Page 132 - Inorganic Mass Spectrometry - Fundamentals and Applications

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Because the severity of the matrix effect varies smoothly with analyte mass, is
possible to use a limited number of internal standards (three, for exmple) across
the mass range and then to interpolate to correct analyte sensitivities of analyte
ions with masses between those of the internal standards [ 1911.
The ideal internal standard is the same element as the analyte because it has
similar mass, ionization energy, and chemical properties. Therefore, isotope dilu-
tion based calibration provides high accuracy as long as isotope equili~ration is
attained and the measured isotopes are free of spectral overlaps [192,€93], Stan-
dards do not need to be matrix-matched. ~uadrupo~e-based ICP-MS inst~ments
can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope
ratio precision can be obtained by using simultaneous MS detection, such as a
multicollector-based inst~ment or perhaps time-of-flight MS. In comparison to
thermal ionization mass spectrometry, ~CP-M~ provides much higher sample
t~oughput and simpler, faster sample preparation.
Because interference-free isotopes are most effective for isotope dilution
analysis, potential sources of interference may need to be removed before anal-
ysis. Separation procedures based on anion exchange [194-197], cation exchange
[ 1981, solvent extraction [ 1991, and anodic stripping E2001 prior to isotope dilution
analysis have been described.
Propagation of errors using isotope dilution ICP-MS has been considered to
determine how to optimize the meas~ements [201]. Comparison of analysis results
from external calibration versus isotope dilution can be used to assess the quality
of external calibration results and the effectiveness of internal standards with
external calibration [202,203]. Because isotope ratio precision depends on the
total ion count rate, the use of high-efficiency sample introduction to generate
larger signals can improve isotope ratio precision and, therefore, analysis preci-
sion [204].

analysis for about '70 elements by using
element response functions built into the instrument software and calibration of
only a few elements [205,206]. Most elements are more than 90% ionized in the
ICP (with the exception of elements with ionization potentials greater than about
8 eV). Ion transmission efficiency is a smooth function of mass. The natural
isotopic abund~ces of the elements are well known. "herefore, it is possible to
predict the relative sensitivities of the elements and any isobaric overlaps.
Semiquantitative analysis is carried out by comparing or fitting spectra for
elemental ions (based on their natural abundances or response factors determined
a
previously) and molecular ions to the experimental mass spectrum acquired over
wide mass range. Day to day changes in sensitivity and mass bias are corrected
by measu~ng signals for a few elements in a standard solution. The keys to