Page 176 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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through which H O passes freely while all the other combustion products
2
are retained in the carrier gas stream. Quantitative water removal prior to
admitting the combustion gases into the ion source is essential because any
+
water residue would lead to protonation of CO to produce HCO , which
2 2
interferes with analysis of CO (isobaric interference). A detailed study of
13
2
this effect has been reported by Leckrone and Hayes. 35
In dual-inlet systems, the analyte gas comes from an adjustable reservoir
(bellow) and only travels a short distance prior to entering the ion source.
For this reason, the gas pulses result in rectangular signals. In contrast, in
CF-IRMS systems used for gas isotope analysis, on-line gas purification steps
and overall interface length lead to bell-shaped signals. This is, evidently,
even more the case in GC/C-IRMS systems, where analyte peaks eluting from
the GC column are fed into an on-line microchemical reactor to produce,
e.g., CO peaks. However, due to the chromatographic isotope effect 36,37 the
2
m/z 45 signal ( CO ) precedes the m/z 44 signal ( CO ) by 150 ms on
13
12
2 2
32
average, an effect not observed in ordinary CF-IRMS systems. This time
displacement depends on the nature of the compound and on chromato-
graphic parameters such as polarity of the stationary phase, column temper-
38
ature, and carrier gas flow. Therefore, loss of peak data due to unsuitably-
set time windows for peak detection and, hence, partial peak integration will
severely compromise the quality of the isotope ratio measurement by GC/C-
IRMS, as will traces of peak data from another sample compound due to
close proximity, resulting in peak overlap with the sample peak to be ana-
lyzed. Due to the fact that isotope ratios cannot be determined accurately
from the partial examination of a GC peak, HRcGC resulting in true baseline
separation for adjacent peaks is of paramount importance for accurate and
precise CSIA.

4.2.3.3 Hyphenated CSIA Systems
In recent years, the research efforts of different groups working in the field
of GC/C-IRMS have focused on extending the scope of on-line CSIA towards
18
2
1
16
the measurement of O/ O and H/ H isotope ratios of organic compounds.
2
In a first step towards on-line measurement of H isotope signatures of
organic compounds, Prosser and Srimgeour coupled a high mass dispersion
IRMS to a GC via a pyrolysis interface including a 5 Å molecular sieve PLOT
2
39
column to achieve CSIA for H of fatty acids. Employing this instrumental
set-up, d H-values for 16:0, 18:1, and 22:6 fatty acids (as methyl esters) from
2
tuna oil were reported as –148.5 ± 4.1‰, –155.3 ± 1.0‰, and –147.7 ±
1.2‰ (vs. VSMOV), respectively. IRMS manufacturers now offer GC cou-
40
pled IRMS systems where the GC effluent is fed into a pyrolysis (Py)/thermal
conversion (TC) reactor, or even systems with a dual reactor set-up, i.e.,
combustion and thermal conversion, giving the user the freedom to connect
© 2004 by CRC Press LLC