Page 249 - Inorganic Mass Spectrometry - Fundamentals and Applications

Isotope Dil~tion Mass S~e~tro~et~ 235

ample injection valve
Guard column
Standard injection
for calibration of
Separation column

Schematic diagram of high-pressure liquid c~omatography inductively cou-
pled plasma mass spectrometry (HPLC-ICP-MS) system with dual-inlet system for isotope
dilution analyses. (From Ref. 32.)

tion, the authors were able to obtain good agreement with standards selenium
for
at the 10-nglmL level. The spike was s2Se, and two isotope dilution ratios (77Se/82Se
the
and 78Se/82Se) were used to improve accuracy of the speciation analyses.
Barshick et al. adopted a different approach for the speciation of mercury
[36] These investigators used a commercial gas chromatograph-mass spectrome-
ter (GC-MS) in their work; the mass spectrometer was a quadrupole ion trap. A
solid phase ~croestraction fiber was employed to collect volatile organic species,
which were then desorbed and subjected to GC-MS analysis. Subsequent analy-
to
sis of organic mercury compounds, inorganic species were converted to volatile
a
organic form and analyzed, thus providing total mercury in the sample. Quantifi-
cation of mercury at the 400-ppb level using calibration curve yielded precisions
a
of about 1596, whereas use of a 204Hg spike and isotope dilution yielded 1 % [3’7].
This is a good example of the power of isotope dilution to deliver improved results
even when a mass spectrometer not designed for isotope ratio measurements is in-
volved.
Joining another type of ancillary device to ICP-MS, Beary et al. developed
a continuous- ow microwave digestion apparatus for isotope dilution analysis of
lead in environmental samples [38]. Although introduction to the ICP torch was