Page 234 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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coupling of gas chromatography with mass ion isolation with the first mass
spectrometer, followed by a second MS stage of dissociation through ion
collisions and subsequent detection of the daughter ions formed. The second
dissociation and new daughter ion spectrum detected by the second MS
operation creates the added specificity that labels GC/MS/MS as perhaps the
ultimate method today for mixture analysis.
In MS/MS, a significant ion from a targeted compound or a characteristic
ion from a chemical class is chosen from a full scan GC/MS mass spectrum.
This is the parent (or precursor) ion. In the MS/MS acquisition, the chosen
parent ion is isolated in the first MS operation. In the second MS stage it
undergoes collision induced dissociation (CID) with helium reagent gas, which
results in a new daughter ion (or product ion) spectrum. The isolation and
CID process makes MS/MS inherently more selective than MS alone, and this
presents numerous advantages for complex samples where analyte and matrix
coelution are common. An ion trap mass spectrometer that includes tandem
mass spectrometry in time also offers advantages of reduced electronics, man-
ifolds, and pumps (and price), compared to a triple linear quadrupole mass
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spectrometer which is based on tandem mass spectrometry in space. The latest
ion trap mass spectrometers include higher scan rates, external ionization, and
the capability of obtaining both full scan and MS/MS information in the same
analytical run. This use of GC/MS/MS is not new. This tandem mass spec-
trometry approach has clearly been shown to be the method of choice for
biological specimens, detection of explosives, and drugs of abuse, as well as
for structure elucidation in pharmaceutical analysis. Its application in forensic
laboratories has been limited in the past by the cost and complexity of the
instrumentation. This has changed with the relatively low cost and high effi-
ciency of conducting MS/MS in an ion trap mass spectrometer.
GC/MS/MS has been used to address problems of analytes in complex
matrices such as THC in blood, trace quantities in difficult matrices such
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as residues of drugs of abuse in hair, or to minimize the matrix interferences
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in the analysis of pesticides in fruits and vegetables. 8
The high sensitivity and selectivity of GC/MS/MS have been used in the
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detection of trace components in explosive mixtures. These are important
in environmental remediation projects or for high-speed security sweeps such
as those done at airports.
GC/MS/MS is one of the laboratory’s “big guns” used when the result
must have the highest defensibility. An example is the use of MS/MS
(although with liquid chromatographic separation) in the much publicized
O.J. Simpson trial. A method using LC/MS/MS was developed to determine
whether the blood stained cloth swatch and socks found at the O.J. Simpson
residence contained the blood preservative EDTA. The analysis successfully
determined that EDTA was not present in the evidence at levels indicating

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