Page 169 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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The second kind of isotope effect is associated with differences in phys-
ico–chemical properties such as infrared absorption, molar volume, vapor
pressure, boiling point, and melting point. Of course, these properties are all
linked to the same parameters as those mentioned for the kinetic isotope
effect, i.e., bond strength, reduced mass, and, hence, vibration energy levels.
However, to set it apart from the kinetic isotope effect, this effect is referred
to as the thermodynamic isotope effect because it manifests itself in processes
where chemical bonds are neither broken nor formed. Typical examples for
such processes in which the results of thermodynamic isotope effects can be
observed are infrared spectroscopy, distillation, and any kind of two-phase
partitioning. This thermodynamic isotope effect or physico–chemical isotope
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effect is the reason for the higher infrared absorption of CO as compared
2
18
to CO , the enrichment of ocean surface water with H O, and the isotopic
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2 2
fractionation observed during chromatographic separations. 16
Due to the high precision of isotope abundance measurements by mod-
ern IRMS instruments, even minute changes in the isotopic composition of
a given compound can be reliably detected, irrespective of whether these
minute changes have been caused by kinetic isotope effects associated with
enzyme-mediated biochemical reactions or batch-to-batch variations of reac-
tion conditions during its chemical synthesis. The resulting variation in the
natural abundance of, e.g., C can be as high as 0.1 atom%. This wide range
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reflects the varying degree of mass discrimination associated with the differ-
ent pathways of carbon assimilation and fixation, but can also reflect different
growing conditions due to climatic and geographic differences.

4.1.3 Discrimination Power

Analytical methods currently applied in support of law enforcement agencies
establish a degree of identity between one substance and another by means
of identifying their constituent elements, functional groups, and by elucidat-
ing their chemical structures. Should the spectroscopic data of two com-
pounds correspond, it may be concluded that they are chemically identical.
However, an argument can be brought forward as a defense that although
two substances in question are chemically identical, they are not the same,
i.e., not sharing the same origin, hence coming from a different source. The
analytical technique at the heart of this chapter, IRMS, permits this conten-
tion to be tested and provides the resolution to the question of whether two
compounds or substances are truly identical. With the help of stable isotope
finger-printing (or stable isotope “DNA”), forensic scientists will be able to
link a person to an event, a crime scene, or a criminal organization (such as
a terrorist group) based on a unique characteristic of physical evidence.
For example, research carried out on cocaine base has confirmed the
potential probative power of the data provided by this analytical technique

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