Page 248 - Inorganic Mass Spectrometry - Fundamentals and Applications

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lution mass spectrometry were put to good use. Quoted precision for oil samples
was 0.5% and for coals, which are more inhomogeneous than oil, 1% to 4%.
ICP-MS dominates the field of environmental assay; most metallic and am-
photeric elements are susceptible of analysis, and it is often a great convenience
(to say nothing of being relatively economical) to be able to assay for all elements
of interest in a single analysis. Another feature of ICP-MS, however, has been ex-
ploited perhaps even more tellingly than multielement analysis. This is that the
sample introduction system lends itself to a wide variety of enhancement schemes,
in part because the sample is introduced to the inst~ment at atmospheric pressure
and in part because samples are most often in a water-based (dilute acid) medium.
These attributes combine to allow various separation and preconcentration
schemes to be implemented on-line or nearly so.
Info~ation of critical importance for many elements of environmental con-
cern is the identification of their species. For example, chromium in the +3 oxida-
tion state is relatively innocuous, whereas in the +6 state it is extremely toxic. Spe-
ciation is such an important issue that an entire chapter in this book is devoted to
it. Only a few aspects related to isotope dilution are mentioned here,
One ineluctable aspect of isotope dilution is that enriched stable isotopes
range in cost from moderately expensive to prohibitively so. It is thus highly de-
sirable to minimize consumption of spike isotopes, and, since environmental sam-
ples are often large in volume, it is undesirable to spike the original sample di-
rectly. Heumann et al. addressed this issue by developing a dual delivery system
for ICP-MS interfaced to a high-pressure liquid chromatography (HPLC) separa-
tion system [32,33]; a schematic drawing of&his system is shown in Fig. 5.4. The
system allows simultaneous introduction of two streams of solution to the ICP
torch. One is the sample, which is the effluent from the HPLC column; the other
of
contains the solution the spike for the analyte element. For species-specific quan-
the
tification, the spike is added to the sample in the usual way. For deter~ni~g
of
total amount of the target element in the sample independent species, the HPLC
is not used, and spike is added through the second introduction line to the ICP
torch. By this means, spike consumption can be minimized and, through HPLC,
speciation achieved. The two different spikes are often in different oxidation states.
One is used to quantify the species of concern (e.g., erv1); the other is not specific
and measures total element concentration. The system was first demonstrated for
copper and molybdenum [32] and extended
to analysis of natural waters with high
humus concentrations, which is a challenging matrix [34]. A mul~element spike
MO,
of 10 n m of each element was used to determine Cr, Ni, Cu, I, and Pb from
several rivers and other natural waters.
Heumann’s group also interfaced a gas chromatograph to an ICP-MS for
analysis of volatile elements in environmental samples [35]. The big advantage of
introducing gas-phase samples over the more conventional solutions is that ele-
ments can be transferred to the ICP without incurring the losses attendant on neb-
ulization. Figure 5.5 is a drawing of the apparatus involved. Using isotope dilu-