Page 313 - Inorganic Mass Spectrometry - Fundamentals and Applications

300 0 all id ay et al.

Schematic for the Micromass Iso-Plasmatrace.

The reduction in energy spread means that double focusing is u~ecess~. Micro-
mass achieves excellent peak shape and reproducibility with single (directional)
focusing. So the lens system is virtually identical to that used in TIMS, accelerat-
a
ing the ions t~ough stack that starts with an extraction plate at -900 V and ends
with a source slit at -8 kV.
The ions travel along a drift length expanding to a z baffle with a 4-mm aper-
ture prior to entering the magnetic field. This ensures that the ion beam is unim-
The
peded as it expands along the 16-mm-high flight tube. magnet is laminated for
fast scanning through the entire spectrum, from mass 6 to mass 238, in 600 ms.
The multiple-collector hardware is sirrtilar to that of the P54 but uses a very wide
of
(-1 10-m) flight tube with a broader array detectors to take advantage of a wide
(~0-m) linear focal plane (Fig. 8.6).
The Faraday detectors are carbon coated metal, like the P54, but they have
demonstrated a particularly long life at high performance (Palacz et al., 1996).
Micromass also supplies multiple multipliers, unlike VC Elemental, However,
these are small continuous dynode ch~neltrons, each with a conversion dynode
and ion counting, rather than the larger discrete dynode type used by Nu Instru-
ments. This is because with the Micromass ins~ment there is no provision for
electrostatically deflecting the ion beams of interest, and therefore the collectors
have to be closely spaced. A total of nine Faraday detectors and eight multipliers
with ion counting can be incorporated (including a Daly detector). abundance
The
sensitivity can be improved with a small deceleration lens that should be extremely
effective on this single focusing instrument.
This ins~ment represents a potentially exciting development the field of
in
MC-ICP-MS. The hexapole not only eliminates isobars, it generates higher sensi-