Page 245 - Inorganic Mass Spectrometry - Fundamentals and Applications

Isotope Dilution Mass ~pe~tro~et~ 231

inition. Elemental assay of biological systems is of such importance that a short
section of this chapter is devoted to it.
of
The following material has been chosen as representative current trends
to
but makes no claim for completeness. It does not even pretend consistency, the
is
the
author consoling himself with Emerson’s famous dictum “Consistency hob-
goblin of little minds” [ 161. Reviews are cited where known, and references should
to
be considered as guides to further information rather than definitive sources.
is
As mentioned, thermal ionization mass spectrometry the area in which iso-
tope dilution developed and in which has received the widest range of applica-
it
tions. One of thermal ionization’s major limitations is that it is essentially a sin-
gle-element technique; in no way can it be considered multielement in the sense
that numerous elements can be assayed in a single analysis. It is thus highly de-
sirable to mate isotope dilution with multielement analysis capability. Spark source
mass spectrometry for years dominated elemental analysis, but the nature of the
of
samples (solids) made use isotope dilution difficult. Use of a multielement spike
was reported as long ago as 1970 by Paulsen et al. [ 171, however, and more re-
cently by Carter et al. [l81 and by Jochum et al. [19,20].
In the last decade or two, the advent of new instrumentation directed at ele-
mental analysis has provided fertile new ground for expanded use of isotope dilu-
tion. Glow discharge mass spectrometry is in many ways the modern replacement
to
for spark source and has similar impediments ready application of isotope dilu-
tion. A recent report of Barshick et al. describes assaying lead in oil residues using
the technique [21]. The obstacles spark source and glow discharge mass spec-
trometry both present to ready use of isotope dilution make it unlikely that wide-
spread application of the technique will occur in conjunction with them.
The same cannot be said for ICP-MS, however. Samples are usually in so-
lution form, ideal for mixing spike and sample. This instrument, described in de-
of
tail in Chapter 3, has had huge impact on many areas science in which trace el-
emental analysis is important. The use of isotope dilution in association with
ICP-MS has been described by Jarvis et al. in their book devoted solely to ICP-
MS [7]. It has become widely accepted as a reference method in fields not nor-
of
mally thought of as mass spectrometric such as in analysis medical specimens
[22], and a review describing isotope dilution’s role in clinical, pharmacological,
and toxicological applications has recently appeared [23]. Fassett has summarized
its application, along with other tra~tionally inorganic techniques, to biological
systems [24]. There has been a spate of papers in recent years describing results
obtained by using isotope dilution in association with ICP-MS in inorganic appli-
cations, many of them dealing with environmental issues; two recent reviews by
Heumann bring the subject into focus [8,9].
Because ICP-MS is multielemental and samples are normally in solution
form, use of a multielement isotope dilution spike has many attractive features,
Spikes for individual elements can be kept separate and mixed in appropriate con-
centrations for the problem at hand. Many samples are from the environment, so