CHROMATOGRAPHY / Overview 89
Helmig D (1999) Review: air analysis by gas chro- Oram et al. (1998) Geophysics Research Letters 25:
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Krska R and Kellner R (1985) Chlorofluorohydrocarbons. DM, Alyea FN, O Doherty S, Salameh P, Miller BR,
In Townshend A (ed.), Encyclopedia of Analytical Huang J, Wang RHJ, Hartley DE, Harth C, Steele LP,
Science, 1st ed. London: Academic Press. Sturrock G, Midgley PM, and McCulloch A (2000) A
Molina MJ and Rowland FS (1974) Stratospheric sink for history of chemically and radiatively important gases in
chlorofluoromethanes: chlorine atom catalysed destruc- air deduced from ALE/GAGE/AGAGE. Journal of
tion of ozone. Nature 249: 810 812. Geophysical Research 105(D14): 17751 17792.
Montzka SA, Butler JH, Myers RC, Thompson TM, Singh HB (1995) Halogens in the atmospheric environ-
Swanson TH, Clarke AD, Lock LT, and Elkins JW ment, Chapter 7. In Singh HB (ed.) Composition,
(1996) Decline in the tropospheric abundance of halogen Chemistry and Climate of the Atmosphere. New York:
from halocarbons: implications for stratospheric ozone Van Nostrand Reinhold.
depletion. Science 272: 1318 1322. WMO (World Meteorological Organisation) (1999) Scien-
Oram et al. (1995) Geophysics Research Letters 22: tific Assessment of Ozone Depletion: 1998. Global
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CHROMATOGRAPHY
Contents
Overview
Principles
Multidimensional Techniques
their respective articles, so here only a short intro-
Overview
duction is given.
V R Meyer, EMPA St Gallen, St Gallen, Switzerland
& 2005, Elsevier Ltd. All Rights Reserved.
Classification of Chromatographic
This article is reproduced from the first edition, pp. 720 729,
Techniques
& 1995, Elsevier Ltd., with revisions made by the Author.
The Type of Mobile Phase
Chromatographic techniques can be classified on the
basis of a number of different criteria. Nevertheless,
Introduction
a logical hierarchy is given by first specifying the type
Chromatography can be used to solve a very broad of mobile phase, second the shape of the chro-
range of analytical problems. This versatility is re- matographic bed and third the type of stationary
flected in the large number of chromatographic tech- phase. In Table 1 the chromatographic methods are
niques that are successfully applied today. They can listed according to this scheme. The type of mobile
be classified according to a number of criteria, the phase is the most important criterion because it de-
most important of which is the type of mobile phase termines the class of samples that can be analyzed
used. Subsequently the shape of the chromatographic with one of the associated techniques. (From a the-
bed and the properties of the stationary phase oretical point of view, each type of mobile phase also
expand the possibilities offered by chromatography. governs a certain range of diffusion coefficients,
Once a particular method is chosen, it is possible to which determines and limits the speed of analysis
influence the separation using programmed elution. and the efficiency of the method.) Therefore a dis-
Finally special techniques can be used to perform tinction can be made between gas, supercritical fluid,
difficult analyses or to obtain short separation times. and liquid chromatography (GC, SFC, and LC,
The individual methods are discussed in detail in respectively).
90 CHROMATOGRAPHY / Overview
Table 1 Classification of chromatographic methods
Type of mobile phase Gas
Supercritical fluid Liquid
Column
Shape of chromatographic Column Column Plane
bed
Open tubular Packed
Open tubular Packed
(Open tubular) Packed
Paper Thin-layer
Type of stationary phase Liquid, Liquid Solid
Cross-linked Solid
Solid Liquid Solid
cross-linked
liquid
liquid
Method Gas-solid
Capillary Packed column
Gas-liquid Column liquid Paper Thin-layer
chromatography
supercritical supercritical fluid
chromatography chromatography chromatography chromatography
fluid chromatography
chromatography
Abbreviation
GC. GC.
SFC
LC PC TLC
GLC GSC
HPLCa HPTLCb
Type of method
Adsorption,
Adsorption, Adsorption,
Adsorption,
molecular sieve,
bonded phase reversed phase,
reversed phase,
porous polymer
bonded phase
bonded phase,
ion exchange,
affinity,
size exclusion
a
High-performance liquid chromatography.
b
High-performance thin-layer chromatography.
The high-performance methods use stationary phases of very small particle diameter.
SFC is less important than GC or LC. In many
cases, it is not difficult to choose between the latter
two methods: the prerequisite for GC is that the
analyte is volatile and thermally stable (although
derivatization in order to obtain these properties is
possible in many cases).
(A) (B) (C)
The Shape of the Chromatographic Bed
Figure 1 Three possibilities for creating a chromatographic
bed: (A) packed column, (B) open capillary, and (C) plane. The
If the mobile phase is a gas or a supercritical fluid, it
third technique can only be used with a liquid mobile phase; here
is necessary to let it flow through a tube, a so-called
a thin-layer plate is drawn but the plane can also consist of a
column, that contains the stationary phase. In the
sheet of paper. In all three cases the stationary phase is shown in
case of liquid chromatography one can choose
gray. The particles of the packed column can be round, as in the
between a column or planar geometry because the
figure, or irregular.
mobile phase can move through a sheet of paper or a
thin layer by capillary action. If a column is used, the
mobile phase is forced through it by pressure gene- instrumentation, liquid chromatography with open
rated by a pump or by a gas stored in a pressurized capillaries is only of theoretical interest.) If the column
cylinder. (As a preparative laboratory technique, liq- contains packing, many possibilities are offered by
uid chromatography is also performed in columns contemporary technology. The stationary phase can be
an inorganic adsorbent, a cross-linked and thereby
packed with coarse stationary phases; in this case
simple hydrostatic pressure may be sufficient.) rigid organic polymer, an inorganic or organic material
The column can be an open capillary or a packed with chemically modified surface, or even a liquid film
tube. In the first case the mobile phase is coated as a coated on a granular carrier material.
Figure 1 shows the three possibilities: packed col-
thin film on the inner wall of the capillary. If the
mobile phase has a certain solvating power, as in umn, open capillary, and plane.
SFC, it is necessary to cross-link this liquid film,
whereas in GC linear polymers are used in many
Terminology of the Methods
cases because the usual carrier gases, helium and
hydrogen, cannot dissolve any stationary phase. Taking the type of chromatographic bed and sta-
(Owing to problems with manufacturing and tionary phase into account, GC, SFC, and LC can
CHROMATOGRAPHY / Overview 91
now be subdivided, although the usual terms do not properties of the methods: the density governs the
follow the same criteria in all cases. Table 1 uses the solvating power of the mobile phase and thereby de-
expressions gas liquid, gas solid, capillary supercrit- termines whether the separation can be influenced by
ical fluid, packed-column supercritical fluid, column a particular choice of eluent; the viscosity influences
liquid, paper, and thin-layer chromatography. The the pressure needed to force the mobile phase
following abbreviations are used: GLC, GSC, but in through the chromatographic bed and sets the
most cases the type of stationary phase is omitted upper limit of the solute diffusion coefficient. This
and both techniques are termed GC; SFC; LC (usu- latter should be high because low diffusivity means
ally only used for column techniques, although thin- slow mass transfer and therefore slow chromatography.
layer and paper chromatography are also  LC ); PC If the sample diffusion coefficient is low, it is neces-
(sometimes also used for  preparative chromatogra- sary to keep low the characteristic chromatographic
phy ); and TLC. For LC and TLC, which both use a dimension, i.e., the capillary or particle diameter.
granular stationary phase, a special term was intro- Therefore, LC with 5 mm particles is more efficient
duced to distinguish the more recent instrumental than with a 100 mm packing.
methods based on very fine stationary phases from Practical aspects of the three methods are listed
the classical ones: HPLC and HPTLC where HP is under Variables and Sample Prerequisites at the end
for  high performance . Here the particle diameter is of Table 2.
not larger than B10 mm, which is the key to obtain-
ing high plate numbers per unit length.
Especially in LC, and also in other fields, it is usual
Gas Chromatography
to distinguish in more detail between very different
If the mobile phase is a gas, the sample needs to be
types of method. This will be discussed below.
volatile. Its boiling point at atmospheric pressure
should not be higher than B3601C. If the tempera-
Comparison of the Methods
ture of the GC column or capillary is adequate, the
Table 2 lists some characteristic features of GC, SFC, sample molecules will be transported by the gas
and LC. In most cases GC is used as open-tubular owing to their volatility. Retention is governed by
GLC, and LC is performed in packed columns. As both vapor pressure and affinity to the stationary
can be seen from the physical parameters of density, phase of a given compound. The gaseous mobile
viscosity, and diffusion coefficient, SFC lies between phase has no direct influence on the separation.
GC and LC and it is no surprise that it can be used GC can be a simple and rapid technique and is the
equally well with open capillaries and packed col- method of choice for the investigation of volatile and
umns. The values in the table are to some extent even very complex samples. An example is given in
arbitrary but are typical. The values of the three basic Figure 2.
physical parameters are not only of theoretical The most frequently used mobile phases for GC
interest but are linked directly to some of the main are hydrogen and helium. The lower the molecular
Table 2 Comparison of analytical column-type chromatographic methods
GC (open-tubular GLC) SFC LC
1
Density of mobile phase (g ml )10 3 0.5 1
4 3 2
Viscosity of mobile phase (poise) 10 10 10
1 7 9
Diffusion coefficient of solute in mobile phase (m2 s ) 10 5 10 10
Diameter of capillary (mm) 320 100
Diameter of packing (mm) 5 5
Length of column (m) 25 25, 0.25a 0.1
1
Number of theoretical plates (m ) 3 000 3 000, 50 000a 50 000
Number of theoretical plates per column 75 000 75 000, 12 000a 5 000
Pressure drop (bar) 1 Variableb 100
Variables Stationary phase Stationary phase Stationary phase
Temperature Mobile phase Mobile phase
Temperature (Temperature)
Pressure
Sample prerequisites Volatility Solubility Solubility
Thermal stability (Thermal stability)
a
For open tubular and packed columns, respectively.
b
In SFC the pressure drop over the column can be chosen.
92 CHROMATOGRAPHY / Overview
45
20 22 25
7 10 58
4 36
77
80
41 49 76 88 99
3
12 14
15
11 51
19
32 89
43
5
12
65
42 90
6
9
60
13 46 94
18
57 86
5961 64 70
26 56 87 91
38 4448 5355 63 104106109 112
8 27 33 95
30
16 6769 85
47 92
37 111
23 31 71 78 98103 108110 115
40 50 62 82
21
9396
24 2829 34 52 54 65 68 72 75 79 81 8384 105
39
97 107
66
7374
010 20304050 6070
Time (min)
Figure 2 Gas chromatographic separation of hydrocarbons found in an urban air sample. Open capillary, 0.32 mm i.d. 60 m length;
stationary phase, DB-1 (dimethyl polysiloxane); film thickness, 0.25 mm; carrier gas, helium; temperature programme, 51C isothermal
1
for 3 min, 5 501Cat arateof 31Cmin , 50 2201C at a rate of 51Cmin 1; detector, flame ionization. With this method, a total of 142
hydrocarbons could be separated and identified; 128 of them were found in the urban air sample. (After Ciccioli P, Cecinato A,
Brancaleoni E, Frattoni M, and Liberti A (1992) Use of carbon adsorption traps combined with high resolution GC MS for the analysis
of polar and nonpolar C4 C14 hydrocarbons involved in photochemical smog formation. Journal of High Resolution Chromatography
15: 75.)
mass of a gas, the lower its own diffusivity as well as Alcohols, amines, amino acids, carboxylic acids, car-
the diffusivity of the sample molecules and the faster bohydrates, and steroids can be trimethylsilylated;
the chromatography. Therefore, hydrogen would be amines, phenols, carbohydrates, and steroids can
the favored carrier gas but it is often barred on safety be acylated with trifluoroacetic acid or a higher
grounds. Sometimes nitrogen is used because it is homolog; carbonic acids and phenols can be alkylated.
cheap but this can only be recommended for simple
analytical problems because the separation perform-
ance is poorer than with gases of low molecular
mass. The fact that some detectors demand the use of Liquid Chromatography
a certain gas must also be taken into consideration.
Liquid chromatography has a number of different
A typical stationary phase for GC is a viscous liq-
configurations with regard to technical (instrumen-
uid with low vapor pressure (at the temperature re-
tal) as well as separation modes. Paper, thin-layer,
quired for a given range of application). The two
and classical column techniques all belong to liquid
most important types of stationary phases are sili-
chromatography and the  high performance tech-
cones and polyglycols; their structures are given in
nique especially (though to a lesser extent the other
Table 3. The silicones especially can be substantially
methods also) offers a great variety of separation
chemically modified in order to obtain a wide range
principles.
of polarities and specialized functionalities (including
chiral groups). The stationary phase is coated as a
thin film (typically 0.25 mm) on the inner wall of the
Paper Chromatography
open capillary or on the surface of a granular,
porous, inert packing material, in this case called a The simplest and cheapest technique is paper chro-
solid support. For special types of analyses the sta- matography, where the chromatographic bed con-
tionary phase is not a liquid but a porous solid sists of a sheet of paper, i.e., cellulose. The stationary
packing. Adsorbents (silica), molecular sieves and phase consists of water adsorbed to the cellulose as
porous polymers are used for the GSC of highly well as of the polymer itself, although ion exchange
volatile samples such as mixtures of permanent gases
and complexation processes may play an important
or low-molecular-mass hydrocarbons. role. The sample solution is applied as a spot near
In GC, the eluted compounds are most often de- one end of the paper. A few centimeters of the sheet
tected with a flame-type detector that generates ions, are dipped into the mobile phase which then ascends
the so-called flame ionization detector, FID; for
(or descends, as descending mode is also possible)
special purposes nitrogen- and phosphorus-sensitive into the stationary phase. When the mobile phase has
FIDs, electron capture, or thermal conductive detec- almost reached the other end of the sheet the paper is
tors and mass spectrometers are used. removed from the developing tank and dried. If the
If a sample is not volatile, several derivatization analytes are not visible because they are not colored,
techniques are known that allow reduction in the the sheet is treated with a reagent to visualize the
boiling points of certain classes of compounds. spots.
CHROMATOGRAPHY / Overview 93
Table 3 Important stationary phases for GLC and (HP)LC
GC
With the proper choice of R and R' a
R
Silicones
wide range of polarities and special
Si O
functionalities is available
n
R'
Polar stationary phase:
(O CH2 CH2 n OH
Polyglycols
n ranges from 4 to 800
LC
Three-dimensional network
Silica
(SiO2)n Si OH
Octadecyl silica (SiO2)n Si C18H37
Reversed phases
}
Octyl silica (SiO2)n Si C8H17
Diol silica (SiO2)n CH2 CHOH CH2OH
Si
Polar bonded phases
Nitrile silica (SiO2)n Si CH2 CH2 CN
}
Amino silica
(SiO2)n Si CH2 CH2 NH2
Three-dimensional network due to
cross-linking with divinylbenzene
Polystyrene (CH-CH2)n
SO3-H+
Strong cation exchanger
COO-H+
Weak cation exchanger Can be silica or polystyrene
NR3+OH-
}
Strong anion exchanger
NH3+OH-
Weak anion exchanger
Thin-Layer Chromatography
is LC using microparticulate packings. Under
these circumstances it is necessary to use a pump
Thin-layer chromatography is more versatile than
for mobile phase transport and a detector for the
paper chromatography since a number of different
observation of the fractions (usually the concentra-
stationary phases are available such as silica,
tion of the analytes in the eluate is low), e.g. UV
derivatized silica, or cellulose (the analogue to pa-
absorbance, fluorescence, refractive index, or elect-
per); also, developing times are much shorter. An
rochemical detectors according to the properties of
immense number of spray reagents have been pub-
the analytes. It is also possible to derivatize the sam-
lished that allow detection of any type of analyte.
ple prior to or after the separation. Precolumn
HPTLC is the  high-performance version of TLC
derivatization can be performed offline or online;
and uses 10 mm or 5 mm stationary phase particles.
postcolumn derivatization is usually carried out on-
The separation performance of these plates is higher,
line. An example of LC is given in Figure 4, which
but to take full advantage it is necessary to use in-
shows the separation of the three stereoisomers of
strumentation for sample application, development,
mivacurium, a neuromuscular blocking agent, in a
and detection. As an example of TLC, Figure 3
plasma extract.
presents the separation of ten rare earths.
Liquid Chromatography Liquid Chromatographic Separation Principles
Whereas chromatography in open columns is mainly Liquid chromatography can be performed in a
used for preparative purposes, the analytical technique variety of modes; the most important ones are
(
94 CHROMATOGRAPHY / Overview
that comes from the decrease in contact area between
the two phases as long as the sample molecules ad-
here to the hydrocarbon chains. Polar analytes are
eluted first and homologs will be retained more
strongly the longer their chain length. Ionic com-
pounds can be separated on reversed phases if a
Er
neutral ion-pair is formed by the addition of a coun-
Ho
ter-ion to the eluent. This was carried out in the sep-
Tb
aration shown in Figure 4: mivacurium is a
quaternary amine, and therefore a sulfonic acid was
Gd
added as an agent to mask its charge.
Eu
Sm
Other bonded phases on silica (not illustrated in
Figure 5) Besides the nonpolar hydrocarbons, other
functional groups can also be bonded to silica. Im-
Nd
portant stationary phases are diol, nitrile, amino (see
Table 3), and a great number of special functionali-
Pr
ties including chiral groups. The retention mecha-
Ce
La nisms are as variable as the stationary phases and are
not known in some cases.
Ion exchange chromatography Ion exchange
Figure 3 High-performance thin-layer chromatographic sepa-
ration of ten rare earths (as nitrates). Sample, 1 mg each of rare groups can be bonded to silica or to polystyrene.
earth; layer, silica, impregnated with ammonium nitrate prior to
 Classical ion exchange is based on ionic equilibria
the separation; mobile phase, 4-methyl-2-pentanone/terahydrofu-
between solute, buffer and stationary phase ions and
ran/nitric acid/2-ethylhexylphosphonic acid mono-2-ethyl hexyl-
counter-ions. Besides this ion exclusion mechanisms
ester 3:1.5:0.46:0.46; developing distance, 5 cm; detection
can also be utilized and special types of ion ex-
reagent, (1) spray of saturated alizarin solution in ethanol, (2)
ammonia vapour, (3) gentle heating. (After Wang QS and Fan DP changers have been developed for the separation of
(1991) Journal of Chromatography 587: 359.)
the ions of strong acids and bases.
Size exclusion chromatography If the mobile phase
presented briefly. Schematic drawings are shown in
has a good affinity for both the sample molecules and
Figure 5, and Table 3 also lists some stationary
the stationary phase and if the latter has a well-de-
phases used in LC.
fined pore structure, such a chromatographic system
will separate the solutes according to their size. They
Adsorption chromatography The stationary phase
will not be retained by the column packing but will
is a polar adsorbent, in most cases silica. The mobile
enter the pores where the mobile phase is stagnant.
phase is nonpolar (usually a solvent with polarity
Large molecules can utilize a smaller fraction of the
within the range from hexane to esters). It competes
pore volume than small ones and will be eluted ear-
with the sample molecules for adsorption at the
lier. Molecules that are too large to enter the pores
active sites of the stationary phase. Nonpolar com-
are excluded and will appear as the first fraction at
pounds are eluted first, followed by solutes of inc-
the column end.
reasing polarity. Steric properties of the sample
compounds can play an important role and there-
fore adsorption chromatography is the method of Affinity chromatography The stationary-phase ma-
choice for the separation of many classes of isomers. trix can be loaded with chemically bonded, biologic-
ally active groups such as enzymes or antibodies. If a
Reversed-phase chromatography The stationary complex sample is injected into an affinity column,
phase here is nonpolar; in most cases it is derivati- only those molecules will be retained that bind to the
zed silica that carries C18 (i.e., C18H37) or C8 (i.e., ligands; in the cases mentioned above these will be
C8H17) groups. The mobile phase is polar, in most substrates or antigens. All other compounds will be
cases a mixture of water (or buffer solution) with swept away by the mobile phase. Afterwards the re-
methanol, acetonitrile, or tetrahydrofuran. Such an tained molecules can be eluted by switching to a
eluent cannot wet the surface of the stationary phase specially designed mobile phase (e.g., change of pH
and the solutes are retained owing to an energy gain or ionic strength). Affinity chromatography is a
CHROMATOGRAPHY / Overview 95
O
OCH3
CH3O
CH3
+ +
O N
O
O
CH3O
OCH3
1 2
H3C
CH2 O CH2
2CI-
CH3O OCH3 OCH3
CH3O
OCH3 OCH3
1
3
4
2
05
10 15
Time (min)
Figure 4 Liquid chromatographic separation of mivacurium stereoisomers in human plasma extract. The drug is a mixture of three
isomers; the structure is drawn without stereochemical preference. Column, 4.6 mm i.d. 12.5 cm length; stationary phase,
1
LiChrospher 60 RP (reversed-phase) select B, 5 mm; mobile phase, acetonitrile/water 40:60 with 0.005 mol l octanesulfonic acid (as
1
ion-pair reagent), 1 ml min ; detector, fluorescence 202/320 nm. Peaks: (1) is the trans trans isomer (1R, 10R, 2S, 20S); (2) is the cis
trans isomer (1R, 10R, 2R, 20S); (3) is the cis cis isomer (1R, 10R, 2R, 20R); (4) is the internal standard, the trans trans analog of
mivacurium with a benzene ring instead of the double bond in the middle of the molecule. (After Brown AR, James CD, Welch RM, and
Harrelson JC (1992) Stereoselective HPLC assay with fluorometric detection for the isomers of mivacurium in human plasma. Journal
of Chromatography 578: 302.)
highly selective method and works by an  on off resistance to the eluent flow) needs to be installed
switching mechanism. after or at the outlet of the column.
Owing to its intermediate position between GC
and LC, SFC can be performed equally well in open
Supercritical Fluid Chromatography
capillaries and packed columns. The separation can
be influenced by the type of stationary phase and of
The phase diagram of a pure compound shows not
only regions of the solid, liquid, and gaseous states, modifier, by pressure, pressure drop, and tempera-
the equilibrium lines and the triple point, but also the ture. In contrast to GC, SFC can also be used for the
critical point. If pressure and temperature exceed the separation of nonvolatile or thermally labile com-
critical values, the compound will be neither a liquid pounds (although some temperature compatibility is
necessary). The separation of enantiomers on chiral
nor a gas, nor will the two phases coexist, but a
supercritical fluid exists. This phase is denser and stationary phases can be very attractive because the
more viscous than a gas without attaining the prop- temperature is lower than in GC, which increases the
erties of a liquid as shown in Table 2. The advantage separation factors. SFC is an alternative to normal-
phase LC because it is fast and carbon dioxide is
of SFC over LC lies in the higher diffusion coefficient,
which allows faster separations; in comparison to ecologically sound. An example of an SFC separation
GC the mobile phase has a large solvating power and can be found in the previous article, Principles, where
thus influences selectivity. Figure 2 shows the separation of orange oil compo-
nents.
In most SFC separations carbon dioxide is used as
the mobile phase; often a modifier (of polarity) such
as methanol, other alcohols or water is added. The Special Chromatographic Techniques
critical data for CO2 are 31.31C and 72.9 bar, values
Preparative Methods
that can easily be handled by instrumental chro-
matography. To keep the column outlet under criti- Chromatography can equally well be used for ana-
cal conditions, a restrictor (a device with a high lytical and preparative purposes.  Preparative is not
96 CHROMATOGRAPHY / Overview
Figure 5 Separation principles of liquid chromatography. (A) Adsorption chromatography: the adsorptive sites of the stationary
phase are symbolized by A; the solute molecules interact with their polar groups X or Y; the mobile phase drawn is hexane, which can
also interact weakly with A. (B) Reversed-phase chromatography: the solute molecules interact via their nonpolar groups with the
nonpolar stationary phase. (C) Ion-exchange chromatography: a styrene-divinylbenzene type cation exchanger is shown; sample ions
þ þ
S and buffer cations K compete for interaction with the exchange sites. (D) Size exclusion chromatography: sample molecules can
occupy the pore volume according to their size, therefore the macromolecule will spend less time in the pores and elute first. (E) Affinity
chromatography: only certain sample molecules can fit to the ligands of the stationary phase, the others are washed out.
reserved to the fractionation of large samples, but If large samples need to be separated, the diameter,
indicates that the separated compounds are collected and also often the length, of the column are in-
and used for a subsequent purpose: identification or creased. (Obviously, open capillaries cannot be used
structure elucidation, chemical modification by syn- for this purpose.) Preparative GC is an attractive
thetic methods, use as a reference material, determi- approach (though the fraction collector needs to be
nation of chemical or biological properties, or for cooled) but few commercial instruments are avail-
sale. If only small amounts of material are needed, able. Preparative LC is the most important techni-
the only difference from analytical chromatography que in organic synthesis, biochemical research,
lies in the use of a fraction collector; for routine sep- downstream processing in biotechnology, and for
arations it should be computer controlled. the commercial preparation of certain chemicals or
If the sample size is increased, the shape of the drugs.
peaks changes to rectangular (in the case of volume
overload) or triangular (with mass overload); mixed
Programmed Elution
forms and distorted peak shapes are also observed.
Displacement effects can occur where a compound is In a complex sample the individual analytes often
 pushed and concentrated by a following one that have very different retention factors in a given chro-
has a stronger affinity to the stationary phase. matographic system. It is therefore not possible to
CHROMATOGRAPHY / Overview 97
separate and elute them efficiently without changing Thermal desorption Volatile compounds in gases
the properties of the system, i.e. under so-called iso- such as pollutants in air can be trapped in a small
thermal (GC) or isocratic (LC) conditions. In this adsorption tube, either by pumping the gas through
case a GC separation is started at relatively low or by passive diffusion. The packing in the trap can
temperature, an LC separation at low eluting power be chosen from a wide variety of adsorbents (molec-
of the mobile phase. Subsequently the temperature or ular sieves, graphitized carbon blacks, organic poly-
mobile phase strength is increased in order to elute mers). After sample collection the adsorption tube is
compounds that were strongly retained under the rapidly heated in a stream of purge gas which trans-
initial conditions. In GC this technique is called a ports the released analytes to the GC column where
temperature program (see Figure 2); the correspond- the separation runs.
ing LC term is gradient elution. Note that in normal-
phase LC the polarity of the mobile phase needs to be
increased (however, gradient elution on silica is al- Pyrolysis chromatography For the GC analysis of
high-molecular-mass samples such as plastics or
most never performed because steep gradients are
wood, the sample can be pyrolyzed (heated until
not possible and it takes a long time to re-equilibrate
breakdown into smaller molecules occurs) online
the column after the separation), whereas in
prior to injection. A  fingerprint of the material is
reversed-phase LC the eluent polarity is decreased.
obtained that can be used for quality control or
A gradient from 10 to 100% acetonitrile in water can
identification purposes.
separate a very broad range of compounds on a
reversed-phase column; pH or ionic strength gradi-
ents are also possible. In SFC, mobile phase, pres-
See also: Chromatography: Principles. Gas Chro-
sure, and temperature gradients are of equal
matography: Column Technology; Pyrolysis; Detectors.
importance.
Headspace Analysis: Static; Purge and Trap. Ion
Exchange: Overview. Liquid Chromatography: Over-
view; Ion Pair; Size-Exclusion. Supercritical Fluid
Chromatography: Overview; Applications. Thin-Layer
Column Switching
Chromatography: Overview.
An alternative to programmed elution can be the
coupling of two (or even more) columns with differ-
ent stationary phases. This technique is known as
multidimensional chromatography. The first column,
Further Reading
for example, will separate the sample according to
Anton K and Berger C (eds.) (1997) Supercritical Fluid
polarity groups. Then selected fractions are switched
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Principles
V R Meyer, EMPA St Gallen, St Gallen, Switzerland
Figure 1 is a simple representation of the process.
In (A) a mixture of seven molecules each of and m
& 2005, Elsevier Ltd. All Rights Reserved.
is introduced into the chromatographic system. In (B)
they are distributed between the upper mobile and
lower stationary phase, and in (C) the mobile phase
has transported the dissolved molecules over a small
Introduction
distance: new equilibria between the phases are es-
Chromatography is one of the most important ana- tablished. When this process has been repeated many
lytical techniques. It allows the separation and sub- times, as in (D), the compounds and m are sep-
sequently the qualitative and quantitative analysis of arated because their preference for one of the two
complex mixtures, as long as the samples are volatile phases differs strongly.
or soluble in a suitable solvent. Since chromatogra- In practice, chromatography takes place on a plane
phy is based on the partition of the sample compo- or in a tube. The plane can be a sheet of paper (paper
nents between two phases, one stationary and one
moving, it is necessary to distinguish between gas,
liquid, and supercritical fluid chromatography,
Mobile phase
according to the type of mobile phase used. Chro-
matography is versatile and can be highly efficient;
full automation is possible. Some basic principles of
its theory are presented here as knowledge of the
Stationary phase
(A)
underlying phenomena is necessary to take real
advantage of all the possibilities offered by chrom-
atographic techniques.
(B)
The Chromatographic Process
In order to obtain a chromatographic separation,
two phases are needed  a moving or mobile phase
and a fixed or stationary phase. The stationary phase
can be either a solid or a liquid, the mobile phase is a
(C)
liquid, a gas, or a supercritical fluid. Both phases
must be able to interact physically or chemically with
the sample molecules; chromatography is based on
transport, solvation, and  adsorption (in a very
broad sense) phenomena. When the mobile phase is
flowing through or over the stationary phase the
(D)
analytes in the sample mixture undergo characteristic
Figure 1 Schematic representation of the process of a chro-
partition between the two phases. The mobile phase
matographic separation. (A) Sample injection; (B) partition be-
transports, the stationary phase retains. A mixture
tween the two phases; (C) progression of the mobile phase and
can be separated if its compounds are retained to
new equilibrium; and (D) separation of the two compounds after a
varying degrees. number of partition processes.