Page 177 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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the capillary coming from the GC column to either reactor, depending on
the analytical task.
Another hyphenated technique for position specific isotope analysis
(PSIA) of fatty acids using an on-line pyrolysis system was described in detail
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for the first time by Corso and Brenna. They coupled one GC (GC-1) for
sample separation prior to pyrolysis to a second GC (GC-2), separating
pyrolytic products of the selected sample compound. Furthermore, they
installed a valve into GC-2 to permit separated pyrolysis fragments to be
admitted to an organic MS for structure analysis of these fragments.
Even more important, from a forensic point of view, is another type
of hyphenated CSIA system. Since organic compounds have to be converted
into simple analyte gases isotopically representative of the parent material,
naturally all structural information is lost that would otherwise be used to
confirm the identity of the organic compound whose isotopic signature
has been measured. One “solution” to overcoming this dilemma is to ana-
lyze another aliquot of the same sample on a scanning GC/MS system
employing the same chromatographic conditions that have been used for
GC/C-IRMS analysis. This approach relies ultimately on a mere comparison
of retention index or retention time of a given peak, a modus operandi that
could cause severe problems if such results would have to be put forward
as evidence in a court of law. This problem was recognized by one of the
authors who, in collaboration with an instrument manufacturer, developed
a hyphenated mass spectrometric hybrid system that enables CSIA, while
at the same time recording a conventional mass spectrum of the target
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compound to aid its unambiguous identification. To this end, a GC/C-
IRMS system was interfaced with an Ion Trap mass spectrometer to facil-
itate splitting of the GC effluent to the conversion interface with simulta-
neous admission to the ion source of the organic MS yet without incurring
isotopic fractionation. 43,44

4.3 Isotope Dilution

The use of mass spectrometry (MS)–based methodologies for quantitative
analysis is now a routine practice in forensic science laboratories. In most
applications, isotope-labeled analogs (ILA) of the analytes are used as the
internal standards (IS), and the MS is operated in selected ion monitoring
(SIM) mode. Although H-analogs are most commonly used, recent com-
2
parative studies suggested C-analogs (see Figure 4.3) could be more effec-
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tive. With practically identical chemical properties and MS fragmentation
characteristics, an ILA is a preferred IS because it offers the following
advantages.

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