Page 69 - Inorganic Mass Spectrometry : Fundamentals and Applications

from 8.7% to 29.5%, which are quite good percentages given the low concentra-
tions in the samples. Detection limits in the picogram per gram (pg/g) (part per
trillion) range were reported. Integration times necessary to obtain these values,
however, ranged up to l hour. The authors noted the need for mass resolving
power in excess of 2 X 106 to analyze 137Cs, g0Sr, HIPu, and 238Pu when inter-
ferences from barium, zirconium, americium, and uranium complicate the spec-
trum [ 1041. The use of Fourier transform ion cyclotron resonance mass spectrome-
try in combination with a glow discharge was suggested.

Isotope dilution. mass spectrometry (IDMS) is a powerful technique for establish-
a
ing the concentration of a target species in sample of unknown elemental compo-
sition. It has received widespread application with gaseous and liquid samples, yet
it has seen little use with solids because of the need to establish isotopic equilib-
rium between the sample and the isotopic spike. The technique has largely been
confined to gas [l051 or thermal ionization mass spectrometry [ 1061, although
some work has also been carried out using spark source [l071 and inductively
coupled plasma mass spectrometries [108]. Recently, isotope dilution has been
used in combination with glow discharge mass spectrometry for analysis of
solution residues. The problem of equilibrating the sample and the isotopic spike
was overcome by mixing the sample and spike as solutions and then analyzing
them as dried residues by GDMS [64]. Cathodes were prepared by pipetting -200
pL of a spiked aqueous oil leachate that had been digested according to EPA
SW-846 Method 3050 into 1.0 g of 99.99+% silver powder. The resulting slurry
was then dried at 100°C for 6 hours, mixed to obtain homogeneity, and pressed in
to a pin 1.5 mm in diameter by 20 m in length. Isotope ratio measurements were
made for the isotopically enriched spike, the unspiked samples, and the mixtures.
Figure 2.14 shows representative spectra obtained from the three sample types [(a)
is the isotopically enriched spike, (b) is the unspiked sample, and (c) is the
mixture]. Table 2.4 lists the ratio for the sample (R,), the spike (also called the
tracer, RT), and the mixture (RM), along with the concentrations for each sample
calculated from the isotope dilution equation. These results were in good agree-
ment with those obtained by inductively coupled plasma atomic emission spec-
troscopy. Internal precisions of better than 5% were obtained, even when the
concentration was just above the detection limit; external precision was about
2%.
These values were 3-10 times better than those normally obtained by using
GDMS sensitivity factors [64].

is
Perhaps one of the most severe limitations of glow discharge mass spectrometry
a
that any conta~nants entering the discharge cell along with the support gas have