Page 174 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C04.fm Page 157 Thursday, November 13, 2003 9:54 AM

silver or tin, and loaded into a carousel for automated analysis. The sample
is dropped into a heated reactor that contains an oxidant, such as copper and
chromium oxide for C or S analysis, where combustion takes place in an He
atmosphere with an excess of oxygen. Combustion products are transported
by flowing He through a reduction furnace for removal of excess oxygen and
conversion of nitrous oxides into N . A drying tube is used to remove any
2
excess water in the system. The gas-phase products are separated on a PLOT
column under isothermal conditions, and detected nondestructively by ther-
mal conductivity before introduction to the IRMS.
Oxygen and hydrogen are the two most recent elements for which
elemental analyzer data for bulk compounds have been presented. Oxygen-
containing samples are converted on-line to CO by pyrolytic reaction with
23
carbon (the “Unterzaucher reaction”) as first shown by Brand et al., using
a GC-based system; several other reports using this principle subsequently
24
appeared, showing an elemental analyzer or direct injection analysis. 25
The report of Farquhar et al. demonstrates the automated on-line con-
26
version of the oxygen in water or nitrogen-containing plant dry matter to
CO, using a pyrolysis-based reaction on carbonized nickel at about 1100˚C.
CO is separated from N , using a GC with a molecular sieve column;
2
precisions of SD (d O) = 0.2‰ are obtained. Begley and Scrimgeour have
18
2
shown the analysis of oxygen (d O) and hydrogen (d H) on a single sample
18
by measuring the isotope ratio of H gas produced in the pyrolytic reactor
2
with a high mass dispersion IRMS, capable of fully resolving analyte HD
(m/z 3) from excess He carrier (m/z 4). Precisions for water, urine, and
volatile organic compounds are about SD (dD) = 2‰ and SD (d O) =
18
0.3‰. 25
Nowadays, thanks to advances in electronics and instrument design, dual
13 C/ N analysis of the same sample in one analytical run is possible on a
15
combustion/reduction EA-IRMS. Allowing for a good separation of the N
2
peak from the CO peak to permit a high precision magnetic field jump, total
2
18
analysis time can be as fast as 7 min per sample. Similarly, dual H/ O analysis
2
from the same sample can be carried out using a high temperature thermal
conversion EA (TC/EA). In addition to the aforementioned reaction on
carbonized nickel at 1100˚C, high temperature conversion on glassy carbon
at 1400˚C is used as an alternative. Both solid and liquid samples, the latter
2
by means of a special liquid injector, can be analyzed for H and O simul-
18
taneously, with total analysis time being as fast as 6 min per sample. In
conclusion, EA-IRMS or TC/EA-IRMS would appear to be the method of
choice for many forensic applications (drugs, explosives, hair, fingernails,
etc.), at least as a quickly performed initial measurement to concentrate the
efforts of more elaborate techniques and analyses on samples seemingly
identical, based on their bulk isotopic composition.

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