Page 136 - Inorganic Mass Spectrometry - Fundamentals and Applications

126 Olesik

lernental Analysis of Eliological Fluids

to
ICP-MS has been used measure a variety of elements in urine, serum, and blood
[230]. The main problems are sample-dependent changes in sensitivity and spec-
tral overlaps due to polyatomic ions. Elements with masses below about 85 are
particularly prone to spectral overlaps.
Nuttall et al. [23 l] reported the use of ICP-MS for routine dete~nation of
i, Cd, Pb, Sb, Te, and TI in a clinical laboratory. Blood was deproteinized,
centrifuged, and passed through a 2-pm filter before analysis. Other specimens
were diluted or digested with dilute mineral acids. Se, Zn, and As suffered from
interferences that could be overcome proper isotope selection or mathematical
by
correction. The authors [231] concluded that Cr and Fe were too prone to inter-
ferences (due to spectral overlaps) to be practically measured by ICP-MS in
clinical samples.
Nixon and Moyer C2321 described the use of ICP-MS for routine screening
measurement of Pb, As, Cd, and TI in urine and blood. As, Cd, and T1 are cleared
rapidly from the blood, so that if significant concentrations are found in the blood,
acute exposure is indicated and immediate action is necessary. Pb is more slowly
cleared from the blood. Significant concentrations found urine indicate chronic
in
exposure or a delay in treatment since exposure. Compared to graphite furnace
atomic absorption analysis, ICP-MS provides better sensitivities and more r
sample throughput. However, two problems must be overcome for the ICP
analysis to be successful: The spectral overlap of 40Ar35C1+ at mass 75, the mass
of the only As isotope, must be corrected. Sample-induced changes in sensitivity,
due to the high concentration of dissolved solids in urine and blood, must be cor-
rected. An empirically determined relationship between 16035Cl+ and 40Ar35C1+
was used to subtract the contribution of 40Ar35C1+ at mass 75. Several internal
standard elements were tested to compensate for the changes in sensitivity. Ga was
the best internal standard for As, Rh was best for Cd, and Bi was best for TI and
Pb, Table 3.6 shows the certified and measured concentrations using the different
internal standards.
The analysis of Cr and V in biological fluids is difficult because of
4OAr'2Cl+ overlapping with 52Cr+, the most abundant isotope of Cr, and 35C1160+
overlapping with the S1V+, the predo~nant isotope of vanadium. Recently, a
procedure using high-pressure thermal and moderate-pressure microwave diges-
tion with vapor-phase acid digestion was reported to reduce the carbon- and
chloride containing molecular overlaps [233]. The vapor phase acid digestion with
microsample quartz vessels reduced the blank signals. However, blank levels still
limited the method to Cr concentrations greater than 2.8 ng/g in biofluids. Moens
et al. [234] measured Fe, Cu, V, and Zn using high-resolution ICP-MS to pre-
of
vent many of the comon molecular ion spectral overlaps. Drying and ashing
the sample before dissolution and analysis can also help to reduce spectral
overlaps [235].