Page 133 - Inorganic Mass Spectrometry - Fundamentals and Applications

I~ductivel~ Coupled Plasma Mass Spectrometry 123

successful semiquantitative analysis include properly choosing the elements to
use in the model or fitting process, choosing the molecular ions to include, and
accounting for changes in mass bias, day to day instrument sensitivity, and sample
matrix-induced changes in analyte sensitivity. Properly identifying and modeling
the molecular ion signals are probably the most challenging problems. Accuracy
obtained is sample matrix-dependent and is typically good to within a factor of
2 to 5 for most elements. Higher accuracy (sometimes within 230%) can be
obtained for many elements that are in “clean7’ solutions or when a few (typically
is
three) internal standards are used across the mass range. However, care neces-
sary to assess accuracy for different sample types.
Different approaches have been used to assign the signals in the measured
mass spectrum to particular elements and polyatornic ions. The Total~uant ap-
proach by PE-Sciex uses a combination of equations and heuristics (“rules of
thumb”) [207]. For exarnple, constraints are included on the relative detected
oxide ion (such as Lao+) to elemental ion (such as La+) signals. The LaO+ signal
is assumed to be no more than 2% of the La+ signal. Instrument response values
the
for each isotope of each elemental ion are stored in computer, as are spectra of
potential interferents. The steps for the se~quantitative determination are shown
in Table 3.5. The approach takes advantage of the isotopic pattern of masses for
elemental, polyatornic, and doubly charged ions. Monoisotopic element

Steps in Semiquantitative Analysis by PE-Sciex TotalQuant

1. Measure spectrum for a blank.
2. Measure spectrum for a standard containing a few elements. This is used to update
elemental response values to account for day to day changes in sensitivity and mass
bias.
3. Measure the full mass spectrum for the sample.
4. Make a preliminary estimate of the intensity for each element, based on isotopic natu-
ral abundances.
5. Initial estimates of signals polyatomic ions are made and are constrained to be less than
a given percentage of a constituent element.
6. Assignments of intensity are made for elements with multiple isotopes in a prioritized
order that depends on the relative signal intensities observed. The assignments are
evaluated and adjusted if necessary for elements with overlapping isotopes.
7. The data are evaluated for apparent inexact isotopic abundances caused by nonideal
measurement precision.
8. Assignments are made for polyatomic and doubly charged ions associated with the
multi-isotope elements. These assignments are constrained to given percentages of the
elemental ion signals.
9. Assignments are made for monoisotopic elements and associated polyatomic and
doubly charged ions.
Source: Ref. 207.