Page 311 - Inorganic Mass Spectrometry - Fundamentals and Applications

et
298 Halli~a~ al.
the
ets. Twelve Faraday buckets are provided with ins~ment, none of which is re-
quired to move because of the use of the zoom ion optics. Unlike with the P54, or
the Micromass instrument discussed later, the flight tube does not need be par-
to
ticularly wide because ions of very different mass can be deflected into the cups
of interest. Also unlike the P54, three multipliers with ion counting can be pro-
vided as an option, utilizing low-noise discrete dynode multipliers capable of
recording signals of greater than 2 X 106 counts per second. The ions are deflected
focal
into the multipliers by using electrostatic deflectors located beyond the plane.
To achieve high abundance sensitivity a small retardation lens is added. This in-
strument achieves specifications for the low-mass tail comparable to that of the 30-
cm energy filter of the P54.
is
The pumping configuration also different, with all vacuum pumps and sen-
sors operated at ground potential. The extraction and lens region is divided into
three regions differentially pumped with three small turbomolecular pumps (Fig.
8.3). So effective is this arrangement that the vacuum the second Einzel lens has
at
already achieved mbar. Therefore the analyzer region is pumped with three
ion pumps (Fig. 8.2), in contrast to the heavy-duty turbomolecular pumping used
on other MC-ICP-~S instruments (Fig. 8.1). Apart from the acoustic quietness
is
with which the inst~ment operates, this system has the advantage that if power
lost, the active surfaces still provide pumping, with little loss of vacuum and no
risk of cont~ination. At the time of writing, this ins~ment provides the most ac-
curate isotopic data of all the commercially available machines.

The high temperature of ICP sources (-7OOOK) inhibits the formation of molecu-
lar interferences. Generally, dimers and simple oxides and hydrides are the most
complex molecules formed. Combinations of major matrix elements, the solute,
and components of the plasma support gas, for instance, oxygen and argon, create
the most serious spectral interferences. So, for example, 4oAr12C+ on 52Cr+,
40Ar14N+ on 54Fe+, 40Ar160+ on 56Fe+, 40Ar2+ on 8oSe+, and 40Ar35C1+ on 75A~+ limit
the applicability of the method. The presence of such interferences gives rise to an
elevated background at the analyte mass that, at best, raises the achievable detec-
tion limits and, at worst, renders the analyte isotope unusable. For this reason most
of
~C-~CP-~S measurements have been restricted to that portion the mass region
(z > 80) above which molecular interferences are absent or negligible.
~icro~ass developed a p~tentially powerful new technique that elirni-
has
nates many of these molecular interferences and also removes ions with an energy
of
that differs from that the analyte, such as components the Ar support gas. This
of
has a dramatic effect on performance of the instrument. The technique deploys
the
a hexapole ion lens (Szabo, 1986) located behind the skimer cone and surrounded
by a gas cell (Fig. 8.5). The hexapole uses a hexagonal array of rods between which
a 400-V rf field is applied, confining the ions of interest to stable trajectories be-