Chemistry Reference
In-Depth Information
Some selenium compounds can cause DNA damage,
probably by inducing oxidative stress. Teratogenicity
has been observed particularly in avian species and
fi sh. Selenite on hamsters and mice and selenomethio-
nine in macaques were essentially negative.
Selenium may prevent or alleviate toxic effects of
arsenic, cadmium, mercury, platinum, and silver. Con-
versely, some of these metals protect against selenium
toxicity.
in all the steps from sampling to the fi nal measurement
(Varo and Koivistoinen, 1981). Apart from instrumental
neutron activation and X-ray fl uorescence analysis, most
analytical methods are based on acid digestion/separa-
tion before determination of the total selenium content.
A variety of analytical techniques have been used, such
as spectrophotometry, atomic absorption spectrometry
(AAS), atomic emission spectrometry (AES), atomic
fl uorescence spectrometry, mass spectrometry (MS),
inductively coupled plasma (ICP)/AES, ICP/MS, gas
chromatography (GC), and activation analysis using neu-
trons or protons (ATSDR, 2003; Bem, 1981; Thomassen,
1994). The spectrofl uorometric methods are based on
extraction of Se(IV) with 2,3-diaminonaphthalene from
the digested sample. Chelated selenium can be meas-
ured by GC/MS. Neutron activation is based on the
direct measurement of 77mSe (half-time, 17.5 seconds)
or 75Se (half-time, 120.4 days). The detection limit is
below the nanogram per gram range (Raptis, 1983).
Isotope dilution mass spectrometry is a highly accurate
method for determination of the selenium.
AAS and ICP-MS are the most widely used ana-
lytical methods for biological samples (ATSDR,
2003; Thomassen, 1994; Verlinden, 1981). AAS with
conventional fl ame atomization does not have ade-
quate sensitivity for biological samples. The hydride
technique, which entails generating SeH
2
from the
digested sample, requires large samples (Raptis,
1983). Graphite furnace (GF)-AAS is a highly sensi-
tive method, but the volatility of selenium requires
stabilization during ashing to avoid loss of selenium.
Furthermore, chemical and spectral interferences from
biological matrices (e.g., Fe, PO
2
−4
) are serious prob-
lems. Whereas a number of metal reagents prevent
volatilization of inorganic selenium, only nickel, pal-
ladium, and silver work in thermal stabilization of
biological forms of selenium (Alexander and Aaseth,
1980; Johannesson
et al
., 1993; Thomassen, Lewis
et al
.,
1994). The spectral interferences are eliminated by
use of Zeeman-based background correction. Deter-
mination of selenium by ICP-MS suffers from spectral
interference with Ar
2
for example. This can be solved
by use of reaction collision cell or high-resolution
MS. ICP-HRMS is a highly sensitive method.
During the past decade, great advances have
been made in the speciation of selenium compounds
in biological matrices such as serum and urine,
plants, and food (Block
et al
., 2004; Francesconi and
Pannier, 2004; Kotrebai
et al
., 2000; Rosenberg, 2003;
Uden, 2002; 2004; Wrobel
et al
., 2003). The most used
methods are various hyphenated methods based on
a coupling of an electrophoretic or chromatographic
separation with an atomic spectrometric or selenium-
specifi c measurement. It can be off-line or on-line
1 PHYSICAL AND CHEMICAL
PROPERTIES
Selenium belongs to subgroup VIa of the periodic
system and has both metallic and nonmetallic proper-
ties. Subgroup VIa also contains oxygen, sulfur, and
tellurium, and the similarities between the two lat-
ter elements and selenium are often pointed out. The
atomic weight is 78.96. Elemental selenium is insoluble
in water and has several allotropes; it may be grey or
“metallic,” a red amorphous powder, or it may have
a vitreous form. The boiling point is 685°C and on
boiling in air, selenium dioxide is formed. Selenium
dioxide (the formal oxidation state +4) is a crystalline-
white powder at room temperature. In contact with
water it forms selenous acid (H
2
SeO
3
), which is a strong
acid. Selenium trioxide (oxidation state +6) is a yellow-
ish white powder, which forms selenic acid (H
2
SeO
4
) in
water. The salts of these acids, selenites and selenates,
respectively, are usually soluble in water.
Another oxide of commercial and toxicological inter-
est is selenium oxychloride (SeOCl
2
). The reduced form
of selenium (selenide) may be formed from water-solu-
ble selenium compounds under acid conditions or in
biological systems. The formal oxidation state is −2, and
of biological interest are hydrogen selenide, dimethylse-
lenide and trimethylselenonium ions, and selenosugars.
The reduced form may also be found in the place of
sulfur in amino acids (e.g., selenomethionine and selen-
ocysteine). A large number of low molecular selenium-
containing compounds are present in plants and other
organisms used for food. Selenide may form heavy
metal complexes, which are practically water insoluble.
2 METHODS AND PROBLEMS
OF ANALYSIS
The low concentrations of selenium (ng/g) usually
occurring in biological and environmental samples
require sensitive and accurate analytical techniques.
Care should also be taken to avoid sources of system-
atic error including loss of volatile selenium compounds
