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1522_C04.fm Page 163 Thursday, November 13, 2003 9:54 AM

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a: m/z 196/200
b: m/z 181/185
30 a
Analyte/IS ion intensity ratio 20 b
25

15

10

5

0
0 1000 2000 3000 4000 5000
Analyte concentration (ng/mL)

Figure 4.4 Linear and hyperbolic calibration results using ion-pairs with dif-
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ferent degrees of cross-contribution æ Secobarbital (SB)/ C 4 -secobarbital ( C 4 -
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SB): m/z 196/200, 181/185. A total of 0.23% of the measured intensity of m/z
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196 (designed for SB) is contributed by C 4 -SB; 0.017% of the measured intensity
of m/z 200 (designed for C4-SB) is contributed by SB; 1.6% of the measured
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intensity of m/z 181 (designed for SB) is contributed by C 4 -SB; and 0.29% of
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the measured intensity of m/z 185 (designed for C 4 -SB) is contributed by SB.
(Reprinted with permission from Liu, R.H. et al., Anal. Chem., 74, 618A, 2002.
© 2002 American Chemical Society.)
the sample of interest is the same, partial degradation or addition of foreign
materials will not alter the analytical results. Many applications of this con-
cept are highly relevant to forensic science and will be discussed in this
chapter. Although stable isotope compositions of oxygen, nitrogen, and
hydrogen have also been utilized, carbon isotope measurement will be
emphasized for its simplicity in sample preparation and the potential for on-
line measurement and practical application.

4.4.1 Detection of Adulteration in Food Products

Atmospheric carbon dioxide is fixated by plants for their biosynthesis of
organic matters. Although the resulting organic matter is generally depleted
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in C as compared to atmospheric carbon dioxide, the extent of depletion
depends on the primary carboxylating enzymatic reaction adopted by the
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plant. 46,47 As shown in Figure 4.1, the d C values found in C3 plants range
from about –22 to about –35‰, while that of C4 plants range from –8 to
about –20‰. Values for crassulacean acid metabolism (CAM) plants, which

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