Page 171 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C04.fm Page 154 Thursday, November 13, 2003 9:54 AM

electron multipliers (EM) and therefore cannot simultaneously detect par-
ticular isotope pairs for continuous isotope ratio measurement. For isotope
ratio measurement, a MS has to be operated in a selected ion monitoring
mode (SIM) to optimize sensitivity to selected masses. Even in SIM mode,
limited accuracy and precision of such isotope ratio measurements impose
13
15
a minimum working enrichment for C and N of at least 0.5 atom% excess
(APE). 19,20 In other words, single collector scanning MS cannot provide reli-
able quantitative information on isotopic composition at natural abundance
level.
Isotope ratio mass spectrometry systems have been designed to measure
isotopic composition at low enrichment and natural abundance levels. In an
IRMS, molecular ions emerge from the ion source and are separated by a
magnetic sector set to a single field strength throughout the experiment. No
energy filtering is used so as to optimize transmission near unity, giving a
–3
typical absolute sensitivity of 10 (about 1000 molecules enter the ion source
per ion detected) and better. Mass-filtered ions are focused onto dedicated
Faraday cup (FC) detectors positioned specifically for the masses of interest.
For example, IRMS instruments equipped to determine CO have three FCs
2
for measurement of m/z 44, 45, and 46, positioned so that the ion beam of
each mass falls simultaneously on the appropriate cup. Each FC has a dedi-
cated amplifier mounted on the vacuum housing to minimize noise pickup,
and dedicated counters/recorders for continuous and simultaneous recording
of all relevant ion beams. FCs are the detectors of choice for IRMS due to
two major considerations. First, the absolute precision required for IRMS
–4
determinations is at least 10 , which is attainable based on counting statistics
with at least 10 particles detected. Ion currents that achieve these levels are
8
well within the range detectable by FCs. For instance, the major ion beam
current for atmospheric CO analysis, m/z 44, will typically be around 10
2
nA, with the minor m/z 46 beam around 40 pA. Second, FCs are highly stable
and rugged, and rarely need replacement, compared to EMs, whose sensitivity
degrades with use and age. This means that minute variations in very small
amounts of the heavier isotope are detected in the presence of large amounts
of the lighter isotope. The abundance A of the heavier isotope n in a sample,
s 2
given in atom%, is defined as:

A = R /(1 + R ) ¥ 100 [atom%] (1)
s s s
where R is the ratio n /n of the two isotopes for the sample. The enrichment
s 2 1
of an isotope in a sample as compared to a standard value (A ) is given in
std
atom% excess (APE):

APE = A – A (2)
s std
© 2004 by CRC Press LLC