Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 278

Arsenic lowered somewhat the peak temperature in atomisation profile for selenium.
Copper tends to suppress the interferences of diverse elements on atomisation of
selenium. However, the interferences from large concentrations of diverse elements and
matrices were not improved even in the presence of copper at the atomisation step.
Therefore, the separation of selenium from matrices was recommended.
2.90.2 Polarography
AC polarography and differential pulse polarography [675] and anodic stripping
voltammetry [676] have been used to determine selenium.
2.90.3 X-ray fluorescence spectroscopy
Robberecht and Van Grieken [677] determined µg L −1 levels of total dissolved selenium
and selenite in non saline waters by X-ray fluorescence spectrometry. Detection limits for
tetravalent selenium and total selenium are 50ng L −1 and 60ng L −1 respectively.
Coefficients of variation are 10% for tetravalent and hexavalent selenium at the 0.5µg L −1
selenium level.
2.90.4 Isotope dilution mass spectrometry
In order to clarify the species composition of selenium in environmental water samples,
Tanzer and Henmann [678] developed analytical methods for the selective determination
of different chemical forms of this element (selenite, selenate, and organic species
including trimethylselenonium) using isotope dilution mass spectrometry. The species
analysis was made possible by means of chromatographic separation procedures and an
82 Se-enriched selenate, selenite, and trimethylselenonium spike for the isotope dilution
process. The total selenium concentration was determined after decomposition of organic
compounds with nitric acid-perchloric acid mixture. Selenium was measured in the mass
spectrometer by producing negative Se− thermal ions for detection. Precise determination
at the parts-per-trillion level was achieved. This new methodology was applied to
different types of non saline water samples (groundwater, pond water, river water,
moorland lake water) with total selenium concentrations in the range of 200pg g −1 to
15ng g −1 . Selenite and selenate have been the only detected species in most of the
investigated samples, with selenate dominating all except one. In samples with high
contents of dissolved organic carbon, however, different organoselenium compounds
including trimethylselenonium ions were additionally quantified in the range of 10−95pg
g −1 . In these cases, the sum of all selenium species agreed well with the independently
determined total element concentration.
2.90.5 Ion chromatography
The application of this technique is discussed under multianion analysis in sections
12.2.2, 12.2.4 and 12.2.5.