Page 314 - Inorganic Mass Spectrometry - Fundamentals and Applications

M~l~iple-Collector KP-MS 301

tivity, a small energy spread, and remarkably stable focusing and peak shape. We
are at the early stages of learning all the possible ra~fications for MC-ICP-~S
provided by hexapoles.

Eleme~t~l Axio
VG Elemental launched another design of mass spectrometer, the Axiom, a small
of
instrument with extreme dispersion, capable multiple collection at variable mass
resolution. Designed by P. Marriott, the Axiom is very compact yet provides the
difficult combination of very high mass resolution (>10,000 when required) with
a linear focal plane suitable for multiple collection. The Axiom is a forward geom-
etry double-focusing, C con~g~ration ins~ment with a 77" electrostatic sec-
580-
tor-90" magnetic sector using non-normal incident and exit angles to achieve
mm dispersion. Unusually, the instrument is arranged with an upright
configuration. As with the P54 and Nu Plasma, the instrument extracts the ions at
high voltage (5 kV) onto and through an extraction electrode at ground potential.
"urbopumps are used throughout the instrument to create ultrahigh vacuum and
to
allow reasonable abundance sensitivity. Two Einzel lenses are used to shape the
trajectory of the ion beam so that it is partially focused down onto an adjustable
source slit. The gradually diverging beam then enters a drift length region in which
it is steered, focused, and slightly shaped, before entering the ESA. At this point
the pressure has been reduced to mbar, As the ion beam passes through the
magnetic analyzer it broadens to some 15 rnrn in the nondispersive plane but fo-
cuses down to a very small height (1 m) at the focal plane. This reduced spread
in the z direction allows a smaller collector slit height to be used. To achieve high
mass resolution, an adjustable collector slit is required. For extremely fast elec-
trostatic scanning over a mass range of 10% A mlm, a potential is applied to the
rear section of the flight tube itself.
At the time of writing, this instrument is still under development. A proto-
type has already demonstrated the power of the ion optics in single-collection
10% h mlm of the focal
mode. Also, the beam size has been measured across some
plane, confirming the 90" focal plane and its linearity-endorsing the detailed ion
optical modeling used in its design. The instrument is designed for rapid high-pre-
cision analytical work, but with the convenient simplicity required by the nonspe-
cialist user.

The trans~ssion efficiency of MC-ICP-MS is high relative to that of most ICP-
MS inst~ments (>lo8 ions secm1 ppm indium using conventional aspiration
methods) but is low relative to that of TIMS. This is because the ions in the plasma