Environmental Engineering Reference
In-Depth Information
Organo-Mercury Species in Other
Environmental Matrices
sterilized either by autoclaving or by gamma irradiation.
This important step prevents bacteriologic activity, which
may otherwise lead to methylation and demethylation
processes. In general, very little is known about the effects
of sterilization on the stability of MMHg compounds. More
studies are needed to investigate the stability of organo-Hg
compounds in biologic samples, particularly under various
sample preparation and long-term storage conditions.
Signifi cant external contamination of biologic samples
with MMHg is unlikely to occur; however, precautions are
necessary to avoid contamination by inorganic Hg.
Blood and hair samples are often analyzed in order to
estimate exposure of humans to Hg and its compounds.
Blood and hair should be collected as described above. After
blood has been sampled without an anticoagulant, it will
clot spontaneously and separate into serum and packed
cells. The process takes 15 to 30 minutes at room tem-
perature. The samples should then be centrifuged within
1 hour. This separation should be done as soon as possible
to avoid hemolysis of blood. Hemolyzed samples cannot
be considered for speciation analysis. The distribution of
the different trace element species between serum and cells
may vary by several orders of magnitude. Serum and packed
cells may be deep-frozen only once, as repeatedly frozen
and thawed samples have shown a remarkable decrease in
MMHg concentrations. There is also some evidence that
MMHg may be destroyed during lyophilization of blood
samples (Horvat and Byrne, 1992; LaFleur, 1973).
Analysis of human hair offers several advantages over
analysis of blood samples, such as ease of sampling and
sample storage. The concentration of MMHg in hair is
approximately 250 times higher than in blood, and analy-
sis of different longitudinal sections of hair can give infor-
mation on the history of the exposure to MMHg ingested
through food. Adhering dust and grease should be removed
by washing with a solvent such as hexane, alcohol, ace-
tone, water, diethylether, or detergents. The International
Atomic Energy Agency (IAEA) and the World Health
Organization (WHO) recommend the use of only water
and acetone (UNEP/WHO/IAEA 1987). Long-term storage
of human hair samples has shown that MMHg is stable for
a period of a few years if stored dry and in the dark at room
temperature (Horvat, 2005).
Sampling and Storage
SEDIMENTS AND SOILS
Monomethylmercury generally does not occur in sedi-
ments at a level of more than about 2% of the total Hg pres-
ent. There appears to be an approximate equilibrium level
between MMHg production and destruction. Methylation-
demethylation reactions are assumed to be widespread
in the environment, and each ecosystem attains its own
steady state with respect to the individual species of Hg.
Dimethylmercury is considered to be unstable in sedi-
ments, but is assumed to be stabilized by a conjunction
of factors, such as high sulfi de levels, salinity, anoxic con-
ditions, and constant inputs of methane into the media
(Weber et al., 1998).
Sediment and soil samples should be collected and stored
with caution because the percentage of MMHg in these
samples is very low (e.g.,
2% of total Hg) and improper
handling and storage may lead to inaccurate results.
Changes in in situ pH, reduction-oxidation potential and
moisture content as a result of sampling can lead to conver-
sion of Hg species (methylation, demethylation, or reduc-
tion) and may signifi cantly infl uence the stability of MMHg
in sediments. This is particularly true in the case of sedi-
ments taken from zones where bottom waters are oxygen-
depleted (Horvat et al., 2004). Samples collected from such
environments are best analyzed fresh or, if long-term stor-
age is unavoidable, samples should be kept in the dark in
an inert atmosphere and deep frozen.
BIOLOGIC SAMPLES
In upper-trophic-level piscivorous fi sh, the percentage of
MMHg to total Hg in fi sh muscle varies from 80 to 100%,
but in other organs its concentration is smaller (typical
values in liver and kidney are
20%). In lower-trophic-level
fi sh and other aquatic organisms, the percentage of MMHg
is much more variable. Relatively high Hg and MMHg
concentrations have been reported for fi sh-eating marine
birds. Birds feeding on wild vegetation generally have much
lower levels of Hg. There have been many studies performed
on terrestrial ecosystems (particularly in Canada, Sweden,
and the United States). Mercury also passes from vegetation
into the food chains of fauna (Gnamuš et al., 2000).
Most biologic samples are preferably analyzed fresh or
after lyophilization. Deep-freezing of fresh samples, especially
with long storage, should be avoided, since it has been
observed that in some organisms MMHg may decompose
with repeated freezing and unfreezing (particularly in
bivalves) (Horvat and Byrne, 1992). Monomethylmercury
and total Hg in freeze-dried biologic samples, such as
biologic CRMs, are stable for years. CRMs are, however,
Determination of Organo-Mercury
in Solid Matrices
Environmental samples can have complex matrices, neces-
sitating that several steps be used to release the organic Hg
from the sample prior to analysis: (1) liberation of MMHg
from its matrix, (2) extraction/cleanup/preconcentration,
(3) separation of Hg species of interest, and (4) quanti-
fi cation. These steps can vary widely from one matrix to
another, depending on the potential interferences present
in the sample and how tightly bound the organic Hg is
within the sample matrix.
