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160 Mass Spectrometry in Forensic Science

Quadrupole A type of mass filter. It has an open space Mass spectrometry is, in short, a method by which one
between four rods where an oscillating electric field is generates charged molecules and molecular fragments and
generated to direct the flow of ions made up by charged then measures the mass of each or, more rightly, their
fragments, formed from an analyte, and accelerated mass/charge ratio. The uncharged gas molecules, which
into the field. Only those ions with mass/charge ratios are introduced into the mass spectrometer, move randomly
that suit the electric potentials of the oscillating field around in space, so to control their motions they have to
pass the rods to become recorded. be ionized (i.e., a charge is added to the molecules). Also,
energyinexcessoftheionizationmustbesuppliedtobreak
some of the covalent bonds holding a molecule together.
MASS SPECTROMETRY is a main tool in forensic This is to split it into specific fragments that may be used
chemistry for the analysis of chemicals in samples taken to obtain structural information about the molecule or to
to reveal a possible crime. Its main value rests on the fact unfold its characteristic mass spectrometric fingerprint.
that mass spectrometry allows for an unbiased search with An outline of the process is shown in Fig. 1. Our dis-
high sensitivity and specificity for a variety of substances cussion will include a general description of the hardware
with wide ranges of different physicochemical properties. to generate the MS raw data (the introduction of the sam-
The principles for making a substance suitable for mass ple at atmospheric pressure into the mass spectrometer,
spectrometric analysis are presented. These include the which is under vacuum), the formation of the mass frag-
sample inlet from atmospheric pressure to the low pressure ments and their separation from one another, focusing, and
in the apparatus, the conversion of the sample molecules detection. The raw data thus obtained require some form
to ionized particles, their separation from one another in a of evaluation to become intelligible, a process that calls
magneticoroscillatingelectricfield,andtheirdetection.In for automatic and/or manual computer work. A few exam-
forensic chemistry the mass spectrometer is most often fo- ples of real-life forensic studies will help explain common
cused on a wide range of different mass fragments, and the approaches used in such a pursuit.
data gotten from a test, therefore, need refinement to be-
come intelligible. Examples from real-life forensic work
I. DATA GENERATION
in toxicology, arson analysis, and environmental forensics
have been chosen to illustrate such processes.
A. Sample Inlet
Forensic science plays a key role for law-enforcing bod-
ies. Its main task is to supply physical evidence pertaining The inlet system is used to introduce the sample into the
to a suspected criminal act. The forensic scientist searches mass spectrometer, to convert it into the gas phase, and
materialscollectedbythepoliceforevidenceapersonmay to reduce its pressure before ionization. Forensic sam-
have left at the site of a crime or brought with him. The ples are often impure, so the analytes, have to be sepa-
test matter is often made up of agents suitable for chemical rated from the matrix before being inserted into the mass
analysis, such as drugs seized by the police or present in spectrometer. The inlet system is most often an interface
a body fluid, residues of arson accelerants or explosives, between a chromatographic device and the mass spec-
poisons in a dead body, or residues of illegally deposited trometer. By this approach, the analytes are separated
spill oil, to name a few examples. from one another and from the contaminants by either gas
Two distinct approaches can be used for the chemical chromatography (GC) or high-performance liquid chro-
exam. In some surveys the analyst, without having to iden- matography (HPLC), and the isolated compounds in the
tifyeachsinglecompound,mayjustcomparethepatternof effluents from the column flow directly into the mass
a group of substances in the sample with that of a reference spectrometer.
product that has some bearing on a crime. In some types The combination of GC and MS (GC/MS) is often used
of suspected crimes (e.g., intoxication or drug trafficking), in forensic science. Since the GC separation today is done
the individual substances in a sample must be pinpointed. in a capillary, and not in a packed column as it used to
Regardless of the analytical aim, a method must be used be, introducing the analytes into the mass spectrometer
that will hold up to scrutiny in a court of law. Another has become rather simplified. The flow rates are much
problem the chemist is faced with is that he does not know lower in a capillary than in a packed column, and the
which compounds to look for amid the wide range of pos- entire volume of the effluents can be let into the mass
sible ones with different physicochemical properties. By spectrometer without any losses simply by pushing the
virtue of its high sensitivity and specificity, as well as its loose end of the GC capillary in close to the ion source of
ability to reveal the general unknown, mass spectrometry the mass spectrometer.
(MS) is the chemist’s primary analytical tool for solving Even though HPLC in combination with mass spec-
many forensic problems. trometry (HPLC/MS) is used less frequently than GC/MS,