Page 167 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C04.fm Page 150 Thursday, November 13, 2003 9:54 AM

4.1 Introduction

Stable isotope ratio mass spectrometry (IRMS) has undergone about 50 years
of development and applications. It is now often adopted as the basis for (1)
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monitoring the fate of selected compounds in the biosphere ; (2) labeling
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2–4
experiments used in biological and organic reaction mechanistic studies ;
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(3) quantitative analysis by the “isotope dilution technique” (IDT) ; (4) sam-
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ple differentiation based on isotope composition distributions ; and (5)
archaeological studies. 8
Isotope dilution techniques are now routinely used in forensic toxicol-
ogy laboratories for the quantitation of drugs and their metabolites in
biological specimens. These applications are based on variations in inter-
molecular isotope abundances at an enriched level. A general purpose mass
spectrometer is adequately used in these applications. Sample differentia-
tions in forensic applications are typically based on intra-molecular isotope
composition variations at natural abundance level in the samples of interest.
An isotope ratio mass spectrometer (IRMS) is required in these and archae-
ological studies.
In this chapter, we shall first address some essential conventions adopted
in IRMS and the theoretical basis and instrumentation of IRMS, followed by
a survey on the fundamental aspects of IDT and a review of the applications
of IRMS in forensic science.

4.1.1 Convention
Differences in stable isotope contents are typically expressed in delta nota-
tions in reference to a standard. For example, C-enrichment levels are
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universally expressed as :

È R ˘
13 sample
(
d C‰) = Í - 1 ˙ ¥ 1000
Î R stan dard ˚
where R is the ratio of the number of atoms of the minor isotope to that of
the major isotope.
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The C-enrichment level found in PDB (calcium carbonate — a fossil
of Belemnitella americana from the Peedee formation in South Carolina) is
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universally used as the reference. Thus, the d C (‰) found in the samples
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of interest are compared to this standard. PDB originates from a marine
carbonate shell and as such contains one of the heavier varieties of carbon
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in the terrestrial environment. Compared with carbon from PDB ( C/ C =
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0.0112372), most natural carbon gives a negative delta value. The range of

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