Page 238 - Inorganic Mass Spectrometry - Fundamentals and Applications

224 Smith

et
Institute of Science and Technology has been described by Bowers a1 [3]. A re-
cent report describes using isotope dilution to calibrate analysis of solid samples
using electrothermal vaporization with inductively coupled plasma mass spec-
trometry (ICP-MS) [4].
It is obvious that, no matter what kind of mass spectrometer or ionization
method is used, correction for isotopic bias is mandatory for successful applica-
tion of isotope dilution, The basis of the whole technique rests on ~stwo~y
meas-
urement of the isotope dilution ratio [R, in Eq. (5.7)], which necessitates applica-
tion of a bias correction. Accuracy of the final result is critically dependent upon
accurate calibration of the inst~mentation, and, as always, it is important to have
analysis of reference materials mimic as closely as possible that of samples.

An absolutely essential step in isotope dilution is to achieve equilibration of spike
is
and sample. The spike is usually added as a solution whose concentration accu-
rately known; since many samples are not solutions, chemical processing is usu-
ally required to equilibrate the two. This invariably involves dissolving the sam-
ple, not always a simple task. The spike solution is added to the sarnple, and the
two are thoroughly mixed. ~quilibration for some elements requires nothing more
like
than to have spike and sample in solution together. Some elements, plutonium,
have a multiplicity of oxidation states and require more effort; this very often in-
cludes drying the sample and redissolving it, and adding an oxidizing or reducing
agent to ensure that the target element in both spike and sample assumes the same
oxidation state. Once equilibration is attained, it is unnecessary to achieve full re-
covery of the analyte element in subsequent chemical processing, one of the big
advantages of isotope dilution. Full recovery is not required because chemical
processes do not cause isotopic fractionation, at least to the degree necessary to af-
fect results significantly. This characteristic of isotope dilution is p~icularls ad-
vantageous when the sample is a complex matrix such as vegetation, soil, or ani-
mal tissue. To exploit this attribute, the spike should be added the sample as early
to
in the chemical prep~ation process as possible. For example, in the author's lab-
oratory the 233U spike is added to the platinum dish cont~ning a vegetation sam-
ple whose uranium content is desired before it is inserted into the muffle furnace;
the spike is thus introduced before any chemical processing has occurred.
Because extensive chemical processing involved in using isotope dilution,
is
it is important to determine the level of the process blank before analyzing sa-
ples. Although there are a few instances in which doing so is unnecessary, the vast
majority of problems require it; it obviously becomes more important with de-
creasing concentration of the analyte element. Many elements are ubiquitous in
nature and occur at some level in virtually all reagents, including purified water.
In numerous applications the blank level rather than the ins~mentation determines