Page 306 - Inorganic Mass Spectrometry - Fundamentals and Applications

Multiple-Colle~tor lCP-MS 293

Naturally, one wants to achieve high transmission and extract as many ions from
40h+ ion beam can be as large as
this source as is possible, and in fact the focused
a microampere. However, high-precision isotopic measurements require ultrahigh
vacuum (<lo"* mbar) in order to minimize collisions between analyte ions and
atoms in the residual vacuum envelope and preserve good peak shapes. Therefore,
all MC-ICP-MS instruments need to be accompanied by a considerable inventory
the
at
of pumps to achieve efficient near-supersonic extraction cone interface, pump
away residual h, and maintain ultrahigh vacuum in the analyzer.
Second, the source of ions is geometrically broad compared with that in
TMS. Therefore, the extracted ions need to be spatially restricted and direction-
ally focused at the source (defi~ng) slit with minimal loss of transmission, in order
for the mass resolved ion beams to fit easily within the openings of the multiple
Faraday detectors (buckets) and produce excellent peak shape.
Third, the ions in such an inductively coupled h plasma source possess a
broad energy spread of up to -15 V, sometimes as high as 20-30 V-roughly two
orders of magnitude greater than in TIMS. Therefore, the ions of interest need to
be focused and restricted in their energy spread before entering the magnetic sec-
tor; otherwise they will not be focused the collectors and the peak shape will be
at
greatly degraded. In practice, the energy spread of all components, including the
plasma support gas, is too great to provide the same focusing for all ions. This does
not matter, however, so long as the ions of interest are focused.
Fourth, all plasma sources are inherently unstable relative to the best TNS
sources. Improving plasma stability assists the task of energy and directional fo-
cusing. However, the single-detector system is too slow to respond to the changes
in signal size resulting from plasma instability if sequential data collection is de-
it
ployed. To achieve high precision is essential to use simultaneous multiple Fara-
day collection to cancel out the effects of plasma instability. Therefore, all instru-
ments need broad dispersion to incorporate multiple collectors and produce ion
to
z direction with minimal aberrations. Hence extended
beams that are focused in the
geometry is the norm for MC-ICP-MS.
to
The only such multiple-collector ICP-MS proved operate in a routine man-
ner with a significant track record at the time of writing is manufactured by VG
Elemental under the model name Plasma 54 (hereafter P54). However, three new
and interesting ins~ments are being developed and marketed, one by Nu Instru-
ments, the second by Micromass Inc., and the third by VG Elemental. Each of these
latter instruments utilizes ICP sources with a large potential difference (rt4 to 6
all
kV) relative to the analyzer region. They differ from the P54 by including a lam-
inated magnet capable of fast scanning and are accompanied by a range inno-
of
vative solutions to the design obstacles discussed. The footprints are smaller. The
of
electronics are greatly simplified and modernized relative to those the P54, with
of
computerized monitoring and control nearly all instrument parameters. The data
Vi-
control software suites are relatively accessible. Many programs are written in