Chemistry Reference
In-Depth Information
and are now monitored in crustaceans (oysters) and
fi sh. Eventually, they could become harmful to man
(Rosenberg, 2005).
tions of arsenite, arsenate, MMA, and DMA. The use
of additives did not improve the stability of the arsenic
species in urine. Moreover, the addition of 0.1 mol/L
1
HCl to urine samples produced relative changes in
the inorganic arsenite and arsenate concentrations.
Another research group (Jokai
et al.
, 1998) investigated
the effect of storage on the stability of solutions of some
organic and inorganic As species at room temperature
and at 4 °C. The results indicated that organic arsenic
species are stable during short-term storage, whereas
solutions of inorganic species were only stable in
refrigerated conditions.
Another example where speciation has become the
rule is the monitoring of mercury. Exposure to the toxic
alkylated species must be discerned from that of expo-
sure to elemental mercury or its inorganic salts (Horvat
and Gibicar, 2005). Methylmercury is bioaccumulating
to mg/kg
1
levels in the top predators of the food chain,
making up 90-100% of the total Hg concentration.
Exposure to mercury vapor is highly toxic, because it is
easily absorbed in the lungs into the bloodstream, from
where a major share crosses the blood-brain barrier
and even the placenta barrier (see Chapter 33).
One more element in which only the measurement
of species is most relevant is tin. The widespread use of
organotin compounds (OTCs) has led to their entrance
into various ecosystems and in the food chain. Because of
the high toxicity at even very low levels, tributyltin and
triphenyltin have received great attention. These com-
pounds (as well as the complete family of OTCs) are
very persistent and represent a signifi cant problem for
the coming years. The analytical method to monitor the
extremely low concentrations of these compounds in
humans is possible, but, at present, the analyses require
substantial preconcentration and sample cleanup.
Therefore, they are time consuming and costly, not to
say impossible, if too large a sample of blood or tissue is
requested (Rosenberg, 2005).
8.3 Biological Monitoring
Biological monitoring consists of the continuous or
repeated measurement of potentially toxic substances
or their metabolites or their biochemical effects in tis-
sues, secreta, excreta, expired air, or any combination of
these to evaluate occupational or environmental expo-
sure and health risk by comparison with appropriate
reference values based on knowledge of the probable
relationship between ambient exposure and resultant
adverse health effects (Duffus, 1993). The purpose is
to obtain an integrated estimate of the uptake of metal
species through all pathways and media of exposure.
The interpretation of the data requires knowledge of
the absorption, metabolism, and excretion of the metal
species in question. It becomes more and more evident
that knowledge of total element concentrations in par-
ticular biological fl uids and tissues is not suffi ciently
relevant. A typical example is arsenic, which is absorbed
by humans as inorganic arsenic. It is methylated for the
larger part fi rst to monomethyl arsonic acid and next
to dimethylarsinic acid. The inorganic and methylated
species are the compounds to be specifi cally monitored
in the urine of people exposed to inorganic arsenic from
drinking water or through inhalation. This will allow
discrimination against arsenic uptake from eating fi sh
and seafood, where the element is mainly present as
nontoxic arsenobetaine and arsenosugars (Buchet, 2005;
Francesconi, 2002). Arsenobetaine progresses unaltered
throughout the gastrointestinal tract and is excreted in
the urine. Arsenosugars, however, are metabolized, and
approximately 12 metabolites have been documented
(Raml
et al.
, 2005). Before starting on speciation of the
arsenic species, one should be aware that this element
is easily subject to contamination from reagents, dust,
and laboratory ware at the
g/L
1
level. If contamina-
tion occurs, it will most probably be in the form of inor-
ganic arsenic and not organic arsenic. Another aspect
to consider is the stability of the species. An extensive
study by Feldmann
et
al.
(1999) on the stability of com-
mon arsenic species such as arsenite (As
III
), arsenate
(As
V
), monomethylarsonic acid (MMA), dimethyl-
arsinic acid (DMA), and arsenobetaine in urine shows
that low temperature conditions (4 and −20 °C) are suit-
able for the storage of samples for up to 2 months. For
longer periods (4-8 months) the stability of the arsenic
species was dependent on the urine matrix. Whereas
the arsenic species in some urine samples were sta-
ble up to 8 months at both 4
°
and − 20 °C, other urine
samples showed substantial changes in the concentra-
µ
9 SEPARATION TECHNIQUES
Species separation is mainly achieved by one of the
following well-known techniques: liquid chromatogra-
phy (LC), gas chromatography (GC), capillary electro-
phoresis (CE), and gel electrophoresis (GE). The choice
will be determined by the chemical properties of the spe-
cies, the available skills and infrastructure in the labora-
tory and, last, but not least, by the available resources.
9.1 Liquid Chromatography
The sample is introduced into a chromatographic
column packed with a stationary phase while a liq-
