Mass spectroscopy in food aplication

Food Applications

F A Mellon

, Institute of Food Research, Norwich, UK

Introduction

Mass spectrometry (MS) is a very versatile technique
with an established and rapidly growing record of
successful applications in the food and nutrition sci-
ences. Major advances in organic MS over the last
decade, include the reﬁnement and widespread adop-
tion of electrospray ionization (ESI), atmospheric
pressure chemical ionization (APCI), and matrix-as-
sisted laser desorption ionization time-of-ﬂight (MA-
LDI-TOF) MS. These techniques have substantially
extended the scope of mass spectrometric applica-
tions in the food and nutrition sciences. Furthermore,
alternative mass spectrometric techniques, including
isotope ratio MS (CIRMS), inductively coupled
plasma MS (ICP-MS), proton transfer reaction MS
(PTR-MS), and accelerator MS (AMS) are having an
increased impact in the study of food authenticity,
nutrient metabolism, and, in the case of PTR-MS,
ﬂavor research. This article will briefly review appli-
cations of MS in the analysis of a wide range of food
components, including nutrients, structural constitu-
ents of foods, ﬂavors, biologically active nonnutri-
ents, and toxicants.

Sample Preparation

Sample preparation techniques vary from rudimen-
tary to elaborate because of the very wide range of
analytical problems presented in food analysis and
research. Preparation methods are highly dependent
on both the nature of the analyte and on the type of
mass spectrometric procedure used. At one extreme,
the processing of food samples prior to pyrolysis MS
(PyMS) may entail little more than the drying of a
few micrograms of material, followed by introduc-
tion of the sample into the mass spectrometer on a
pyrolysis probe. Flavor components are frequently
isolated by solvent extraction, headspace sampling,
or solid-phase microextraction (SPME), or in the
case of PTR-MS analysis, simply by direct sampling
of volatile components of the analyte. In contrast,
linkage analysis of cell wall polysaccharides in food
plants requires extensive chemical derivatization
before samples are analyzed by gas chromato-
graphy–mass spectrometry (GC–MS). Combined
liquid chromatography–mass spectrometry (LC–MS)
and tandem mass spectrometry (MS/MS or MS

)

can sometimes be conducted on samples that have
undergone minimal preparation. Finally, sample
preparation for inorganic MS, zinc or chromium in
food or biological ﬂuids, for example, can be highly
demanding because it is essential to avoid contam-
ination.

Major Food Components

Amino Acids, Peptides, and Proteins

The analysis of amino acids by GC–MS of suitable
volatile derivatives is a well-established technique.
Despite the availability of amino acid analyzers, GC–
MS is still occasionally used because it is useful for
qualitative and quantitative analysis of unusual (i.e.,
nonprotein) amino acids. GC–MS is also valuable for
conducting studies of the racemization of amino ac-
ids during cooking or food processing, when used in
conjunction with chiral GC columns and deuterium
labeling. GC–MS methods are also often used for
determining stable isotope labeled amino acids in
nutritional, metabolic studies. ESI LC–MS may also
be used for determining amino acids.

Peptides and proteins are of interest in several ar-

eas of food and nutrition science, including allergenic
response, food material properties, ﬂavorings, toxic-
ology, and nutritional value. An increasing number
of analytical papers in the food and nutrition ﬁeld
now describe applications of modern MS techniques
such as ESI and MALDI-TOF for molecular weight
determination or (in MS/MS mode) full or partial
sequencing. The variety of applications appearing in
the current literature is also impressive: represen-
tative examples include the effects of irradiation on
egg proteins, pH-related conformational changes in
egg lysozyme, measurement of lactosylation of whey
proteins, determination of the heterogeneity of casein
proteins, authentication of cheeses by determining
protein proﬁles, characterization of ﬂavor peptides in
protein hydrolysates, quantiﬁcation of wheat glia-
dins, characterization of soya and other legume pro-
teins,

and

the

characterization

the

food

preservative peptide antibiotic nisin, its genetically
modiﬁed variants and its inactivation by glutathione
by Fourier transform ion cyclotron resonance (FT-
ICR) MS and MALDI-TOF, respectively. Figure 1
shows a comparison of the sustained off-resonance
irradiation collisionally activated decomposition
(SORI-CAD) spectra of ‘wild-type’ nisin A and three
genetically modiﬁed variants. The ions shifted in
mass (marked by an asterisk) relative to the wild-type

476

MASS SPECTROMETRY

/ Food Applications

molecule are clearly visible and were used to aid
structural conﬁrmation of the variant molecules.

Carbohydrates and Sugars

Carbohydrate analysis is of great importance in the
food sciences because of the signiﬁcant role of
polysaccharides as macronutrients, as major constit-
uents of dietary ﬁber, and as food structure compo-
nents contributing to textural properties, and food
additives. Because of structural similarities between

many monosaccharide residues, detailed mass spec-
trometric analysis is more difﬁcult than for other
biopolymers. Furthermore, the anomeric conﬁgura-
tion and the nature of the linkages between sugar
residues cannot generally be determined directly by
MS. Many different schemes are available to convert
sugars to derivatives sufﬁciently volatile for GC–MS
analysis. These often involve trimethylsilylation or a
combination of this derivatization technique with
oxime formation. A classic method for compositio-
nal and linkage analysis of large polysaccharides

0.1

0.0

(A)

(B)

(C)

(D)

1300

1350

1400

1450

1500

1550

1600

1650

m /z

Intensity

NISIN A

( y + 2)

+ S

0.10

0.05

0.00

1300

1350

1400

1450

1500

1550

1600

1650

Intensity

( y + 2)

+ S

0.10

0.05

0.00

1300

1350

1400

1450

1500

1550

1600

1650

Intensity

( y + 2)

+ S*

0.1

0.0

0.2

1300

1350

1400

1450

1500

1550

1600

1650

Intensity

( y + 2)

+ S*

+ S

S5A

+ S*

H27K

130W-H

Figure 1

Comparison of parts of the SORI-CAD ESI FT-ICR mass spectra of the [M

þ 3 H]

precursor ions of (A) nisin A (wild-type

nisin), (B) nisin S5A, (C) nisin H27 K, and (D) nisin I30W-H, showing the region containing doubly charged fragment ions. The ions in
the transgenic variants that are shifted in mass compared to nisin A are marked with asterisks. (Reprinted from Lavanant H, Heck A,
Derrick PJ, et al. (1998) Characterization of genetically modiﬁed nisin molecules by Fourier transform ion cyclotron resonance mass
spectrometry. European Mass Spectrometry 4: 405–416, with permission;

& IM Publications.)

MASS SPECTROMETRY

/ Food Applications

477

involves GC–MS of partially methylated alditol
acetates (PMAAs). Polysaccharides are methylated,
hydrolyzed, and reduced with NaBD

to deuterated

O-methyl alditols, which are then acetylated and
analyzed qualitatively and quantitatively by GC–MS.
PMAAs yield characteristic mass spectra that allow
determination of the number, nature, and linkage
position of the monosaccharides that comprise the
polysaccharide.

The molecular weights of large polysaccharides

can be measured with the aid of ESI, in LC–MS,
capillary electrophoresis/mass spectrometry (CE–
MS), or direct injection modes, and by MALDI
MS. For example, MALDI-TOF analysis of fruc-
tooligosaccharides has enabled the development of a
rapid method for analyzing these molecules in food
plants. Examples of other important applications of
organic MS in food carbohydrate analysis include
determination of the structures of plant oligosaccha-
rides that may be involved in eliciting allergic
response and direct analysis of plant cell wall
xyloglucans.

Concerns over the adulteration of high value

foods, for example, dilution of maple syrup, fruit
juices, or of protected denomination of origin honeys
with corn syrup, have been addressed for some time
by combusting food samples and then measuring

ratios

high-precision

IRMS.

Subtle differences in isotope ratio are found in sugars
derived from different sources and this property can
be used to conﬁrm their origin. Figure 2 exempliﬁes
this by demonstrating the linear relationship between
d

C% and the ratio of an adulterant sugar, high

fructose corn syrup, to honey sugar.

C% is a measure of natural carbon-13 levels

against a reference standard and is deﬁned by the
equation:

sample

standard

1000

Lipids

Dietary lipids may be analyzed mass spectrometri-
cally as long-chain fatty acids, esters, triglycerides,
and phospholipids. Although the degree of unsatu-
ration of fatty acids may be determined readily from
electron ionization (EI) mass spectra, double bond
location presents a more challenging problem. Long-
chain unsaturated fatty acids rearrange extensively
under EI, so fragment ions do not yield reliable in-
formation regarding double bond position. A num-
ber of strategies have been devised to overcome these
problems: one of the most popular is derivatization
followed by GC–MS. Several different procedures
are available, for example, oxidation followed by si-
lylation, epoxidation, and the analysis of methoxy or
dimethyldisulﬁde derivatives. Alternatively, chemical
derivatives that localize charge may be generated,
thus preventing charge-induced double bond migra-
tion. A good example is the formation of volatile
2-substituted 4,4-dimethyloxazoline derivatives by
condensation of the fatty acid with 2-amino-2-me-
thylpropanol. The ‘charge-localizing’ derivative is
sufﬁciently volatile for GC–MS analysis and yields
ion clusters separated by 14 Da, reducing to 12 Da
when a double bond interrupts the chain. Alternative
techniques for locating double bond position include
high collision energy ‘charge-remote’ MS/MS frag-
mentation of molecules ionized by negative-ion
chemical ionization (CI). Possible alternative tech-
niques employ ESI of lithiated adducts of unsatura-
ted fatty acids and low-energy MS/MS and acetonit-
rile CI ion trap MS/MS of polyunsaturated fatty acid
methyl esters.

Capillary GC–MS is useful for identifying the

carbon and unsaturation number of acylglycerols.
However, APCI LC–MS can be used to analyze less
volatile acylglycerols that may be unsuitable for GC–
MS. APCI LC–MS has become the method of choice
for qualitative and quantitative analysis of acylgly-
cerols and, when combined with MS/MS, is capable
of distinguishing fatty acid chains in the sn-2 position
from those in the sn-1/3 positions.

The sensitivity, selectivity, and convenience of po-

sitive and negative-ion ESI LC–MS makes this the
current benchmark technique for analyzing phos-
pholipid mixtures. Additional structural information
can be obtained by MS/MS. Where these advanced

−10

−20

−30

100

% Honey (by volume)

Figure 2

Plot of changes in

ratio (d13C%, de-

ﬁned in text) of honey against concentration of high fructose corn
syrup adulterant. (Reproduced with permission from PDZ
Europa, Norwich, UK.)

478

MASS SPECTROMETRY

/ Food Applications

techniques are unavailable, a more laborious ap-
proach of chromatographic class separation, chem-
ical degradation, and GC–MS analysis of fatty acid
can be used to generate useful structural information.

Complete Foods and Food-Related Materials

PyMS is capable of generating useful data on intact
foodstuffs or associated microorganisms (food poi-
soning bacteria, for example). PyMS is based on the
controlled thermal degradation of samples under in-
ert conditions to produce mixtures of volatile com-
pounds that are swept into the mass spectrometer ion
source and ionized by EI or CI. The resulting ‘ﬁnger-
print’ spectrum of the analyte contains characteristic
features that can be classiﬁed by chemometric meth-
ods. Pyrolysis is sometimes combined with GC–MS
in order to extract more information from the ana-
lyte. Applications of PyMS in the food sciences in-
clude quality assurance and authentication of food
and drinks, analysis of cell wall material in food
plants, and identiﬁcation of food microorganisms.

Direct analysis of microorganisms, including food

poisoning microorganisms, has also been conducted
by MALDI-TOF MS. This promising technique can
be used to characterize bacteria rapidly by genus,
species, and strain.

Flavors and Taints

Because most ﬂavor components are highly volatile,
EI and CI GC–MS have been primary techniques in
ﬂavor analysis and research since the early 1960s.
For example,

B1000 discrete compounds have been

identiﬁed in coffee volatiles using capillary GC–MS
methods.

The ease with which SPME GC–MS studies may

be conducted has made this an important and useful
technique for sampling ﬂavors and taints. GC–MS
has been supplemented by LC–MS techniques for
studying involatile ﬂavor precursors or semivolatile
or involatile food components that have important
ﬂavor characteristics.

The availability of a large knowledge base of ﬂavor

proﬁles (largely deﬁned by GC–MS analysis) and re-
cent instrumental advances have resulted in a shift in
emphasis of mass spectrometric applications in ﬂavor
research. Instead of characterizing complex mixtures
of volatiles, several researchers are now focusing on
ﬂavor release and, more specifically, on sampling
volatiles released into the mouth and nose. By using a
special APCI probe coupled to a mouth or nose
piece, it is possible to conduct dynamic, breath-by-
breath analyses of air expired during eating. The re-
cent development of PTR-MS for online trace gas
monitoring has considerable potential for application

to ﬂavor research because of its ability to sample
air directly. PTR-MS has already been applied to
analysis of ﬂavor compounds. Other food-related
applications include the control of food produc-
tion by determining volatile organic compounds
produced during fruit ripening and aging, the
study of coffee volatiles and monitoring of meat
degradation.

Mass spectrometric methods are also useful for

authenticating ﬂavor components, using methods for
accurately measuring

ratios, similar to

those already described for authenticating sugars
(above). If more detailed analysis is required GC/
combustion/isotope ratio MS will yield accurate iso-
tope ratios on individual components of a ﬂavor
sample.

Nutrition Studies

Stable Isotope Methods

Interest in the use of stable isotope MS for studying
both the nutritional value of foods and diets and
fundamental aspects of nutrient metabolism in hu-
mans has increased considerably. The only major
drawbacks of stable isotope studies are associated
with the presence of endogenous isotopes of the
elements under investigation. Sufﬁcient label must
be administered to generate a measurable increase
in isotope ratio. However, care is needed to ensure
physiological dosing as administration of excessive
doses

yield

physiologically

contentious

data.

Conversely, administration of small quantities of la-
bel requires careful and accurate measurement of
isotope ratios because the enriched material is diluted
by the endogenous nutrients. In the case of mineral
metabolism studies, additional precautions are nec-
essary because of the possibility of contamination
during sample processing.

Stable isotope studies usually involve administra-

tion of the enriched stable isotope in or with a meal.
The method of labeling depends on the type of study
undertaken. For elements such as selenium, where
absorption and metabolism are highly dependent on
chemical form, it is usually necessary to use an in-
trinsic label (i.e., one that is biosynthetically incor-
porated into the food). Conversely, minerals believed
to form a ‘common pool’ in the digestive system may
be mixed directly with the food (extrinsic labeling).
In some cases a second isotope is injected or infused
intravenously to correct for endogenous losses. Sam-
ples of breath, blood, urine, saliva, or feces are
then collected for an appropriate period and subject-
ed to isotopic analysis. Isotope ratio measurement
often requires specialized instrumentation such as

MASS SPECTROMETRY

/ Food Applications

479

high-precision IRMS or dedicated inorganic instru-
ments such as ICP-MS.

Mineral Nutrients

Although several different mass spectrometric meth-
ods have been deployed to determine enriched stable
isotopes in human studies of nutrient mineral me-
tabolism, thermal ionization mass spectrometry
(TIMS) and particularly ICP-MS are now used al-
most exclusively. ICP-MS is rapid, very sensitive, and
sample preparation and introduction is often simpli-
ﬁed. Furthermore, ICP-MS can be coupled directly to
separation techniques such as size-exclusion chro-
matography (SEC), high-performance liquid chro-
matography (HPLC), or CE so that speciation, the
determination of the chemical form of particular ele-
ments, may also be studied. The two major draw-
backs of ICP-MS, low precision relative to TIMS and
interference from polyatomic ions in the argon plas-
ma, have largely been overcome by new generations
of instruments equipped with multiple collectors and
collision/reaction cells, respectively.

A wide range of human studies has been conducted

using enriched stable isotopes of nutrient minerals.
Applications include the determination of the ab-
sorption and metabolism of iron, zinc, calcium, cop-
per, selenium, and molybdenum. An example of the
type of information that can be obtained is provided
by a study of iron absorption from different weaning
foods, and the effects of vitamin C on iron absorp-
tion. These measurements, conducted using enriched
stable isotopes of

Fe and

Fe, demonstrated a do-

ubling of iron absorption when a drink containing
50 mg of vitamin C was administered with the food.

A recent development, especially useful for con-

ducting long-term studies of the effect of diet on
metabolism, is to administer extremely low levels of
long-lived radioisotopes, e.g.,

Ca, that are then

measured by AMS. Because the activity of the radi-
oisotope is extremely low (typically

o10

of an-

nual background radiation dose), it is considered to
be safe for human use. AMS opens up the exciting
possibility of conducting long-term metabolic and
dietary intervention studies where required; for ex-
ample, in studies of bone metabolism.

Vitamins

Although vitamins can be determined, both quali-
tatively and quantitatively, by MS, routine analysis is
usually best conducted by other means (e.g., HPLC
with ultraviolet (UV) or ﬂuorescence detection,
immunoassay methods, or microbiological methods).
Analytically, MS does have an important role as a
reference technique, especially when used in isotope

dilution-based analyses, for evaluating and calibra-
ting alternative, non-MS techniques. However, the
most prominent role for MS in vitamin research is in
studies of the metabolism of vitamins in humans,
especially by LC–MS or LC–MS/MS. The newer LC–
MS techniques of APCI and ESI are particularly
useful in this respect because many vitamins are too
labile for EI and CI MS.

Vitamins D

and D

and their major metabolites

have been studied extensively by GC–MS of volatile
derivatives; these studies include quantitative deter-
mination by isotope dilution. More recently, ESI LC–
MS and LC–MS/MS have been used in the qualitative
and quantitative measurement of vitamin D and
vitamin D analogs.

The association of folates with reduced chronic

disease risk and prevention of neural tube defects has
generated increased interest in studies of the metab-
olism of these B group vitamins. Recent activity has
focused on developing improved mass spectrometric
methods for determining the absorption, metabolism,
and bioavailability of these molecules. The folates are
polar, involatile molecules that are unstable in solu-
tion and must be derivatized if they are to be analyzed
by EI or CI MS. Pioneering studies of the human
metabolism of folates were ﬁrst conducted by selected
ion monitoring (SIM) GC–MS of derivatized extracts
from biological ﬂuids. Although the GC–MS method
has provided useful data on the kinetics of folate
metabolism and on urinary excretion of folate and
metabolites, the method has several drawbacks. Im-
provements both in mass spectrometric techniques
and in speed of analysis were clearly desirable and a
number of groups have focused on developing LC–
MS methods. Negative-ion ESI techniques developed
to determine the four main food and supplementary
folates, folic acid, 5-methyltetrahydrofolic acid, tet-
rahydrofolic acid, and folinic acid, in selected food-
stuffs and a vitamin supplement demonstrated the
feasibility of using ESI LC–MS in folate analysis.
Other laboratories have subsequently reported suc-
cessful measurement of stable isotope labeled folates
in human plasma and/or urine by positive-ion ESI
LC–MS, negative-ion LC–MS/MS (in multiple reac-
tion monitoring mode), and SIM negative ion
LC–MS. Limits of quantiﬁcation were sufﬁcient to
conduct human metabolic studies using

and

H folates, labeled both extrinsically and intrin-

sically, on spinach and fortiﬁed cereal grain. The
low-collision-energy negative-ion tandem ESI mass
spectrum of 5-formyl tetrahydrofolate is shown in
Figure 3.

AMS has also been used to determine the meta-

bolic fate of folates labeled with low levels of the
radioactive isotope

480

MASS SPECTROMETRY

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The ability to conduct bioavailability studies on

foods labeled intrinsically with stable isotopes is par-
ticularly important as this type of labeling is the ‘gold
standard’ for metabolic studies, i.e., intrinsic label
should mimic the behavior of endogenous nutrients
most closely. However, the mass spectrometric meas-
urement of the metabolism of intrinsically stable iso-
tope labeled materials is far more challenging than
measuring extrinsically labeled nutrients. Low levels
of isotope incorporation must be determined in small
samples and it is only quite recently that modern LC–
MS and LC–MS/MS methods, as demonstrated by
the folate studies described above, have begun to rise
to this challenge.

Vitamin A is a generic descriptor for a family of

fat-soluble vitamins that has the biological activity of
retinol, one of the most active and bioavailable
members of the group. Retinol, retinoids, and their
derivatives yield characteristic EI spectra, a property
that has been exploited for over 25 years by GC–MS
methods that determine liver stores of retinol non-
invasively, following oral administration of deute-
rated retinol. A simple mathematical formula derives
liver stores of retinol from isotope composition in
plasma. Various EI and positive ion CI GC–MS pro-
cedures have been used; the most successful tech-
niques involve measurement of silylated derivatives
of retinol. Although these methods have provided
useful nutritional data, further improvements in sen-
sitivity were still desirable for conducting serial in-
fant studies or when sampling under nonideal ﬁeld
conditions. A technique based on negative ion

electron capture GC–MS of trimethylsilyl derivatives
fulﬁlls these increased demands and yields. The
method worked on 200

ml blood samples, the amount

of blood typically collected in a heel-prick sample
from an infant, and yielded measurements on en-
richments as low as 0.01% of circulating retinol-D

More recently, both APCI LC–MS and LC–MS

methods have been developed to measure retinol
conjugates such as retinyl palmitate (which appears
in the blood soon after vitamin A containing meal is
consumed). These techniques have advantages over
GC–MS methods because there is no need to hydro-
lyze samples to release free retinol, or for derivat-
ization for GC–MS. Carotenoid (pro-vitamin A)
metabolism is also being studied by LC–MS tech-
niques because of epidemiological evidence of their
putative role in cancer prevention. Although ESI LC–
MS has shown promise in preliminary mass spectro-
metric studies, APCI LC–MS is now the method of
choice because of its robustness and tolerance of a
wider range of organic solvents.

Many other vitamins have been determined by

mass spectrometric methods; representative exam-
ples include APCI LC–MS/MS measurement of toco-
pherols and LC–MS measurement of K vitamins.

Trace Components in Foods

Food Additives

Food additives comprise a wide variety of com-
pounds that are generally monitored by techniques

100

150

175

200

225

250

275

300

325

350

375

400

425

450

475

500

m/z

472

444

400

357

343

272

315

228

164

Figure 3

The MS/MS product ion spectrum of m/z 472, the [M

ion of 5-formyl tetrahydrofolate, at a collision energy of 35 V.

(

& Institute of Food Research, reproduced with permission.)

MASS SPECTROMETRY

/ Food Applications

481

other than MS; however, occasional quantitative
applications do occur, exempliﬁed by GC–MS deter-
mination of antioxidants in stored products.

Biologically Active Nonnutrients (Dietary
Phytochemicals)

Biologically active (bioactive) nonnutrients in foods
comprise a wide range of phytochemical substances.
Lack of space permits discussion of mass spectro-
metric analysis of only a small range of these com-
pounds here.

Glucosinolates (Figure 4) and their biologically

active breakdown products are found in many plant
foods. More than 100 different types of glucosinolate
have been isolated from plants. Although some
glucosinolates may have toxic (e.g., goitrogenic)
properties, they are known to be potent inducers of
Phase II enzymes that protect against carcinogens
and other toxic electrophiles.

The structural variability is in the aglycone, R that

may comprise linear or branched alkyl and alkenyl
side chains, alcohols, methylthioalkyl, methylsulﬁnyl,
aralkyl, or heterocyclic groups. GC–MS is a useful
mass spectrometric technique for analyzing volatile
glucosinolate breakdown products. Negative-ion ESI
LC–MS is now the method of choice for determining
intact glucosinolates and positive ion APCI LC–MS
for analyzing the more thermally labile breakdown
products (e.g., sulforaphane) and their human
metabolites. ESI LC–MS and MS/MS are also
useful

for

conducting

metabolic

studies,

for

example, by detecting and determining glutathione

conjugates of isothiocyanate breakdown products of
glucosinolates.

Phenolics are a distinctive feature of all plant tis-

sues and are of interest because they can affect pal-
atability, taste, nutritional value, and particularly the
health properties of foods. For example, the ﬂavono-
ids occur very widely in plants and there is great
interest in their role as protective factors in the diet.
More recently, the modern methods of LC–MS ana-
lysis, APCI and electrospray, have been applied to
the detection and quantiﬁcation of ﬂavonoids and
isoﬂavonoids. Although early mass spectrometric
techniques for analyzing these molecules focused on
GC–MS methods (after appropriate derivatization),
the newer LC–MS methods are now widely used. For
example, isoﬂavones and their conjugates have been
determined in soy foods by positive and negative ion
APCI LC–MS. ESI and APCI LC–MS methods are
also now used for conducting studies of ﬂavonoid
metabolism, by following the disappearance and
deconjugation of ﬂavonoid glycosides and the ap-
pearance of glucuronides, sulfates, and methylated
metabolites.

Natural Toxicants

Mycotoxic secondary metabolites produced by As-
pergillus (aﬂatoxins) and Fusarium (tricothecenes,
etc.) food-spoilage molds comprise two of the most
prominent groups of natural toxicants. Many differ-
ent mass spectrometric techniques, including EI,
positive and negative ion CI, GC–MS, LC–MS, super-
critical ﬂuid chromatography/mass spectrometry, and

S-

-Glucose

Myrosinase

Sulfate +
Glucose

NOSO

−

R-NCS
Isothiocyanate

R-CN
Nitrile

R-SCN
Thiocyanate

R group

Semisystematic name

Methylglucosinolate

3−

−

(CH

)

4−

2−

4-Methylsulfinylbutylglucosinolate

2-Phenylethylglucosinolate

3-Indolylmethylglucosinolate

Common dietary source

Capers (Capparis spinosa)

Broccoli (Brassica oleracea)

Watercress (Nasturtium officinalis)

Most Brassica Crops

2−

Figure 4

General structure of the glucosinolates and their common myrosinase hydrolysis products. (Reproduced with permission

from Institute of Food Research;

& Institute of Food Research.)

482

MASS SPECTROMETRY

/ Food Applications

MS/MS have been devised to monitor the levels of
mycotoxins in foods, body ﬂuids, and tissues. The
most recent developments include ESI LC–MS deter-
mination of aﬂatoxins down to low picogram levels.
Additional selectivity was provided by MS/MS se-
lected reaction monitoring. Examples of other natu-
rally occurring food toxicants determined with the
aid of MS include mutagenic compounds related to
quinoxaline that may be formed in cooked meats.
The most notorious recent example of a food natural
toxicant is the discovery, made with the aid of GC–
MS, ESI LC–MS, and LC–MS/MS, that the genotoxic
carcinogen acrylamide is formed during some coo-
king processes.

Anthropogenic Toxicants

Many examples of mass spectrometric methods for
determining toxic or potentially deleterious ant-
hropogenic compounds in foods can be found in
the scientific literature. The range of compounds
analyzed is wide and includes dioxins, polyaromatic
hydrocarbons, pesticide and veterinary drug residues,
plasticizers from packaging materials, and environ-
mental contaminants. LC–MS, MS/MS, GC–MS,
and GC–MS/MS techniques are used widely for de-
termining these compounds down to parts per billion
or even parts per trillion levels. For example, polar
organophophorus pesticides can be quantiﬁed by
LC–MS/MS down to levels of 0.01 mg kg

in fruits

and vegetables and dioxins are regularly determined
in food matrices at femtogram levels.

Isotope dilution mass spectrometry is an accurate

and sensitive technique for determining toxic trace
elements in food matrices. Lead, cadmium, and
thallium have been analyzed rapidly down to very
low levels by ICP-MS. The latter technique is par-
ticularly useful for simultaneous measurement of a
wide range of elements. Because the toxicity of an
element can be highly dependent on its chemical
form, ICP-MS is also useful in the speciation of toxic
minerals in foodstuffs by combination with HPLC or
SEC.

Future Trends

Enormous advances in the scope and sensitivity of
mass spectrometric techniques have occurred in the
last decade. The use of modern techniques of organic
MS, particularly APCI and ESI LC–MS, CE/MS, and
MS/MS, has grown apace in the food and nutritional
sciences. Applications of MALDI-TOF in the food
sciences are also increasing, although at a lower rate
(the frequent need for chromatographic separation in
food-related applications has led to an understand-
able bias toward LC–MS methods).

The importance of ICP-MS in multielement anal-

ysis of foodstuffs is already well established, as is the
use of this technique in human metabolic studies.
Both types of application are being augmented by
increased applications of combined chromatography/
ICP-MS for conducting speciation studies. The
advent of a new generation of high-precision isotope
ratio ICP-MS instruments also opens up the exciting
possibility of conducting metabolic studies without
using enriched labels, by observing isotopic fraction-
ation between different body compartments. High-
precision ICP-MS also has considerable potential in
food authentication (by linking isotopic composition
to geographical origin, for example).

Fundamental studies of the metabolism of pro-

teins, fats, and starches by GIR-MS are increasing
and it is anticipated that practical applications to the
metabolism of extrinsically labeled foods will make
an impact in the future.

Mass spectrometry has a central role in the

postgenomic science of proteomics, the qualitative
and quantitative comparison of the entire protein
complement of a genome under different conditions.
The new science of metabolomics, the global, quan-
titative analysis of all the low and intermediate mo-
lecular weight metabolites expressed by a genome
under specific conditions, has obvious applications in
food science, for example, in assessing the safety of
transgenic plants or in studying environmental ef-
fects on food poisoning microorganisms and mass
spectrometry is one of the pivotal analytical tech-
niques in metabolomics. This type of application is
expected to increase in the future.

See also: Carbohydrates: Overview. Elemental Specia-
tion: Overview. Food and Nutritional Analysis: Over-
view.

Gas

Chromatography:

Mass

Spectrometry.

Lipids: Overview. Liquid Chromatography: Liquid
Chromatography–Mass Spectrometry; Food Applications.
Mass Spectrometry: Overview; Principles; Ionization
Methods

Overview;

Atmospheric

Pressure

Ioniza-

tion Techniques; Electrospray; Matrix-Assisted Laser
Desorption/Ionization; Pyrolysis. Proteins: Traditional
Methods of Sequence Determination. Vitamins: Over-
view.