Page 172 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C04.fm Page 155 Thursday, November 13, 2003 9:54 AM

Since the small variations of the heavier isotope habitually measured by
IRMS are of the order of –0.07 to +0.02 APE, the d-notation in units of per
mil (‰) has been adopted to report changes in isotopic abundance as a per
mil deviation compared to a designated isotopic standard:

d = ([R – R ]/R ) ¥ 1000 [‰] (3)
s s std std
where R is the measured isotope ratio for the sample and R is the measured
s std
isotope ratio for the standard. To give a convenient rule-of-thumb approxi-
13
mation, in the d-notation, a C abundance in the range of –0.033 to +0.0549
13
APE corresponds to a d C value range of –30‰ to +50‰. A change of +1‰
is approximately equivalent to a change of 0.001 APE and 0.0003 APE for
15
13 C and N, respectively.
Natural abundance applications for the most part require the high-pre-
cision techniques introduced in the late 1940s; these techniques dominated
IRMS for four decades. They focused initially on the analysis of bulk materials
of high chemical complexity such as plant extracts or petroleum reduced to
simple gases by combustion to CO or conversion to N or H . Analysis of
2 2 2
chemically pure materials was much more common for compounds that
occur in relatively pure form in nature, such as water, because off-line sample
preparation is a time-consuming affair fraught with the risk of contamination
and subtle isotopic fractionation.

4.2.2 Dual-Inlet Isotope Ratio/Mass Spectrometry (IRMS)
However, until the commercial availability of continuous-flow isotope ratio
mass spectrometer (CF-IRMS) systems enabling on-line isotope analysis of
organic compounds in the late 1980s, initial attempts to exploit the informa-
tion locked into stable isotope ratios at natural abundance level for forensic
purposes were confined to employing dual-inlet IRMS systems. Typically, off-
line preparation involves multiple steps on custom-designed vacuum lines
equipped with high-vacuum and sample-compression pumps, concentrators
using cryogenic or chemical traps, reactions in furnaces using catalysts or
true reagents, and microdistillation steps. Contamination and isotopic frac-
tionation are a constant threat at any step, and, in general, manual off-line
7
methods are slow and tedious, usually requiring large sample amounts. The
quality of the results depends considerably on operator skill and dedication.
The dual-adjustable volume inlet system facilitates sample/standard
comparison under the most nearly identical of circumstances. It is necessary
for highest precision to compensate for (a) normal fluctuations in instru-
mental response and (b) ion source nonlinearity which produces isotope
ratios that depend on the source gas pressure. The latter phenomenon can

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