Page 308 - Inorganic Mass Spectrometry - Fundamentals and Applications

ICP-MS
M~l~i~~e-~ollector 295

after the ESA with an exit lens. This inverts the image. Therefore, in order to main-
tain energy focusing at the collector it is necessary for the analyzer to be arranged
with an S configuration.
set
The magnet pole faces are at an angle that is non-normal to the optic axis,
providing double dispersion equivalent to that of a 54-cm-radius magnet. The ex-
z focusing and sufficient dispersion to per-
tended geometry magnet also provides
mit high transmission of the ions at the source slit at a mass resolution of about
400 (10% valley definition). The exit pole face is adjustable to rotate the ion beams
the
and produce an ion image perpendicular to optic axis. The University of Michi-
gan (U-M) PS4 has a total of 1 l Faraday collectors providing true flat-topped peaks.
Such peaks, in which each mass-resolved ion beam is entirely collected, are re-
quired for high-precision isotope ratio measurements. It is also essential that the
transmission through the analyzer region and the detector gains remain constant
as the beam is scanned across the interior of the Faraday detector. Unlike with
TIMS, the instability of the ICP source prevents the normal measurement peak
of
flatness. For practical purposes is currently specified as a static measurement rel-
it
ative to the signal another isotope measured simultaneously in another detector;
of
it is to be within rk 100 ppm over SO0 ppm of mass change as the collector is moved.
The main collector assembly has eight independently adjustable Faraday cups and
a fixed axial Faraday cup that can be lowered to allow the ion beam access to a
Daly detector with ion counting (Fig. 8.1).
The U-M instrument also has a wide flight tube with an off-axis high-mass
is
Faraday cup for the measurement of U at the same time as Pb measured with the
multiple-collector block (Halliday et al., 1993). This instrument has been further
for
equipped with a second-stage 30-cm energy filter enhancing abundance sensi-
tivity before the ion beam reaches the Daly detector (Fig. 8.1), permitting highly
accurate Th isotopic measurements (Luo et al., 1996, 1997). The abundance sen-
sitivity (relative size of the peak tail at l mu) of the U-M instrument in this mode
is about 0.3 ppm for Th. An additional~Faraday cup is located beyond the Daly de-
tector. This rear cup is normally only used for alignment purposes because the beam
is somewhat defocused at this point.

The Nu Instruments Nu Plasma displays some general similarities to the PS4, but
critical features are radically different in concept. Most importantly, the instrument
deploys novel variable dispersion ion optics that are applied to the ion beams ex-
iting the magnetic analyzer, permitting use of fixed multiple collectors and rapid
the
peak switching.
Like the PS4, the Nu Plasma is a forward geometry double-focusing mass
spectrometer with double dispersion, but it has the standard C configuration and
defines a smaller footprint (Fig. 8.2). The instrument has a source that is similar to