g L

–1

a miniaturized column with a reduced i.d. of 1.5 mm [78].
The authors proposed the development of stronger (adsor-
bent-type) coatings but this will result in loss of the sorp-
tion behavior, which is not to be recommended for a static
extraction technique.

SPME relies on equilibrium between the sample and

the sorbent-coated fiber. To increase the speed at which this
equilibrium is reached the sample is almost always stirred
during extraction. Although this is a rather straightfor-
ward approach, it is not ideally suited to automated sam-
pling, because of the large number of stir-bars needed, the
manual recovery of these, and possible carry-over prob-
lems. Alternative mixing techniques, e.g. vibration of the
SPME fiber, were investigated by Eisert et al. [80]. It
should, however, be noted that more than 95% of all SPME
experiments are currently performed with stir-bar agita-
tion.

The use of SPME as a tool for investigation of pro-

tein–drug interactions was recently described [81]. By
use of very small sample sizes – as low as 150

L – the

amounts of free and bound drugs could easily be deter-
mined. Vercammen et al. illustrated the effect of SPME
fiber polarity on the enrichment of volatile solutes of dif-
ferent polarity [82]. The headspace of a single rose petal
was analyzed by SPME with both PDMS and PA fibers.
The film thickness of the PDMS fiber was 100

m and

that of the PA fiber 85

m; for both fibers the coated length

was 1 cm. The SPME fiber was exposed to the headspace
for 30 min.

Although the same sample was used for both fibers, the

chromatograms obtained were markedly different, as is il-
lustrated in Fig. 8. The amount of the polar compound
phenylethyl alcohol (peak 1) on the PA fiber was a factor
of ten higher than on the PDMS fiber and that of the me-
dium-polarity compounds, e.g. peaks 2, 3, and 4, was
higher by a factor of ca. 1.5; for the apolar sesquiterpenes
(peaks 6 to 13), however, the amount on the PDMS fiber
was at least a factor of five higher than on the PA fiber.
This discrimination can be tiresome in real-life SPME
sampling, because optimization is required for specific
samples. Dynamic sampling is much more promising in
this respect. Stronger trapping materials might help over-
come this problem, but particular care must be taken with
such materials – if they are adsorbent in nature the bene-
fits of sorption and the ease of static sampling no longer
apply.

In SPME accurate knowledge of the equilibrium parti-

tioning coefficients (K

PDMS/W

) of all compounds before

experimentation can be most helpful in the selection and
optimization of the sampling procedure. It would be very
convenient if these coefficients could be deduced directly
from octanol–water partitioning coefficients (K

O/W

), which

are tabulated for numerous compounds [83]. In several
publications K

PDMS/W

values have been correlated with

O/W

literature data [84, 85, 86]. It was found that over a

specific polarity range K

O/W

and K

PDMS/W

data correlate

very well, especially for low-molecular-weight (MW) an-
alytes such as BTX [87, 88]. The agreement between

O/W

and K

PDMS/W

data illustrate that PDMS and octanol

behave similarly as extracting liquids.

Although this correlation seems to apply to a wide

range of solutes, for high-MW and very apolar solutes,
e.g. polyaromatic hydrocarbons (PAH) and polychlori-
nated biphenyls (PCB) [89], the correlation between K

O/W

and K

PDMS/W

no longer seems valid. For PCB it was found

[90] that with decreasing polarity (increasing K

O/W

by a

factor of 4

) the measured K

PDMS/W

decreased by a

factor of 40, resulting in misprediction of K

PDMS/W

of a

factor 2

. The authors explained their data on the basis

of surface adsorption by the PDMS fibers rather than bulk
partitioning (sorption); they emphasized the validity of
this model by comparison of data from two PDMS fibers
(7

m and 100

m). Despite these seemingly convincing

results, serious doubt arose and several groups performed
detailed investigations into this topic [91, 92, 93]. PCB
(and PAH) are among the compounds with the lowest
water solubility [94], often in the ng L

–1

range, and are

known to be readily adsorbed by glass and other surfaces
(e.g. the stir-bars used in SPME). It was suspected that
this was occurring in the experiments and data presented
by Yang et al. [90].

The SPME experiments of Yang et al. [90] were re-

peated by our group, but in contrast with the experiments
reported earlier not only was the SPME fiber analyzed but
also the (Teflon-coated) stir-bar. SPME recoveries similar
to those reported by Yang et al. [90] were also found in
this study, with recoveries of the most apolar PCB being
lower than for the more polar (less chlorinated) com-
pounds. This effect was, however, strongly counteracted
by the discovery of substantially higher amounts of PCB
on the stir-bar. This clearly illustrated the limits of static
extraction – when extractive competition between the sor-
bent and interfering adsorbent surfaces (e.g. stir-bar, glass
vial, Teflon septum) occurs, quantitative work will be-
come impossible. Here, very large partitioning constants
do not prove beneficial to very sensitive analysis. By use
of fluorescence microscopy Mayer et al. [92] confirmed
that large apolar compounds such as PCB and PAH do in-
deed diffuse through the full thickness of the PDMS film
coated on to the SPME fiber, thereby unambiguously
proving the sorbent nature of PDMS.

Gum-phase extraction

Gum-phase extraction (GPE) strongly resembles SPE or
adsorbent-based air sampling techniques, in that a trap-
ping material is used in a packed-bed configuration. The
applicability of a packed bed containing 100% sorbent
particles (most often PDMS) for use as a means of sorp-
tive enrichment has been illustrated in a number of contri-
butions in the literature [95, 96, 97]. The analytes of in-
terest are first trapped on a sorptive preconcentration trap
which is subsequently desorbed either with a liquid (fol-
lowed by injection of an aliquot into a GC or LC system)
or by heating the trap to desorb the analytes on to a GC
column for analysis. The thermal desorption approach is

Fig. 8 TICs obtained from a single rose petal. A. SPME with
PDMS-coated fiber. B. SPME with PA-coated fiber. The com-
pounds were separated on a 30 m

0.25 mm

0.25

m HP-5MS

capillary column. The oven was programmed from 40 to 325 °C at
15 ° min

–1

. Peak identification: 1, phenylethyl alcohol; 2, 1-ethenyl-

4-methoxybenzene; 3, 2-phenylethyl acetate; 4, 1-(ethylthio)-2-
methylbenzene; 5, 3,7-dimethyl-2,6-octadienoic acid, methyl es-
ter; 6,

-cubelene; 7, copaene; 8–13, terpenes. Published in Jour-

nal of High Resolution Chromatography [82]

often preferred, because it usually ensures higher sensitiv-
ity; applications described in this paper all utilize thermal
desorption.

In principle, GPE can be used for enrichment of both

liquid [98, 99, 100, 101] and gaseous samples. For liquid
samples, however, the packed bed must be dried between
the sampling and (thermal) desorption steps. Because this
causes losses of volatile compounds, for liquid samples
we recommend the use of either SPME or stir-bar-sorptive
extraction (SBSE), neither of which has this disadvantage.
Here only applications of GPE to gaseous samples are
given, because for these samples GPE had the same ad-
vantages as open-tubular trapping and SPME, in terms of
inertness and thermal desorption characteristics, but en-
ables sampling flow rates as high as 2.5 L min

–1

(air sam-

ples), thereby significantly shortening sampling time; this
can also result in a substantial increase in sensitivity. Be-
cause a much larger amount (100–500-fold increase) of
stationary phase is present in a GPE trap compared with
an OTT trap or an SPME fiber, proportionally greater sen-
sitivity is obtained in many applications.

Gum-phase extraction –
reactivity and inertness for sulfur compounds

The performance of packed PDMS traps as an alternative
to adsorbents for air sampling has been evaluated and com-
pared with that of the adsorbents Chromosorb, LiChrolut
EN, Carbotrap 300, and Tenax [102]. For many polar
(and/or reactive) compounds substantially better recovery
was obtained by use of PDMS, because of its high inert-
ness. PDMS degradation results, moreover, in a series of
cyclic siloxane oligomers which are readily detected and
identified by mass spectrometry and do not interfere with
the solutes of interest. Permanent adsorption and reactiv-
ity of PDMS are also suspected to be minimal.

In another contribution [103], adsorbent reactivity and

its influence on analyte conversion/stability was studied
by using several sulfur-containing analytes as model com-
pounds. The enrichment performance of three materials,
PDMS, Carbotrap 300, and Tenax, was compared. Carbo-
trap 300 and Tenax are the two adsorbents most com-
monly employed for air sampling.

Initial recovery studies were performed with a 4-

film PDMS column, a thick-film column specifically de-
signed for the analysis of volatile sulfur compounds. In-
complete recoveries on the adsorbent materials could not
be attributed to permanent adsorption or artifact forma-
tion, because no other peaks were observed in the chro-
matogram. Because relatively high-molecular-weight arti-
facts, which cannot easily be eluted from the thick-film
column, might have been formed, the same experiments
were repeated on a 1

m film column.

Several chromatograms illustrating the enrichment of

the analytes under investigation on the three enrichment
materials, and a chromatogram obtained by direct injec-
tion of a standard solution, are shown in Fig. 9. It should
be noted that to prevent breakthrough a lower sample vol-

ume was chosen for PDMS, because the retentive capac-
ity of PDMS is markedly lower than that of the adsorbent
materials. With Tenax, good results were indeed obtained
for 2-propanethiol and 1-propanethiol, but 2-methyl-2-pro-
panethiol was partly lost. On Carbotrap 300 recovery of
both 2-propanethiol and 1-propanethiol was low. It was
remarkable that the performance of the adsorbents Tenax
and Carbotrap 300 was very poor for these analytes, espe-
cially as the concentration levels were relatively high
(ppm level).

On both adsorbents an early eluting compound (A) was

observed, as is clearly illustrated in Fig. 9. This compound
was identified as 2-methyl-2-propene which originates
from compound 2 by elimination of H

S. This artifact tends

to occur more on Carbotrap 300 than on Tenax, although
the loss of compound 2 is more prevalent on Tenax. The
2-methyl-1-propene formed is probably too volatile to be
trapped on Tenax and is lost immediately when formed.
When Carbotrap 300 was used several peaks were de-
tected between 15 and 17 min; these were identified as the
dimers of the thiols (disulfides) in the mixture and were
formed by elimination of H

from two thiol molecules.

Six different dimers could be formed from the three thiols
present in the mixture; all were found to be present.

The primary advantages of PDMS, its inertness and the

absence of catalytic activity and adsorptive sites, are clear-
ly illustrated by this example. For these highly volatile
solutes, however, breakthrough volumes on PDMS are
much lower than for Tenax or Carbotrap 300, which can

Fig. 9 Chromatograms obtained from volatile sulfur compounds
injected directly (standard) or enriched from a spiked gaseous sam-
ple on Tenax, Carbotrap 300, or PDMS. The sample volume was
150 mL (Carbotrap 300 and Tenax) or 50 mL (PDMS). Peak iden-
tification: 1, 2-propanethiol; 2, 2-methyl-2-propanethiol; 3, 1-pro-
panethiol; A, 2-methyl-1-propene. The chromatogram obtained af-
ter use of Carbotrap 300 also reveals the presence of dimers. A Gers-
tel TDS-2 Thermodesorption system mounted on an HP6980–
HP5972 GC–MSD system (Hewlett–Packard, Little Falls, DE,
USA) was used. A CIS-4 programmable vaporizing injector (PTV,
Gerstel) was used to cryofocus the analytes. The system was used
in a modified form suitable for split desorption only. Published in
Journal of High Resolution Chromatography [103]

be regarded as a disadvantage. EGPE, a slightly modified
technique described below, can help to overcome this
problem.

Gum-phase extraction compared
with solid-phase microextraction –
analysis of plant volatiles

Dynamic sampling on PDMS and Tenax was compared
for sampling of the volatile solutes emitted by living
plants [82]. Dynamic sampling was performed at a flow
rate of 100 mL min

–1

for 10 min. Thermal desorption was

at 225 °C for 5 min, in the splitless mode, with helium as
carrier gas at a flow rate of 150 mL min

–1

. Before injec-

tion on to the analytical column the thermally released an-
alytes were re-focused in a cold trap. Figure 10 shows the
total-ion chromatograms obtained from the headspace of
Jasminum officinale with Tenax (A) and PDMS (B) as ad-
sorbent and sorbent, respectively. Although the chro-
matograms are very similar, several important differences
are apparent. Other peaks, denoted T

and arising from

Tenax decomposition, occur the chromatogram obtained
after enrichment on Tenax (Fig. 10A) and the time win-
dow between 13 and 17 min contains several unidentified
peaks which are typical of Tenax and are not present in
the PDMS profile. Because of the high Tenax background
accurate quantification of two important flavor compounds,

benzaldehyde and acetophenone, which, moreover, have
biological activity as attractants for insects [104, 105, 106,
107], becomes impossible. For comparison, benzaldehyde
was also identified in the PDMS trace by use of ion ex-
traction at m/z 105, but its concentration was ten times
lower than for Tenax sampling (Fig. 10 insert). Careful
evaluation of the chromatograms in Fig. 10 also reveals a
boiling point discrimination effect in the Tenax profile
compared with the PDMS profile; for example, peaks 13,
14, and 18 are larger in the chromatogram obtained from
PDMS than in that from Tenax.

Equilibrium gum-phase extraction

A primary limitation of GPE (on PDMS) is the low break-
through volumes sometimes obtained for volatile com-
pounds. Equilibrium gum-phase extraction (EGPE) is an
extension of normal breakthrough GPE. In EGPE sam-
pling is continued until all the compounds of interest are
in equilibrium with the sorptive material, i.e. beyond the
breakthrough volume. This can be of particular advantage
for volatile analytes, for which the breakthrough volume
(and thus the sensitivity achievable) is low. Because all
analytes partition independently into the sorbent, after an
infinite sampling time a specific equilibrium is reached
for each analyte. Calibration can be no longer be based on
the amount of trapped analyte and the volume sampled –
because analyte is lost during the sampling process, an al-
ternative calibration procedure is required.

In EGPE sampling is performed until the compounds

of interest are in equilibrium with the sorbent. Sampling is
thus stopped (far) beyond the breakthrough point for (most)
compounds. As mentioned above, one of the main advan-
tages of sampling on PDMS is that this material has been
extensively studied in the past, because it is the most
widely applied stationary phase in GC. Literature data on
the retention of many compounds are available as Kováts
retention indices [108]. A simple means of calculation of
equilibrium constants (K) from retention indices (RI) has
been presented in the literature [109]. In the equilibrium
sorption mode the gas phase is in complete equilibrium
with the PDMS sorbent and the sorbed volume of air is
equivalent to the retention volume (=KV

; Eq. 4) of the

trap. The concentration of a compound in the gas phase, C
(in kg m

–3

) can be calculated by use of the equation:

sorbed

≈

sorbed

PDMS

× K

for K

>> 1

(11)

Special care must be taken in the calculation of the equi-
librium-sorbed amount, because it can vary when the com-
position of the sampled gas is not constant over the length
of the trap. For gaseous samples sampled at high flow
rates, however, this is no longer true. Sampling is per-
formed with a reduced pressure at the trap outlet and the
sample pressure (and thus concentration) therefore de-
creases through the bed. As a consequence of the sorption
mechanism, at lower sampling pressures (where the gas

Fig. 10 TIC traces obtained from the headspace of Jasminum of-
ficinale. A. with Tenax as enrichment material. B. with PDMS as
enrichment material. For conditions see text. Peaks labeled T

are

Tenax degradation products. T

is benzaldehyde, T

is acetophe-

none, and T

and T

correspond to the Tenax monomer structure

(see text). The insert shows the ion chromatogram for m/z 105 ex-
tracted from the TIC, which is specific for benzaldehyde. Peak iden-
tification: 1, 3-hexenyl acetate; 2, 2-hexenyl acetate; 3, benzyl al-
cohol; 4, 4-methylphenol; 5, linalool; 6, benzyl cyanide; 7, benzyl
acetate; 8, 3-hexenyl butyrate; 9, 2-hexenyl butyrate; 10, 2-me-
thoxy-4-methylphenol; 11, methyl salicylate; 12, 3-hexenyl 2-meth-
ylbutanoate; 13, phenylethyl acetate; 14, eugenol; 15, methyl cin-
namate; 16, isoeugenol 1; 17, trans-caryophyllene; 18, isoeugenol
2; 19, nerolidol; 20, hexadecanoic acid, methyl ester. Published in
Journal of High Resolution Chromatography [82]

phase concentration, expressed as kg m

–3

, is lower), the

amount of analyte sorbed by the sorbent (m

sorbed

) will be

proportionally lower. For practical purposes this is not
usually a problem when flow rates are kept below 100 mL
min

–1

The nature of the EGPE sorption profile has been re-

ported in the literature [111]. Different volumes of a gas-
eous sample containing five alkanes were sampled on to a
packed PDMS trap. For each individual compound the
measured peak area after enrichment was plotted as a
function of the relative sample volume (actual sample vol-
ume divided by the retention volume of the analyte). In
this way similar sorption profiles were obtained for all
compounds; this is illustrated in Fig. 11.

In the sorption profiles two regions of interest can be

distinguished. The part of the curve, for which V/V

<0.5,

is linear and represents ‘normal’ breakthrough sampling.
Here, the peak area is directly proportional to the volume
sampled. The part of the curve for which V/V

>2 repre-

sents the equilibrium enrichment region in which the
amount of compound sorbed no longer depends on the
volume sampled.

After the theoretical studies, some challenging com-

pounds in air were sampled by EGPE to illustrate its per-
formance [110]. Epichlorohydrin, a not very volatile solute
(RI=696.3), is not easily enriched on classical adsorption
materials, because of the instability of the epoxide ring.
On a 0.45-mL PDMS trap the equilibrium volume of epi-
chlorohydrin is calculated from theory to be 280 mL
[109]. One liter of air was sampled at 100 mL min

–1

guarantee equilibrium in the absence of a pressure drop.
Analysis was accomplished by GC–MSD selected ion
monitoring (SIM) in the positive chemical ionization (PCI)
mode with methane as reagent gas. The enrichment of an
air sample containing 0.2

g m

–3

epichlorohydrin was

presented; 56 pg epichlorohydrin was collected on the
PDMS trap. Epichlorohydrin was clearly detected without
interferences. Detection limits were approximately 10 ng m

–3

(ppt).

In the same paper the monitoring of ethylene oxide

(EO) was described. This highly volatile (RI=424.4 on a
PDMS column) and unstable compound is difficult to an-
alyze by GPE or other techniques. EO from air samples is
normally enriched on HBr-impregnated silica cartridges;
these convert the analyte into 2-bromoethanol which is
subsequently eluted from the cartridge with acetone and
an aliquot is analyzed by GC–ECD. Disadvantages of this
approach include reduced sensitivity (only an aliquot is
injected) and many manual handling steps. For enrich-
ment by EGPE an equilibrium sorption volume of 15 mL
was calculated from the retention index of EO. By use of
mass spectrometric detection in electron-impact (EI) mode,
scanning from 10–200 amu, a detection limit of 20

g m

–3

was obtained by using extraction and quantitation on ions
29 and 44. It should, however, be noted that in this analy-
sis acetaldehyde interferes with the determination of EO,
because the compounds were not completely separated on
the analytical column used (Porabond Q). In practice,
therefore, detection limits might be affected by this inter-
ference.

Stir-bar-sorptive extraction

For aqueous samples a primary limitation of the sorptive
sampling techniques discussed so far is the low sample
capacity (OTT), low sensitivity (SPME), and the loss of
volatile compounds (GPE and EGPE). The theory and
practice of a novel approach for sample enrichment, the
application of sorbent-coated stir-bars – referred to as stir-
bar-sorptive extraction (SBSE), was recently described
[112]. In SBSE an aqueous sample is extracted by stirring
for a predetermined time with a PDMS-coated stir-bar.
The stir-bar is subsequently removed from the aqueous
sample and the absorbed compounds are then either ther-
mally desorbed, and analyzed by GC(–MS), for very high
sensitivity, or desorbed by means of a liquid, for improved
selectivity or for interfacing to an LC system. Although
this technique is still relatively new, several interesting
applications have already been reported in the literature;
these will be summarized briefly here.

In SPME, the maximum volume of PDMS coated on to

the fiber is ca. 0.5

L (film thickness 100

m). For a typ-

ical sample volume of 10 mL the phase ratio is 2

, im-

plying that quantitative extraction is obtained only for
compounds for which K

O/W

>10

(calculated by use of Eq. 2).

Very few analytes have K

O/W

values this high and it has

also recently been shown that this type of apolar solute is
strongly adsorbed by the stir-bar and glass vial used in
SPME [113]. In conclusion, therefore, in SPME there is
no real opportunity of realizing quantitative extraction. In
SBSE, on the other hand, the situation is much more fa-
vorable. A stir-bar coated with 100

L PDMS can easily

be used to extract 10 mL water, leading to a

of 100,

which implies that solutes with a K

O/W

in excess of 500

are extracted quantitatively into the PDMS-coated stir-bar
(Eq. 2). This not only renders quantification straightfor-
ward but also ensures significantly better sensitivity for

Fig. 11 Enrichment of an air sample spiked with five alkanes
(n-pentane to n-nonane). Different volumes were sampled at a flow
rate of 50 mL min

–1

. Peak areas are expressed relative to the peak

areas obtained for infinite sampling time. Sampling volume is
expressed relative to a compound’s retention volume. Data are
based on a 0.45-mL PDMS trap. Published in Analytical Chem-
istry [110]

compounds for which K

O/W

<10

. In conclusion, therefore,

SBSE enables quantitative extraction of many analytes
from aqueous samples, thereby resulting in much higher
sensitivity than SPME. Also, because it is not necessary to
dry the stir-bar before desorption, volatiles are not lost.
This is a substantial improvement over GPE and EGPE.

The performance of SBSE has been compared with

that of a standard SPME procedure for extraction of PAH
from aqueous samples [112]. For SBSE extraction exper-
iments a 100-mL water sample was spiked at the 30 ng
L

–1

level whereas for the SPME experiments the spike

level was 3

g L

–1

. Figure 12 shows the chromatograms

obtained after SBSE and SPME extraction. Although the
principle of extraction and the phase are identical for both
techniques the difference between the recoveries obtained
is striking. In the SBSE experiments all compounds are
extracted to a similar extent whereas in SPME the most
apolar compounds are extracted in significantly larger
amounts than the more polar compounds. This can be en-
tirely attributed to the phase ratio for the PDMS extraction
phase and the water sample, which in this experiment was
ca. 100 times larger for SBSE than for SPME. It is, how-
ever, clear that calibration and quantitation are much more
straightforward for SBSE, because most experimental
conditions, e.g. stirring time, sample temperature, and the
exact mass of PDMS used, do not play such an important
role, because extraction is already more or less quantitative.

An interesting application of SBSE is the extraction of

fatty matrixes (milk, fresh cheese, yogurt, etc.) [114]. A

typical example, the profile of a strawberry-flavored yo-
gurt, is shown in Fig. 13. For this application the yogurt
sample was diluted 1:1 with water before extraction.
Compounds responsible for the strawberry flavor, ethyl-3-
methyl butyrate and

-decalactone, were clearly present. It

is surprising that the lipid matrix did not disturb SBSE en-
richment in this quality-control application.

Trace analysis by SBSE has been illustrated by analy-

sis of 200 mL surface water [114], the chromatogram
from which is shown in Fig. 14 (non-spiked). The largest
peaks in the total-ion chromatogram (TIC) correspond to
the PDMS-degradation products D3, D4, and D5 (cyclic
siloxane oligomers). It is apparent that the breakdown sig-
nals are remarkably low for the large amount of PDMS
applied. A series of aldehydes (n-hexanal, A6, to n-de-
canal, A10) is also present. Several priority pollutants
were detected in the TIC by means of ion extraction. The
upper traces in Fig. 14 are the ion traces for benzene (m/z
78), toluene, ethylbenzene, the xylenes (m/z 91), and 1,3-
and 1,4-dichlorobenzenes (m/z 146). The concentrations
in the surface water were 5.1 ng L

–1

benzene, 3.9 ng L

–1

toluene, 0.4 ng L

–1

ethylbenzene, 0.7 ng L

–1

m+p-xylene,

0.5 ng L

–1

o-xylene, 2.9 ng L

–1

1,3-dichlorobenzene, and

1.4 ng L

–1

1,4–dichlorobenzene. These very low concen-

trations clearly illustrate the high performance of SBSE as
a technique for trace analysis, particularly as the mass
spectrometer was operated in the full scan mode.

Several applications of SBSE to the analysis of food

and beverage samples have been reported [114]. A partic-
ularly interesting example is the use of SBSE in combina-
tion with GC–AED (atomic emission detection). The GC
was operated under retention-time-locking (RTL) condi-
tions for pesticides; this enabled matching of pesticide re-
tention times with those in a library. The AED was set for
simultaneous monitoring of chlorine, bromine, and carbon.
The carbon channel yields an “universal” response, similar
to that of an FID or of a mass spectrometer in the scanning
mode, whereas the chlorine and bromine channels show

Fig. 12 Analysis of a 60 mL water sample spiked with PAH by
use of SBSE (upper chromatogram) and SPME (lower chromato-
gram). An equilibration time of 30 min was used for both. In the
SBSE experiment the spiking level was 30 ng L

–1

whereas for

SPME it was 3

g L

–1

. Peak identification: 1, naphthalene; 2, ace-

naphthylene; 3, acenaphthene; 4, fluorene; 5, phenanthrene; 6, an-
thracene; 7, fluoranthene; 8, pyrene [113]

Fig. 13 Analysis of a strawberry flavored yogurt by SBSE–TD–
CGC–MS. The sample was first diluted 1:1 with water and then
extracted with a 10 mm stir-bar for 60 min at 1400 rpm. Identified
compounds: 1, methyl 2-methylbutyrate; 2, ethyl butyrate; 3, ethyl
3-methylbutyrate; 4, cis-3-hexenol; 5, ethyl caproate; 6, cis-3-hex-
enyl acetate; 7, isoamyl butyrate; 8, methyl cinnamate; 9, vanillin;
10,

-decalactone [114]

the response for compounds containing these elements on-
ly. Figure 15A shows the chlorine-trace obtained from a
French dry white wine in which four chlorine-containing
compounds were detected at

g L

–1

and sub-

g L

–1

levels.

The main compound (22.1 min) was identified as pro-
cymidone by use of the RTL pesticide library. This was
confirmed by re-analysis of the same sample by GC–MS,
also under RTL conditions. The corresponding spectrum
(Fig. 15B) and the library search spectrum (Fig. 15C) are
also illustrated. The concentration, determined by inter-
nal standard addition, was 21

g L

–1

. The repeatability,

as RSD, of SBSE for this particular pesticide was 6.1
(n=6).

A recent paper by Vercauteren et al. [115] reported the

use of SBSE in combination with inductively coupled plas-
ma mass spectrometry (ICP–MS) for the speciation of
organotin compounds. The compounds were derivatized
in the aqueous sample by use of sodium tetraethylborate
and subsequently extracted on to a stir-bar coated with
55

L PDMS. The applicability of the technique was dem-

onstrated both for environmental aqueous samples and for
speciation of organotin compounds in mussel tissue. With
tetracyclohexyltin as internal standard, detection limits down
to pg L

–1

levels were reported; the relative standard deviation

was 10% – very good considering the complex sample pre-
treatment procedure and the low concentration determined.

The use of SBSE to monitor benzoic acid in lemon-fla-

vored beverages was described by Tredoux et al. [116]. The
accepted technique for analysis of benzoic acid is based on
ether extraction, successive partitioning between sodium
hydroxide and dichloromethane, then conversion of the free
acid into its trimethylsilyl ester and analysis by capillary
GC. Compared with this very complex and time-consuming
method SBSE enabled much faster analysis of this target
compound in soft drinks. Direct extraction from soft drinks
resulted in a pronounced matrix effect, because many inter-
fering analytes compete with benzoic acid for absorption by
the stir-bar. The soft-drink matrix was also found to bind
strongly to dissolved benzoic acid. Both effects could be
circumvented by diluting the sample 1:10 with water before
extraction. Because of the very high intrinsic sensitivity of
SBSE, the detection limit was 8

g L

–1

(S/N=10) and calibra-

tion curves were linear for concentrations up to 400 mg L

–1

SBSE combined with thermal desorption-GC–MS for

the analysis of PCB in sperm was recently described in

Fig. 14 SBSE–TD–CGC–MS of surface water. Bottom chromato-
gram, total ion current; upper traces, selected-ion chromatograms
at specific ion masses. Abbreviations: A6 to A10, linear aldehydes
hexanal to decanal; D

, cyclic siloxane degradation products; B, ben-

zene; T, toluene; EtB, ethylbenzene; oX, mX, and pX, ortho-, para-
and meta-xylene; DCB, dichlorobenzene [114]

Fig. 15 Identification of procymidone in a French white wine. A.
SBSE–TD–CGC–AED chromatogram of the chlorine emission
line. B. Spectrum of the procymidone peak obtained from separate
SBSE–TD–CGC–MS analysis. C. Library spectrum of procymi-
done [114]

Fig. 16 Determination of seven PCB congeners in spiked human
sperm. A, 10 ng L

–1

; B, 1 ng L

–1

. Published in Journal of High Res-

olution Chromatography [117]

the literature [117]. Focus was on the seven Ballschmiter
congeners, with octachloronaphthalene as internal standard.
Sperm samples (3 mL) were treated ultrasonically for
3 min then diluted with 1:9 water–methanol. The diluted
samples were extracted for 45 min at 1000 rpm by use of
a stir-bar coated with 50 mg PDMS. By use of mass spec-
trometric detection in the selected ion monitoring mode
detection limits at the low ng L

–1

level were achieved. The

chromatograms obtained from a spiked sperm sample,
shown in Fig. 16, illustrate the extremely low concentra-
tions that can be handled.

The use of SBSE for the determination of polyaromatic

hydrocarbons (PAH) in water samples was described by
Popp et al. [118]. In contrast with the applications men-
tioned above these authors combined SBSE with liquid
desorption and HPLC analysis. Detection limits between
0.2 and 2 ng L

–1

were achieved by use of fluorescence de-

tection. Repeatability was 4.7–13.5%, illustrating the via-
bility of SBSE combined with liquid desorption. This trend
might continue in the future, because liquid desorption–
HPLC enables analysis of polar compounds that cannot be
handled by gas chromatography.

Sandra et al. [119] described the use of SBSE for the

determination of dicarboximide fungicides in wine. Pro-
cymidone and iprodione were detected at 65

g L

–1

where-

as vinclozolin was found at a concentration of 3

g L

–1

Because iprodione is known to be thermally unstable, these
concentrations were confirmed by separate SBSE analysis
combined with liquid desorption and LC–MS. The same
group illustrated the use of sorptive enrichment by means
of a sorbent-coated stir-bar, which was present in the head-
space of the sample [120]. In comparison with SPME, the
presence of a significantly larger amount of sorbent on the
stir-bar resulted in extraction efficiencies up to 100 times
higher; the limit of detection was at the ng L

–1

level. Ap-

plication of this extraction technique to the headspace
analysis of aromatic plants [121] resulted in high-sensitiv-
ity profiles; sensitivity was clearly better than that of
headspace-SPME. This technique has high potential for
rapid profiling of complex samples and will probably be
further developed in the near future.

Kreck et al. [122] described the use of SBSE combined

with multidimensional gas chromatography for the enan-

tioselective analysis of flavor compounds in strawberries.
By use of SBSE, laborious and time-consuming tech-
niques, e.g. liquid–liquid extraction or solid-phase extrac-
tion, were avoided. Seven chiral compounds were identified
and quantified by use of an SE-52–

-cyclodextrin column-

combination. By use of this approach the occurrence of
non-natural, racemic analytes could be detected in straw-
berry-derived products. The results indicated that SBSE–
MDGC–GC–MS is a reliable technique for monitoring
the authenticity of food samples.

Two recent contributions have described the use of

SBSE for the determination of off-flavor compounds in
drinking water [123, 124]. The naturally occurring com-
pounds 2-methylisoborneol (2-MIB) and trans-1,10-di-
methyl-trans-9-decalol (geosmin) are responsible for mud-
dy or earthy odors in water supplies throughout the world.
In addition to these naturally occurring compounds, tri-
chloroanisole (TCA) is also a highly odorous compound;
it is formed by biomethylation of 2,4,6-trichlorophenol.
The odor-threshold concentrations for these three com-
pounds are at or below the ng L

–1

level, illustrating the ex-

tremely low levels at which these compounds should be
determined. Use of SBSE enabled extraction of up to
50 times more of the analytes from the aqueous phase
than was possible with SPME, illustrating the increased
sensitivity that can typically be achieved by use of SBSE
under real-life sampling conditions. Figure 17 shows re-
sults from analysis, by mass spectrometric detection in the
SIM mode, of 40 mL water spiked at the 1 ng L

–1

level.

All three compounds could be clearly identified with de-
tection limits as low as 0.02–0.04 ng L

–1

. In addition to

spiked samples several unfortified samples were also ana-
lyzed. All three compounds were detected at levels be-
tween 0.16 and 3.1 ng L

–1

with RSD values of 0.6–7 %,

again illustrating the high precision of SBSE for the deter-
mination of ultra-trace amounts of contaminants in a wide
range of matrixes.

Conclusions

As has been illustrated on many occasions, both in the lit-
erature and in this review, sorption materials have clear

Fig. 17 SIM chromatograms obtained
by SBSE–GC–MS of a 40 mL natural
water sample spiked at the 1 ng L

–1

level. SBSE extraction time was
120 min. Peak identification: 1, MIB;
2, TCA; 3, Geosmin

advantages in terms of inertness, stability, and versatility
over adsorbents and perform particularly well when com-
bined with thermal desorption–GC(–MS). OTT and SPME
are still the sorptive techniques most often used, but often
cannot provide adequate performance, because of severe
sensitivity restrictions and practical limitations. Essen-
tially OTT is not used in practice and many attempts are
being made to improve the sensitivity of SPME. Most
work on SPME focuses on the development and applica-
tion of more absorbent extraction materials. Because of to
the very limited amount of sorbent that can be coated on
to an SPME fiber (0.5

L or 0.5 mg), the only means of

improving extraction recoveries (and thereby sensitivity)
is to use more retentive materials. In practice, however,
these are often adsorbent materials and, therefore, many
practical problems arise, e.g. competitive adsorption and
matrix effects. All the advantages of sorption are lost and
special care must be taken not to end up with all the typi-
cal problems of static adsorbent-based extraction. Al-
though this point deserves attention it is rarely considered
in the literature or in the practical application of SPME.

To overcome the sensitivity problems of the older sorp-

tive techniques, two alternative methods, GPE and SBSE
have been introduced in the last few years.

Both techniques employ a significantly increased

amount of sorbent, which ensures correspondingly greater
sensitivity, for both gaseous (GPE) and aqueous (SBSE)
samples. By use of GPE gaseous samples can be effec-
tively enriched and analyzed, in combination with thermal
desorption–GC, with a large reduction in the risk of ana-
lyte decomposition. This is a significant improvement over
adsorbent materials, for which incomplete recovery, de-
composition, and artifact formation are often observed for
polar and/or reactive compounds. SBSE, on the other hand,
is a very effective technique for trace enrichment of liquid
(aqueous) samples. Recoveries are up to 500 times more
than those of SPME, because the amount of PDMS on the
stir-bar of is much larger than that on an SPME fiber.
When combined with high-sensitivity thermal desorption
detection limits in the sub-ng L

–1

range can be achieved

by use of this relatively simple technique.

The development of sorptive extraction techniques is

still in progress. Many papers emphasizing instrumental
aspects and/or applications, are published every month.
Exciting developments include new or improved sorptive
materials (i.e. for SPME and the other techniques), SBSE
in combination with liquid desorption–LC for analysis of
more polar analytes, applications to complex matrixes,
and full automation of the SBSE technique. The commer-
cial availability of sorptive sampling is a key factor in its
adoption by the analytical laboratory; this is expected to
progress further in the near future.

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