Page 239 - Inorganic Mass Spectrometry - Fundamentals and Applications

Isotope ~il~tion Mass ~pe~tromet~ 225

is
the process detection limit. A process ~Za~k one that tests the whole chemical
procedure, including the instrument and all reagents. A known amount of calibrated
spike is added to the solvent (usually acid) and the mixture subjected to all steps
a real sample would undergo. The amount of the target element is measured in the
same way a normal isotope dilution analysis is carried out. If the blank is unac-
of
ceptably high, the source conta~nation must be identified and eliminated. This
is
can be a tedious, time-consu~ng operation. Once the blank level established, sam-
ples must be corrected for its contribution unless it is deemed insignificant.
Isotope dilution involves isotopes the same element, thus eliminating differ-
of
ences in chemical behavior that can plague techniques that use different elements
for the spike than the one being analyzed. This, along with the fact that quantita-
tive recovery is not necessary once spike and sample have been equilibrated, makes
el-
isotope dilution less subject to error arising in chemical processing than many
emental assay techniques, a fact of great importance at low concentration levels.

.3 I

[l].
Isotope dilution was first developed by Inghram in 1954 Since that time its ap-
in
plication has become very widespread, and has been used numerous disciplines.
The following derivation is based on that of Hintenberger [5]. The following no-
tation is used:
C = concentration
W = weight
A = atomic weight
a = atomic abundance
R = ratio
N = number of atoms
The subscripts are defined as follows:
S = unspiked sample
t = tracer (spike)
m = mixture of spike and sample
i = major isotope in the sample
k = major isotope in the spike
The ratios (Rx )are defined as follows:

R, = ax /axk