Page 134 - Inorganic Mass Spectrometry - Fundamentals and Applications

124 Olesik

do
concentrations are assigned last because they not have a unique isotopic pattern
to aid in their identification. This approach was used to reduce the computation
time that would be necessary to solve a complete set of simultaneous equations
and to deal with species in the spectrum that might not be anticipated in the model.
Recently, a full multicompo~ent analysis approach for automated semi-
quantitative survey analysis by ICP-MS has been described [153,208,209]. The
computing power of personal computers (PCs) has increased sufficiently that
modeling a matrix of 250 masses by 105 species can be done quickly (about 10 sec
on a 100-MHz Pentium PC). A database of the isotopic patterns of all elemental
is
the
ions and interferent ions used. The database can be expanded beyond current
400 interferent ions. Elemental and polyatomic ions with signals below the
estimated standard deviations in the total measured intensities are excluded from
the model. To assess which elemental and polyatomic ions are in the experimental
spectrum a relative standard deviation (RSD) function that has been modeled in
is
terms of source and shot noise used [208]. Diagnostics for goodness of fit of the
model include graphical comparison of mass spectra with elemental ion or inter-
ferent ion spectra and precision for each element or interferent ion. The measured
intensity at each isotope for a given elemental or polyatomic ion is divided by the
corresponding natural abundance to calculate a set
of element i~te~~ities. "he RSD
of the element intensities is a measure of the quality of the model fit. The precision
information can also be used to determine true detection limits in the sample.
There are three potential limitations of the MCA approach. If the signals at
some masses (such as mlz = 56) are over range because of large signals (ArO+,
Fe+, andlor CaO+), it is more difficult to assign their element intensities at other
masses. Monoisotopic elements have more uncertainty because there is no iso-
topic pattern to assess the model fit. there are more species than masses over a
If
region of the spectrum, there is not a unique solution. For example, Cr+ may
require correction for Ti+, V+, Fe+, and ArO+.
Semiquantitative analysis procedures have also been used with laser abla-
tion ICP-MS [7 1,2 10-2 131. The effect of experimental parameters and potential
improvements in accuracy of semiquantitiative analysis using ICP-
also been discussed recently [208,214-2161, along with the use of se~qu~tita-
tive analysis with a double-focusing ICP-MS in both low- and high-resolution
modes [2 171.

S is used for elemental analysis of every type of sample imaginable,
including gases, liquids, and solids. Element concentrations in natural, process,
and waste waters; biological tissues, hair, and fluids; metalloproteins; and drugs