Page 151 - Inorganic Mass Spectrometry - Fundamentals and Applications

~~du~ti~ely Coupled Plasma Mass Sp~~t~o~et~ 141

For some specific cases such as Cr(V1) versus CR(II1) measurement in
waters, comercial products, based on short columns that can be used in a flow
injection mode with ICP-MS, have become available, Separation times of less
than 3 minutes, low cost, and convenience make this approach viable for routine
use in some applications.
The importance of elemental speciation in environmental systems, bio-
is
medicine, and other areas becoming more widely realized. Initial regulations for
elemental speciation [such as regulations specifying Cr(V1) measurement rather
than total Cr measurement-EPA 7 196A and 3060A, for example] are being
proposed and adopted.
One of the main problems with speciation measurement is that the specia-
tion may change during sampling, sample preparation, or analysis. ~u~he~ore,
spikes added to samples for standard addition calibration or spike recovery mea-
surements may not remain as the same species once in the sample. For example,
oxidation or reduction can take place, depending on the sample chemistry. Re-
cently, speciated isotope dilution mass spectrometry (SE") using an ICP ion
source has been developed to account for these changes in speciation [399]. The
SIDMS method was used to show that Cr(II1) could be oxidized during alkaline
extraction using EPA method 3060A and Cr(V1) could be reduced by using EPA
method 7 196A, leading to errors [400].
Speciation of As has been of great interest because of the wide range of
sp~cies-dependent toxicity. €PLC coupled with ICP-MS has been used to mea-
sure arsenite and arsenate, monomethylarsonic acid, dimethylarsinic acid, arseno-
A
betaine, and arsenocholine in mussels and fish [401]. Dionex AS7 column with
an on-guard AC7 column was used. A gradient mode separation was carried out
by using a bicarbonate buffer mobile phase. Detection limits were in the 0.001- to
to
O.O03-~M range. Cation- and anion-exchange HPLC-ICP-MS were used mea-
sure arsenic speciation in shrimp, crab, fish, fish liver, shellfish, and lobster [402]
as well as human urine [403]. Ion exchange HPLC-ICP-MS was used to monitor
the bioavailability and speciation of As in carrots grown in As contaminated soil
[404]. HPLC-~CP-~S was also used to study arsenic speciation in soils as a
function of pH [405]. Capillary electrophoresis-ICP-~S has also been used for As
speciation in drinking water [406].
Several potential problems with As speciation have been described [4-071.
ArCl+ causes a spectral overlap in ICP-MS detection. Speciation can change
between the time the sample is obtained and when is measured. Field separations
it
or preservations can be developed to prevent that problem. Particulates in drinking
water can contain a significant portion of the As, so filtering the sample without
also analyzing the composition of the particulates can lead to measurements that
are too low [407].
Selenium speciation has also been measured by a variety of approaches for
several different applications. Size exclusion chromatography was coupled with
ICP-MS detection to measure selenium speciation (especially Se-cont~ning pro-