Page 26 - Inorganic Mass Spectrometry - Fundamentals and Applications

16 Smith

ure 9 Magnetic sector pulse-counting mass spectrometer: (a) ion source, (b) ion lens,
(c) source defining slit, (d) collector slit, (e) electron multiplier. (From Ref. 47.) (Comesy
of P. H. Hemberger, Los Almos National Laboratory.)

function of this magnet is not to effect mass separation, which is accomplished in
the first magnetic field, but to clean up the tails of the peaks and thus improve
abundance sensitivity. Because it is energy spread that is being addressed, the
same result can be acco~plished through use of an electrostatic analyzer [50] or
a quadrupole filter [S 11.
Most analytical problems do not require an abundance sensitivity of more
than 500,000, which is about the masimum available with a single magnetic sector
of reasonable size. Ins~ments with addi~ional stages, either magnets or electro-
static analyzers or both, have been constructed to provide abundance sensitivity
at Oak Ridge
greater than lo*; Fig. 1.10 is a photograph of one such ins~men~
National Laboratory [SO].

Accuracy of isotope ratio measurement is critically dependent on having the
ins~ment properly calibrated and following correct analytical protocol. Mass
bias is present to some degree in dl thermal ionization analyses, and a lot of
ingenuity has been invested in mitigating its effect. Mass bias arises from a