Page 179 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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significant amount of the analyte in a true negative sample and may
also introduce errors in quantitation.
3. The labeling isotopes must be positioned at appropriate locations in
the molecular framework of the compounds so that, after the frag-
mentation process, sufficient number of high-mass ions (that retain
the labeling isotopes) are present with significant intensities and will
not contribute to the intensities of the corresponding ions derived
from the analyte. These ions and their counterparts in the analyte
may then be monitored for ion ratio evaluation to facilitate qualitative
compound identifications and quantitative determinations.

The critical matter is to establish a linear calibration line; the intensities of
the ions designated for the analyte and the IS must not be cross-contributed.
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For example, in a secobarbital/ C -secobarbital study, calibration lines were
4
evaluated using two pairs of ions, m/z 196/200 and 181/185 (Figure 4.4). For
m/z 196 (designed for the analyte), 0.23% of the measured intensity is con-
tributed by the IS, while 0.017% of the measured intensity of m/z 200
(designed for the IS) is contributed by the analyte. On the other hand, 1.6%
of the measured intensity of m/z 181 (designed for analyte) is contributed
by the IS, while 0.29% of the measured intensity of m/z 185 (designed for
the IS) is contributed by the analyte. Figure 4.4 clearly reflects the difference
in linearity of the calibration lines established by these two ion pairs. When
cross-contribution occurs, the nonlinear (such as hyperbolic) model can best
describe the observed data and should be seriously considered.

4.4 Sample Differentiation

Analytical work performed in forensic science laboratories primarily aim for
the identification and comparison of various samples with the intention of
linking the samples of concern to a specific person or event. Identifications
are often achieved through the characterization of specific compounds, while
comparisons often involve the identification and quantitation of multiple
components in the samples of interest. With automated instrumentation
widely available, highly specific MS–based technologies are now, in most
modern forensic science laboratories, the most valuable tool used to achieve
the analytical goals: compound identification, or sample comparison.
Since sample decomposition, successive dilution, and contamination
during the storage and distribution process may hinder sample differentia-
tion based solely on component identification and quantitation, isotope ratio
measurement provides a complementary approach. The inherent strength of
this approach is that as long as the source of the chemicals used to prepare

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