Page 23 - Inorganic Mass Spectrometry : Fundamentals and Applications

The~a~ Ionization Muss Spectrometry 13

1.5 x

0.5 x

0 .5 1
Pulse Helght (Volts)
Figure 7 Operation of a discriminator.

[39] and ionic mass [40]. This matter has been discussed in some detail by Hayes
and Schoeller [41].
Because instability in beam intensity is a primary cause of imprecision in
isotope ratio measurements, it is a big advantage to monitor all isotopes of interest
simultaneously. Commercial inst~ments are available with multicollector arrays
that effect this [42,43], a drawing of one of these is given in Fig. 1.8 [43]. Note that
the axial collector is recessed behind the focal curve. Since the ion beam spreads
after crossing the focal point, the result is that a smaller fraction of the beam is
intercepted by the axial collector than by the others. This fact must be addressed
when calibrating the instrument. Multicollector arrays provide precision that, with
proper care, can be better than 100 ppm [44]. One price paid for this advantage is
the necessity of calibrating the detectors with respect to each other. Constant
signal sources are provided in commercial instruments to effect cross-calibration
of detectors. The more subtle problem of loss of linearity of individual pockets
with use has been addressed by Fiedler and Donohue [45]. The concern of these
authors was that, as sample elements accumulate in the Faraday pockets with
time, the ampli~cation characteristics might change; this effect is a small one and