Page 237 - Inorganic Mass Spectrometry - Fundamentals and Applications

Oak ~i~ge ~~tional ~a~oratory
Oak Ridge, Tennessee

Isotope dilution mass spectrometry a powerful method for determining the quan-
is
tity of an element or an associated compound in a sample. It requires that a spike
of the same element but with an isotopic composition different from that of the
sample be introduced to it in a controlled manner. The mass spectrum of the mix-
ture of spike and sample is then used to determine the concentration of the target
element in the original sample. In most cases, a single ratio is used, one that in-
corporates the major isotope in the spike and the major isotope in the sample; these
must be different isotopes. The difference in the value of this ratio in the sample
and in the mixture of sample and spike is proportional to the amount of the target
element in the sample.
Isotope dilution has been in use for well over 40 years, having been devel-
oped by Ingham in 1954 for determination of minor and trace level elements [l].
It had its first major application in conjunction with thermal ionization mass spec-
of
trometry but is today used with virtually every kind ins~mentation and in vir-
tually every analytical area in which elemental conce~trations are of importance.
It also has been applied the determination of compound concentrations by using
to
an appropriate molecular spike containing an enriched isotope of one of its ele-
ments. It has gone beyond its original application in elemental analysis of inor-
ganic materials and is used routinely in fields far removed from its origins.
Isotope dilution is usually the most accurate and most sensitive quantifica-
tion technique when sample size is limited, It has in recent years even been used
in certifying the compositions of reference materials [Z]. The use of isotope dilu-
tion mass spectrometry in the standard reference materials program of the National

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