Page 71 - Inorganic Mass Spectrometry : Fundamentals and Applications

Glow Discharge Mass Spectro~et~ 61

Bias Corrected Isotope Ratio ~easurements in the Unspiked
Sample, Isotopically Enriched Spike, and Spiked Sample for Three Oil
Leachate Samples as Determined by Glow Discharge Mass Spectroscopy
Sol~tion Residue Method
R, R, RM
zo7Pb+PPbf 207Pb+P~Pb+ 207Pb+/z04Pb+ Concentration
Sample (sample) (spike) (mixture) (PPm)
A 14.9 4 0.5 0.029 4 0.001 0.192 4 0.004 2.69 4 0.05
B 7.5 4 0.2 0.029 4 0.001 0.130 4 0.001 3.5 4 0.1
c1 15.2 It: 0.2 0.029 5 0.001 1.44 4 0.01 82.5 4 0.7
c2 15.9 4 0.3 0.029 4 0.001 1.46 It: 0.02 83.2 4 0.8
c3 15.1 4 0.2 0.029 4 0.001 1.41 4 0.02 80.2 4 0.8

Source: Ref. 64.

deleterious effect on the resulting mass spectrum. This effect is multiplied by
several orders of magnitude because the support gas is far and away the most
abundant constituent of the discharge. Complicating this effect further is the
production of polyato~c species such as CO+, Arc+, ArN+, and ArO+. Several
methods have been used to reduce interferences in the glow discharge to accept-
able levels. One is to dissociate the molecular ions into their component elements
through the use of gas-phase collisions [ 109- 1 1 l], a second is to resolve the
[ 1 121, and a third is to used
interferences with a high-resolution mass spectrometer
gated detection with a pulsed discharge
[ 1 13,1141. None of these methods, how-
0). Among the methods
ever, reduces the cont~butions of residual gases (C, N, or
specifically designed to remove cont~nants, heating the gas to drive them out,
cryocooling to condense them from the gas phase [ 1 151, using getters as a
conductive binder [ 1 161, and using a high-purity support gas [ 1 171 hold the most
promise. Mykytiuk et al. 1691 and Valiga et al. [ 1 181 have used a combination of
approaches to analyze C, N, and 0 at the tens of part per million level and lower in
samples, In particular, Valiga's work was impressive because he used a low-
resolution quadrupole mass spectrometer, an inherently less expensive instrument
than a sector. Table 2.5 shows these results for the analysis of a National Institute

Thee glow discharge mass spectra taken over the mass range mlz 200-210
pL of
for three different cathodes doped with (a) 500 a 40.9-ppm 2oQPb spike, (b) 500 pL
of an unknown waste oil sample, and (c) 500 pL of a mixture consisting of 1 .O nk of the
u~own and 100 pL of the 40.9-ppm z04Pb spike. The discharge conditions of all three
cathodes were -1000 V dc, 2.0 mA, and 133.3 Pa argon.