MALDI

See

MASS SPECTROMETRY: Matrix-Assisted Laser Desorption/Ionization

MARINE

See

WATER ANALYSIS: Seawater – Organic Compounds; Seawater – Dissolved Organic Carbon;
Seawater – Inorganic Compounds

MASS SPECTROMETRY

Contents

Overview

Principles

Ionization Methods Overview

Electron Impact and Chemical Ionization

Atmospheric Pressure Ionization Techniques

Electrospray

Liquid Secondary Ion Mass Spectrometry

Matrix-Assisted Laser Desorption Ionization

Mass Separation

Ion Traps

Time-of-Flight

Selected Ion Monitoring

Multidimensional

Stable Isotope Ratio

Pyrolysis

Archaeological Applications

Clinical Applications

Environmental Applications

Food Applications

Forensic Applications

Gas Analysis

Peptides and Proteins

Polymerase Chain Reaction Products

Overview

R Sleeman and J F Carter

, Mass Spec Analytical Ltd.,

Bristol, UK

& 2005, Elsevier Ltd. All Rights Reserved.

Introduction

Mass spectrometry (MS) is an analytical tech-
nique that is used to identify unknown compounds,

MASS SPECTROMETRY

/ Overview

337

quantify known materials, and elucidate the struc-
tural and physical properties of ions. The technique
is associated with very high levels of specificity and
sensitivity, somewhat offset by a high degree of tech-
nical complexity. Analyses can be accomplished with
minute quantities – sometimes less than picogram
(10

12

g) amounts of material. MS is highly suited to

the identification of individual components in ex-
tremely complex mixtures.

The history of MS began when the existence of

electrons and ‘positive rays’ was demonstrated by J.J.
Thomson in the early part of the twentieth century.
Thomson suggested that the technique could be used
to perform chemical analysis, but this was not real-
ized for several decades. MS was initially used to
determine the relative abundances of gaseous iso-
topes and to measure their ‘exact masses’, i.e.,
atomic masses with a precision of 1 part in 10

6

or

better. These fundamental measurements led to
developments in a wide range of physical sciences.

Principles

A mass spectrometer is an instrument that separates
charged atoms or molecules according to their mass-
to-charge (m/z) ratio. The unit of mass used is the
unified atomic mass unit (symbol u), defined as 1/12
of the mass of a single atom of the isotope of carbon-
12 (

12

C). The term Dalton (Da) has become widely

accepted in MS as the unit to represent atomic mass.
The charge on an ion is denoted by the integer
number (z) of the fundamental unit of charge, the
magnitude of the charge of an electron. In many
cases, the ions encountered have just one charge
(z

¼ 1) so the m/z value is numerically equal to the

molecular (ionic) mass in Daltons. Mass spec-
trometrists sometimes refer to the ‘mass of an ion’
when they really mean the m/z ratio. The name
Thomson (Th) has recently been proposed as a unit
of mass-to-charge ratio in an attempt to alleviate the
confusion caused by the increasing importance of
multiply charged ions in MS, but does not yet enjoy
widespread acceptance.

Formation of gas-phase sample ions is an essential

prerequisite to mass selection and detection. Ions are
produced and are accelerated toward an analyzer
region that is maintained under vacuum. Either po-
sitive or negative ions are selected for analysis. In
many cases a high proportion of the molecules do not
ionize and are simply pumped away and not detected.

The sample, which may be a solid, liquid, or vapor,

enters the vacuum chamber. Early mass spectrome-
ters required a sample to be gaseous, but the appli-
cability of MS has been extended to include samples
in liquid solutions or embedded in a solid matrix.

The analyte may already exist as ions in solution, or
it may be ionized by a variety of methods within the
ion source.

Gas-phase ions are separated in the mass analyzer

according to their mass-to-charge (m/z) ratios and
impinge on a detector, where the ion flux is converted
to a proportional electrical current. A data system
records the magnitude of these electrical signals as a
function of m/z and converts this information into a
mass spectrum, a graph of ion intensity as a function
of mass-to-charge ratio, often depicted as simple his-
tograms.

The mass scale is calibrated by introducing a ref-

erence compound that yields a well-characterized
mass spectrum comprising known masses at suitable
intervals. A range of calibrants is available for var-
ious techniques and applications.

Compound Identification

The mass spectrum will typically establish the mo-
lecular weight and structure of the compound being
analyzed. Mass spectra recorded under controlled
conditions are highly reproducible such that the
spectrum derived from an unknown may reasonably
be compared to that of an authenticated standard.
When combined with a chromatographic technique
mass spectral identification is often regarded as
providing unequivocal characterization of a chemi-
cal compound.

Large databases or libraries of spectra are com-

mercially available to assist in compound identifica-
tion in a range of applications, although currently
these are largely restricted to electron ionization (EI)
spectra.

Since the rules governing the fragmentation of ions

in the gas phase are well established, it is also pos-
sible to elucidate the structure of an unknown com-
pound solely from its mass spectrum.

Quantification

Quantification may be achieved by comparing the
response of the mass spectrometer from an analyte of
interest to the response obtained from the introduc-
tion of a known amount of a standard. Standards are
typically a closely related substance or may be chem-
ically identical but synthesized by substituting an
isotope of one of the elements.

Since the identity of the analyte is already known

and the requirement is to measure how much is
present, it is not necessary to record the full mass
spectrum. Selected ion monitoring (SIM) is often
used in such circumstances, where the mass spectro-
meter (generally coupled with a chromatographic
technique) monitors only the sample ion and the

338

MASS SPECTROMETRY

/ Overview

equivalent ion for a suitable internal standard. In this
way, very selective conditions for quantifying a
known sample can be devised.

Resolution

The ability of a mass spectrometer to distinguish be-
tween ions of different mass-to-charge ratio values is
termed as resolution. There are many definitions,
depending on specific applications and instrument
types being used. Traditionally, multisector mass
spectrometers are considered as high-resolution in-
struments and quadrupole instruments as medium
resolution.

A typical multisector instrument can be set to re-

solve ions at the 20 parts per million level, that is to
say an ion of mass-to-charge ratio 500.00 can be
separated from an ion of m/z 500.01. Time-of-flight
(TOF) instruments can also attain or exceed this
resolution.

Exact mass measurement can aid in determining

chemical composition. Every isotope (except carbon-
12 which is assigned exactly 12.000 00 Da) has a
unique, noninteger mass. Exact mass measurement
thus allows determination of chemical composition.
With sufficient resolution it is possible to distinguish
between carbon monoxide (CO, 27.995 Da) and nit-
rogen (N

2

, 28.006 Da) by exact mass measurement.

Mass Analyzers

Sector Field Instruments

Magnetic sectors deflect the trajectories of ions
into circular paths of radii that depend on the

momentum-to-charge ratios of the ions. Ions of
larger m/z values follow larger radius paths than
ions of smaller m/z values, so ions of differing m/z
values are dispersed in space. By changing the ion
trajectories through variations of the magnetic field
strength, ions of different mass-to-charge ratios can
be focused onto a detector.

Double focusing sector field instruments incorpo-

rate a combination of electromagnetic fields (B) and
electric fields (E).

A common configuration (‘forward geometry’) for

a sector instrument is the geometry in which a
magnetic sector follows an electric sector analyzer
(ESA). This ‘double focusing’ combination of energy
focusing and ‘angular’ or ‘directional’ focusing and
energy focusing provides mass resolution high
enough to separate ions of the same nominal mass
but different chemical formulas. In ‘reverse geome-
try,’ the magnetic sector precedes the electric sector.
Figure 1 shows a reverse-geometry mass spectrometer
coupled to a gas chromatograph (GC). Such instru-
ments are commonly used for the highly specific de-
tection of environmental contaminants such as
dioxins or performance-enhancing drugs in athletes.

Quadrupole Filters

Quadrupole mass filters consist of four parallel rods,
illustrated in Figure 2. In such instruments, mass se-
lection depends on ion motion resulting from simul-
taneously applied DC and RF electric fields. Scanning
is accomplished by systematically changing the field
strengths, thereby changing the m/z value that is
transmitted through the analyzer. Quadrupole mass

Magnet

Electron multiplier

ESA

Exit

slit

Entrance (source) slit

Ion source

GC inlet

Ion source

Magnetic sector

Electric sector

Figure 1

Photograph and schematic diagram of a modern ‘reverse geometry’ mass spectrometer coupled to a GC. (Reproduced

with permission from Thermo Electron (Bremen).)

MASS SPECTROMETRY

/ Overview

339

spectrometers provide lower resolution than double
focusing instruments but are less costly. The high
scan speeds of quadrupole mass filters render them
highly suited for use in combination with chro-
matographic inlet systems.

Ion Traps

Ion trap mass spectrometers operate on a principle
similar to a quadrupole mass filter. However, it does
not operate as a filter; the ions are stored for sub-
sequent experiments and analysis. Electric fields are
applied to electrodes arranged as a ring electrode in
the middle with cap electrodes on each end. Con-
ceptually, an ion trap can be considered as a convent-
ional quadrupole folded on itself to form a closed
loop. A comparison with a quadrupole mass analyzer
is shown in Figure 2. Within a selected range of m/z
ratios determined by the applied voltages, the device
traps ions in the space bounded by the electrodes. A
mass spectrum is produced by scanning the applied
RF voltages to eject ions sequentially of increasing
m/z ratio through an end cap opening for detection.

Fourier Transform Mass Spectrometry

In a Fourier transform ion cyclotron resonance
(FT-ICR) spectrometer, ions are trapped electrostat-
ically within a cell in a constant magnetic field. An
orbital (‘cyclotron’) motion is induced by the appli-
cation of a pulse between the exciter (or emitter)
plates. The orbiting ions generate a faint signal in the
detector (or receiver) plates of the cell. The frequency

of the signal from each ion is equal to its orbital
frequency, which in turn is inversely related to its m/z
value. The signal intensity of each frequency is pro-
portional to the number of ions having that m/z
value. The signal is amplified and all the frequency
components are determined, yielding the mass spec-
trum. Since the pressure in the cell is very low, the ion
orbital motion can be maintained over many cycles
and the frequency can be measured with very high
precision. FT-ICR instruments can therefore be used
to generate exceptionally high resolution spectra
with great mass accuracy.

Time-of-Flight Mass Spectrometry

TOF mass analyzers separate ions by virtue of their
different flight times over a known distance. Pulses of
ions are ejected from a source and accelerated so that
ions of like charge have equal kinetic energy. They
are then directed into a flight tube where lower mass
ions have greater velocities and shorter flight times.
The travel time from source to detector can be trans-
formed to the m/z value. All ion masses are measured
for each pulse, so TOF mass spectrometers offer high
sensitivity as well as very rapid scanning. They can
provide mass data for very high-mass biomolecules.

Ionization Methods

Compounds are converted into gas-phase molecules
either before or during the charging or ionization
process, which takes place in the ion source.

Many types of ionization mode are available; the

type of compound to be analyzed and the specific
information required determine which ionization
mode is the most suitable. The ionized molecule
may subsequently fragment, producing ions of lower
mass than the original precursor molecule. These
fragment ions are determined by the structure of the
original molecule.

Electron Ionization

In the commonly used EI source (earlier referred to as
‘electron impact’), ions are generated by bombarding
the gaseous sample molecules with a beam of
energetic electrons. EI produces a mixture of positive
and negative ions, as well as neutral species. Positive-
ion EI mass spectra are more commonly recorded
because these ions form more readily.

The energy of the electrons (typically 70 eV) is

generally much greater than that of the bonds that
hold the molecule together. Ionization by electrons is
a highly energetic or ‘hard’ process that may lead to
extensive fragmentation that leaves very little or no
trace of a molecular ion. Because molecular mass and

Ion

source

Ion

source

Quadrupole

Electron

multiplier

Electron

multiplier

Cap

electrodes

Trapped

ion region

Ring

electrode

Figure 2

Schematic diagram of quadrupole and ion trap mass

analyzers.

340

MASS SPECTROMETRY

/ Overview

structure are not easily determined in the absence of
a molecular ion, lower energy or ‘soft’ ionization
techniques have been developed based on chemical
and desorption ionization processes.

Chemical Ionization

Chemical ionization (CI) source is very similar to the
EI source but the beam of electrons is used to create a
plasma of ionized reagent gas (e.g., isobutane, meth-
ane, ammonia).

Transfer of a proton to a sample molecule M, from

an ionized reagent gas such as methane in the form of
CH

5

þ

, yields the [M

þ H]

þ

positive ion. This process

is less energetic than EI and generally produces less
fragmentation. The fragmentation patterns are not
necessarily the same as those of molecular ions, M

þd

.

Negative ions can also be produced under CI con-

ditions. Transfer of a proton from M to reagent gas
or ions can leave [M

H]

, a negatively charged

sample ion. Addition of an electron to M, a process
facilitated by collisionally moderating the energy of
electrons generated in the source, can yield an intense
M

ion. Such ions are often the only ion generated

and can be used to detect species with great sen-
sitivity.

Desorption Ionization

Samples may be desorbed and ionized by an impact
process that involves bombardment of the sample
with high-velocity atoms, ions, fission fragments, or
photons of relatively high energy. The impact depos-
its energy into the sample, either directly or via a
matrix, and leads to both sample molecule transfer
into the gas phase and ionization.

Field desorption (FD) is perhaps the simplest tech-

nique, the sample is coated as a thin film onto a spe-
cial filament placed within a very high intensity
electric field. Ions created by field-induced removal of
an electron from the molecule are extracted into
the mass spectrometer. Field ionization (FI) is the
equivalent process whereby gas-phase molecules are
ionized by a high electric potential. These techniques
are sometimes applied to relatively large, polymeric
molecules but are not commonplace.

Fast Atom Bombardment/Liquid SIMS

The techniques of fast-atom bombardment (FAB)/
liquid secondary ionization (LSIMS), developed in
the early 1980s, revolutionized the range of com-
pounds amenable to analysis by MS and opened up
the field to many areas of biomedical research. Al-
though now considered insensitive by comparison
with more recently introduced ionization modes,
FAB still has a role as a rapid, reliable, and robust

technique for samples where quantity and purity are
not a problem. The sample is first dissolved in a liq-
uid matrix. This is typically a viscous, low vapor
pressure liquid. A few microliters of this liquid are
placed on a small metal target at the end of a probe
that is inserted into the mass spectrometer. The liquid
surface is then bombarded with a beam of high ki-
netic energy atoms (xenon) or ions (cesium). Mole-
cules sputtered from the surface enter the gas phase
and ionize, either by protonation, deprotonation, or
adduction. The resulting ions tend to be stable and
exhibit little fragmentation.

Matrix-Assisted Laser Desorption Ionization

Unlike FAB/LSIMS, matrix-assisted laser desorption
ionization (MALDI) uses a crystalline, rather than
liquid, matrix, and a beam of photons, rather than
atoms or ions. The net result is a dramatic increase in
both sensitivity and mass range of compounds that
may be analyzed. The sample is dissolved in a matrix
and is allowed to crystallize on a stainless-steel
target. The target is then inserted into the mass spec-
trometer and the surface bombarded with a pulsed
laser beam. Molecules are desorbed from the surface
and ionize, usually by protonation or deprotonation.
Any fragment or multiply charged ions are generally
of low abundance in this ionization mode. The
pulsed nature of the laser excitation renders this
technique compatible with TOF, and the combined
technique enjoys an almost limitless mass range.

Atmospheric Pressure Ionization Techniques

Atmospheric pressure ionization (API) techniques
encompass a range of techniques in which ionization
occurs external to the mass spectrometer vacuum.
Ionization can be achieved by a variety of methods,
including photoionization, corona discharge at the
tip of a needle, or by the use of radioisotopes such
as

63

Ni.

Atmospheric pressure chemical ionization (APCI)

is a simple and robust technique routinely used to
interface the eluent from a high-performance liquid
chromatography (HPLC) to a mass spectrometer.
The liquid stream passes through a heated nebulizer
into a corona discharge region. Analyte molecules
are ionized and extracted into the mass analyzer.

Electrospray

Electrospray (ESI) is another example of an API
technique.

The sample is dissolved in a mobile phase and

pumped through a fine stainless-steel capillary main-
tained at high potential. This creates an electrostatic
spray of multiply charged droplets containing the

MASS SPECTROMETRY

/ Overview

341

sample. At higher solvent flow rates, heat and drying
gas may be needed to increase the rate of droplet
evaporation. This technique is sometimes referred to
as pneumatically assisted ESI or Ionspray.

After desolvation and subsequent charge concen-

tration, gas-phase ions are produced and propelled
toward the high vacuum mass analyzer. ESI is con-
sidered to be one of the ‘softest’ ionization techniques
available, i.e., little energy is transferred to the mol-
ecule other than that required for ionization. Thus,
protonated, deprotonated, or cationized molecules
that undergo very little fragmentation are generated,
even from highly polar, thermally labile molecules.

ESI can impart many charges (z), usually in the

form of protons, to amenable large molecules such as
proteins, and thus compounds with molecular masses
in the tens of thousands can be analyzed with mass
spectrometers with m/z ranges of a few thousand.
Molecular mass can often be determined to a preci-
sion in the order of one part in 10 000 or better. ESI is
particularly compatible with liquid separation meth-
ods and has become a widely used method in
biological and pharmaceutical analysis, where iden-
tification is achieved through deconvolution of the
envelope of peaks formed with multiple charge states.

Choice of Ionization Technique

EI and CI are generally the techniques of choice for
small (

o800 Da), volatile, thermally stable com-

pounds.

CI tends to give molecular weight information and

EI, with the greater fragmentation, provides struc-
tural information.

FAB/LSIMS is useful for larger (

t5000 Da)

involatile, polar, thermally unstable molecules, such
as peptides, small proteins, and other biopolymers.
However, this technique has now largely been super-
seded by ESI and MALDI.

MALDI is suitable for similar compounds to those

amenable to FAB, but affords much greater sen-
sitivity. Biopolymers with molecular weights above
300 000 Da have been successfully analyzed.

ESI is suitable for similar compounds to MALDI,

with possibly a slightly reduced sensitivity and mass
range. The tendency to produce multiply charged
ions brings the mass-to-charge ratios of high molec-
ular weight proteins well within the range of inex-
pensive mass spectrometers. This has fuelled an
explosion in biochemical applications of MS and
has spawned the developing fields of proteomics,
genomics, and metabonomics.

ESI is frequently interfaced with chromatographic

techniques such as HPLC, capillary zone elect-
rophoresis (CZE), and capillary electrochromato-
graphy (CEC).

Sample Introduction Techniques

Probe Inlets

A direct insertion probe may be used for reasonably
pure volatile solids. The sample is loaded into a
quartz tube on the tip of a rod that is inserted into the
evacuated source region. The sample is then evapo-
rated or sublimed into the gas phase, usually by hea-
ting. The gaseous molecules are then ionized (often
with accompanying fragmentation) and the ions are
mass analyzed. In some techniques, volatilization
and ionization occur at the same time.

Septum Inlets

Heated reservoir septum inlets may be used for pure
gases or volatile liquids, comprising a heated re-
servoir with a small restriction ‘bleed’ into the ion
source. The sample is injected into the reservoir
through a septum. This method is commonly used to
introduce reference materials for calibration.

Chromatographic Techniques

To obtain the mass spectrum of a single constituent
of a mixture, the individual components often need
to be separated prior to analysis. Separation is nec-
essary for unambiguous identification because two
compounds present in the source region simultane-
ously create a mixed spectrum and even simple com-
pounds can generate many fragment ions. The
historical combination of GC and MS (GC–MS) al-
lows compounds in the vapor phase to enter the mass
spectrometer so that the components of mixtures can
be detected and analyzed sequentially.

The challenge in interfacing a mass spectrometer

to a separation system like a gas or liquid chro-
matograph is maintaining the required vacuum in the
mass spectrometer while introducing flow from
the chromatograph. Interfaces that restrict or reduce
the gas flow into the mass spectrometer (e.g., flow
splitters or devices that differentially remove carrier
gas from the GC effluent) initially made the combi-
nation of GC and MS an extremely widely used
technique. The low gas flows typical of capillary GC
now permit direct connection to mass spectrometers.

More recently, liquid chromatography (LC), su-

percritical fluid chromatography (SFC), and CZE
devices connected to mass spectrometers have been
used to separate components of complex mixtures
prior to mass analysis. When vaporized, the solvent
from an LC represents a volume of 100–1000 times
greater than that of a carrier gas used in GC. Inter-
faces developed commercially have solved the
problem of eliminating this gas load by using com-
binations of heating and pumping, sometimes with

342

MASS SPECTROMETRY

/ Overview

the assistance of a drying gas stream. The inlets for
higher flow rates (as in analytical LC) employed in
LC/MS systems in routine use are primarily APCI
and ESI.

Particle beam interfaces, thermospray, and ‘dy-

namic FAB’ have also been used as LC continuous-
flow injection techniques, but these have largely been
superseded.

For GC–MS, LC–MS, or other combinations, the

data consist of a series of mass spectra acquired
sequentially in time. To generate this information,
the mass spectrometer scans the mass range repet-
itively during the chromatographic run. The intensi-
ties of all the ions in each spectrum can be summed,
and this sum plotted as a function of chromatogra-
phic retention time to give a total ion chromatogram
(TIC) whose appearance is similar to the output of a
conventional chromatographic detector. Each peak
in the TIC represents an eluting compound that can
be identified by interpretation of the mass spectra
recorded for the peak. The intensity at a single mass-
to-charge ratio over the course of a chromatographic
run can be displayed to yield a selected ion current
profile or mass chromatogram. This technique can be
used to find components of interest in a complex
mixture without having to examine each individual
mass spectrum.

Mass Spectrometric Techniques

Selected Ion Monitoring

SIM is frequently used for the quantitative determi-
nation of specific analytes by MS, usually in combi-
nation with a chromatographic separation. The mass
spectrometer is used to monitor a limited number of
ions characteristic of target compounds, rather than
to acquire a complete spectrum. The effect is that the
instrument spends a greater time recording ions from
the analytes of interest with a resulting increase in
both sensitivity and selectivity. This is a very sensitive
technique and for some compounds it is possible to
detect at the femtogram (10

15

g) level.

Tandem or Multistage Mass Spectrometry
(MS/MS, MS

n

)

Tandem MS is used to provide more information than
can be afforded by a single mass spectrometer and is
widely used for screening complex matrices such as
blood and urine. Analysis is achieved, in effect, by
performing two stages of SIM. The first mass spec-
trometer is set to transmit the ‘precursor’ ion of in-
terest into a region where fragmentation occurs. One
of the ‘product’ ions is monitored by a second mass
spectrometer. Selection of an appropriate internal

standard and switching between the gas phase tran-
sitions can lead to very high specificity. This technique
is known as selected reaction monitoring (SRM), and
is frequently used for quantification. Fragmentation is
usually achieved in a collision cell pressurized with an
inert gas such as argon. Collision of ions with atoms
in the cell produces fragments by a process known as
collision induced dissociation (CID). Other approach-
es have been used to cause fragmentation, such as
lasers, electron beams, and surface collisions.

In cases where ‘soft’ ionization techniques are

used, the molecular weight of the sample may be
observed but the lack of in-source fragmentation
means that little structural information is available.
A product ion mass spectrum acquired with a tan-
dem mass spectrometer can yield this structurally
significant information.

In the technique of precursor ion scanning the sec-

ond mass spectrometer is set statically to transmit
product ions of only one selected mass-to-charge ra-
tio. This mass is monitored continuously whilst the
first mass spectrometer is scanned. A signal will be
detected only when a precursor ion fragments to form
the product ion that is monitored. This technique is
often used to screen for compounds of related struc-
ture, such as the metabolites of a known drug.

Another tandem screening method is known as

constant neutral loss scanning. Here, both mass spec-
trometers are scanned simultaneously but are offset
corresponding to the difference between precursor
and product ion masses. A signal only appears when a
precursor ion yields a product ion with the mass dif-
ference selected. This technique can be used to screen
for compounds that contain a specific structural fea-
ture that yields a common fragmentation process.

Tandem MS can be performed using sector, quad-

rupole, and TOF instruments. However, each stage
of mass analysis requires a separate mass analyzer.
Different mass analyzers are often combined to form
tandem instruments for specific applications, e.g.,
Q-TOF.

Ion trap or ICR mass spectrometers permit MS/MS

product ion experiments to be conducted sequenti-
ally in time within a single mass analyzer. A number
of sequential experiments, termed MS

n

, may be per-

formed.

Modern TOF mass spectrometers incorporate a

reflectron unit and the facility to analyze what are
known as postsource decay ions. The spectra pro-
duced are similar to product ion spectra and can be
enhanced by the inclusion of a collision cell.

Stable Isotope Ratio Mass Spectrometry

Although often presumed to be constant, natural
isotope abundance ratios show significant and

MASS SPECTROMETRY

/ Overview

343

characteristic variations when measured very pre-
cisely. In stable isotope ratio mass spectrometry
(IRMS), element isotope ratios are determined very
accurately and precisely. Typically, single focusing
magnetic sector mass spectrometers with fixed mul-
tiple detectors (one per isotopomer) are used. Com-
plex compounds are reduced to simple molecules
prior to measurement, for example, organic com-
pounds are combusted to CO

2

, H

2

O, and N

2

. Iso-

tope ratio measurements are useful in a wide range of
applications, for example, metabolic studies using
isotopically enriched elements as tracers; climate
studies using measurements of temperature-depend-
ent oxygen and carbon isotope ratios in foraminifera;
rock age dating using radiogenic isotopes of elements
such as lead, neodymium, or strontium; and source
determinations using carbon isotope ratios (for ex-
ample, to discriminate between naturally occurring
substances and petroleum-based synthetic materials).

Pyrolysis Mass Spectrometry

Pyrolysis is the thermal degradation of complex ma-
terial in an inert atmosphere or a vacuum. Molecules
cleave into smaller, volatile fragments called pyroly-
sate. In pyrolysis MS (PyMS), the pyrolysate is di-
rectly analyzed by MS to produce a chemical profile
or fingerprint of the complex material analyzed. The
development of PyMS was largely driven by its ap-
plicability to the characterization of microorganisms
and has now largely been supplanted by the appli-
cation of MALDI in this field. In contrast, pyrolysis
GC/MS (Py-GC/MS) still finds numerous applica-
tions in the analysis of complex synthetic and
biological polymers.

Elemental Mass Spectrometry

Elemental MS is applied mostly to inorganic mate-
rials, to determine the elemental composition of a
sample rather than the structural identities of its

chemical constituents. Elemental MS provides quan-
titative information about the concentrations of
those elements. The decomposition of the sample
into its constituent atoms and ionization of those
atoms occurs in a specially designed source. The ion
source used in elemental MS is ordinarily an atmos-
pheric-pressure discharge such as inductively coupled
plasma (ICP) or a moderate-power device such as
glow discharge (GD). The resulting atomic-ion beam
is then separated by a mass analyzer and the signal
used to determine the sample composition. With an
ICP employed as an ion source, solution detection
limits down to the parts per trillion level are possible
in favorable cases, while with the glow-discharge
source, solid metal samples can be analyzed directly
and their elemental composition determined over a
million-fold range of concentrations. Isotopic infor-
mation is readily available.

See also: Gas Chromatography: Mass Spectrometry.
Liquid Chromatography: Liquid Chromatography–Mass
Spectrometry. Mass Spectrometry: Ionization Methods
Overview; Mass Separation; Stable Isotope Ratio.

Further Reading

American

Society

for

Mass

Spectrometry

website,

www.asms.org

Armentrout PB (2003) In: Gross ML and Caprioli RM

(eds.) Encyclopedia of Mass Spectrometry, vol.1: Theory
and Ion Chemistry. Amsterdam: Elsevier.

Ashcroft AE (1997) Ionization Methods in Organic Mass

Spectrometry. Cambridge: Royal Society of Chemistry.

British Mass Spectrometry Society website, www.bmss.

org.uk

Chapman JR (1993) Practical Organic Mass Spectrometry,

2nd edn. New York: Wiley.

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Principles

K J Welham

, University of Hull, Hull, UK

& 2005, Elsevier Ltd. All Rights Reserved.

This article is a revision of the previous-edition article by
A Roberts, pp. 2787–2795,

& 1995, Elsevier Ltd.

Introduction

The mass spectrometer is an instrument capable of
producing a beam of ions from a sample under

investigation, separating these ions according to their
mass-to-charge ratio and recording the relative abun-
dances of the separated ion species as a mass spec-
trum. Many types of mass spectrometers have been
developed to fulfill the above definition. The widely
differing nature and physical state of sample mate-
rials has resulted in a variety of techniques for sa-
mpling, ionization, ion storage, mass separation, and
ion recording. This article on the basic principles of
mass spectrometry (MS) will cover the historical

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MASS SPECTROMETRY

/ Principles