Environmental Engineering Reference
In-Depth Information
DIFFERENTIAL REDUCTION
As described above for aqueous samples, hyphenated
techniques have also been developed for the determination
of Hg species in solid phases. Shade (2008) describes a tech-
nique for the determination of inorganic Hg and MMHg
in biotic samples using Hg-thiourea complex ion chro-
matography with CVAFS detection. Mercury species were
extracted using an acidic leaching solution containing thio-
urea. The cationic Hg-thiourea complexes are separated by
ion chromatography, followed by sequential oxidation of
the methylmercury ion (CH 3 Hg ) to inorganic Hg(II), and
stannous chloride reduction of Hg(II) to Hg 0 to permit
detection by CVAFS. The method achieved detection limits
of ~0.5 ng/g for both MMHg and inorganic Hg(II) using
sample masses as low as 100 mg. Hight and Chen (2006)
describe an HPLC-ICP-MS technique using L -cysteine
extraction for the determination of MMHg and total Hg
in seafood. They achieved a detection limit of 7 ng/g for
MMHg and 5 ng/g for inorganic Hg using a 0.5-g sample.
Chang et al. (2007) describe an HPLC-ICP-MS technique for
the determination of Hg (and lead) speciation in fi sh sam-
ples using 2-mercaptoethanol and L -cysteine microwave-
assisted extraction. They achieved a detection limit of
10 ng/g for both inorganic Hg and MMHg. Even though
these latter methods do not have the sensitivity of the
Shade method, they are nonetheless adequate for mea-
suring Hg species in biologic samples. Other hyphenated
methods have been described by Santoyo et al. (2009),
Margetínová et al (2008), and Chen, J. et al. (2009), but
the sensitivity of these methods may not be adequate for
some biologic samples from lower trophic levels, where Hg
concentration levels are low.
Magos (1971) reported a method in which the inorganic Hg
i in a in a l k a l i in e - d i ge is t e d is a mp l e i is is e l e c t i v e l y r e d u c e d b y is t a in -
nous chloride, while organo-Hg compounds are reduced
to elemental Hg by a combination of stannous chloride
and cadmium chloride. The released elemental Hg can be
measured by CVAAS. The method has been successfully
applied to biologic samples in toxicologic, epidemiologic,
and clinical studies. May et al. (1987) developed a method
using CVAAS for the detection of organo-Hg compounds
after preseparation of organo-Hg by anion exchange. Other
researchers have used volatilization and trapping on cyste-
ine paper (Zelenko and Kosta, 1973) or water vapor distilla-
tion (Horvat et al., 1993). Organo-Hg compounds must be
destroyed by either UV irradiation or acid digestion prior to
detection by CVAAS. In most biologic samples, the organo-
Hg concentrations usually correspond to MMHg. In some
environmental samples, such as sediment, soil, and water
samples, the concentrations of organic Hg (particularly if
separated by anion exchange) have been found to be much
higher than those of MMHg compounds. This is probably
due to presence of some other organic mercury compounds
that have not, as yet, been identifi ed.
SEPARATION OF MERCURY SPECIES OF INTEREST
In addition to choosing proper extraction techniques,
it is essential to choose proper chromatographic condi-
tions for the separation of the organo-Hg species. Apart
from the above-mentioned problems associated with the
extraction of organo-mercurials, problems also exist in
the chromatography of organo-mercurial halides. Many
investigators have recommended that columns packed
with 5% DEGS-PS on 100-120 mesh Supelco support be
used. Some other polar stationary phases have also been
used (e.g. PEGS, Carbowax 20M, Durapak, Carbowax 400,
PDEAS and HIEFF-2AP). In order to prevent ion-exchange
and adsorption processes on the column (which cause
undesirable effects such as tailing, changing of the reten-
tion time, and decrease of peak areas/heights) passivation
of the packing material is needed with Hg(II) chloride in
benzene (O'Reilly, 1982). Although the more inert nature
of capillary columns would be expected to minimize such
effects, improved chromatographic performance over
packed columns cannot be readily achieved. Some work-
ers still prefer to use packed columns because the analyti-
cal protocols using capillary columns require additional
research to optimize performance. The following capillary
columns have so far been reported to give good results:
OV-17 WCOT, Beijing Chemical Industry Works; Superox
20M FSOT, and OV 275. Several workers have chosen to
derivatize Hg species to their corresponding nonpolar,
alkylated analogues such as butyl derivatives, which
can then be separated on nonpolar packed or capillary
columns (Bulska et al., 1992).
QUANTIFICATION
Various detectors can be used in combination with chro-
matography for the determination of Hg species. The
ECD is a very sensitive detector with an absolute detec-
tion limit of approximately a few picograms. It does not,
however, measure Hg directly, but responds to the halide
ion attached to the CH 3 Hg ion. The identifi cation of
small MMHg peaks can sometimes be subject to positive
systematic error owing to co-eluting contaminants. The
use of an ICP-AES detector, a mass spectrometric detector,
CVAAS, CVAFS, or ICP-MS can avoid such problems, since
Hg is measured directly. Miniaturized automated specia-
tion analyzers have been developed for the determination
of organo-Hg compounds, based on microwave induced
plasma emission detector (Slaets and Adams, 2000).
Other Methods
Gage (1961) presented the fi rst practical method for dif-
ferentiating between organic and inorganic Hg. It was a
colorimetric method in which organo-Hg compounds were
extracted into an organic solvent and determined spectro-
photometrically to be dithizone complexes. The method
suffers from low sensitivity. Simple extraction procedures
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