Page 27 - Inorganic Mass Spectrometry - Fundamentals and Applications

Ther~al Ionization Mass S~ectro~et~ 17

ure 10 Three-stage sector mass spectrometer with high-abundance sensitivity.

number of different causes. Evaporation of the sample is to some degree mass-
dependent; light isotopes evaporate more readily than heavy, presumably as the
inverse square root of their masses. Extraction of the ions into the source-focusing
lens has similar mass dependence. Transmission through some types of mass
analyzers (quadrupoles) is a function of mass. Conversion of the ion to a pulse of
electrons at the collector is also mass-dependent. It is experimentally extremely
difficult to isolate these eEects individually. The usual way of dealing with them is
to lump them into one bias correction factor and not worry about the values of
the individual cont~butions.
Certified isotopic standards are available for some elements but by no means
all. Such standards are available from the National Institute of Standards and
Technology [52] and New Brunswick Laboratory [53] in the United States, and
from the Institute for Reference Materials and Measurements [54] in Belgium. De
Bievre et al. have published a review of reference materials available for isotope
ratio ~easurements [551. It is clearly desirable to use certified materials for
ins~ment calibration if at all possible. If no standard is available for the element
in question, the analyst has little choice but to obtain a pure sample of it and
assume it has the composition listed by WPAC [l]. A few elements, with lead
being the outst~ding example, have isotopic compositions that vary widely in