Environmental Engineering Reference
In-Depth Information
LIBERATION OF MONOMETHYLMERCURY FROM
SOLID MATRICES
This extraction method is frequently used in conjunction
with EPA Method 1630 (EPA, 1998b) for the determination
of M MHg in sediment and soil samples. Regardless of which
extraction technique is used, results should be checked
regularly by the use of appropriate CRMs, if available, or by
comparison of the results from different laboratories and/
or the use of different analytical approaches.
The use of ICP-MS with isotope dilution analysis (IDA)
can help overcome problems associated with incomplete
recovery of organo-Hg species, particularly in biologic sam-
ples. If equilibration of the isotopically modifi ed spike and
the sample MMHg is achieved, the spike material acts as
an ideal internal standard. So far, such a protocol has been
successfully applied to numerous environmental and bio-
logic samples (Falter, 1999; Hintelmann, 1999; Snell et al.,
2000; Clough et al., 2003).
Most methods are based on the method originally devel-
oped by Westöö (1966), which involves the extraction of
organo-Hg chloride from acidifi ed homogeneous samples
into benzene (however, the use of toluene is strongly
recommended, for health and safety reasons). Organo-
Hg compounds are then back-extracted into an aqueous
cysteine solution. The aqueous solution is then acidifi ed
and organo-Hg compounds are reextracted with benzene
or toluene. This double partitioning facilitates the removal
of many interferences such as benzene-soluble thiols.
Finally, MMHg is analyzed by GC with ECD.
More recently, several modifi cations have been made to
the Westöö protocol for the separation and identifi cation of
organo-Hg in biologic and other samples. In tissue matrices,
such as homogenized fi sh muscle tissue, a simple digestion
using 25% KOH in methanol (Bloom, 1989) will completely
dissolve the sample, which can then be derivatized and
analyzed as described previously for aqueous samples. In
more complex matrices, such as plant matter and soil/
sediment samples, more rigorous cleanup and extraction
techniques are required. For example, in the initial step the
addition of Cu(II) ions enhances the removal of Hg bound
to sulfur. The method has also been modifi ed in terms of
the quantity of chemicals used. A semi-micro scale method
developed by Uthe et al. (1972) has been widely applied.
However, inorganic Hg cannot be determined using this
procedure unless a reagent is added to form, for example,
alkyl- and aryl-derivatives, which can then be extracted
and determined by GLC (Zarnegar and Mushak, 1974).
In general, solvent-extraction procedures are time
consuming, corrections for the recovery of the procedure
vary from sample to sample, and with some sample types
(e.g., those rich in lipids) phases are diffi cult to separate
because of the presence of persistent emulsions, particu-
larly during the separation of the aqueous cysteine phase.
To overcome these problems, MMHg can be adsorbed on
cysteine paper (instead of into cysteine solution) during
the cleanup stage (Horvat et al., 1988). Using additional
preseparations prior to extraction, such as volatilization of
MMHg in a microdiffusion cell (Zelenko and Kosta, 1973)
and distillation (Horvat et al., 1988, 1994) may also facili-
tate the separation of phases during extraction. Because
various extraction and cleanup procedures are used to
extract organo-Hg from its matrix, it is essential to quan-
tify recovery, especially when speciation is performed on
insoluble samples such as sediments and soils.
It is a lso impor ta nt to note t hat some e xt rac t ion protocols
may lead to artifact MMHg production, especially in proce-
dures in which MMHg is isolated at higher temperatures
(Falter, 1999). A study by Bloom et al. (1997) investigated
artifact formation of MMHg and proposed an extraction
technique using cold acidic bromide and extraction into
methylene chloride that appears to avoid this problem.
EXTRACTION/CLEANUP/PRECONCENTRATION
DERIVATIZATION METHODS
Most methods use the formation of a volatile organo-Hg
derivative (through ethylation, propylation, butylation,
hydration, and iodination) in order to separate the organo-
Hg from the bulk of the sample by simple purge and trap
techniques. The same ethylation method as described
above for water samples has also been applied to biologic
and sediment samples (Bloom, 1989). An aliquot of the
digested or extracted sample is subjected to ethylation by
sodium tetraethylborate, which transforms MMHg into
methylethylmercury. At the same time Hg(II) is trans-
formed into diethylmercury. The two species can be quan-
tifi ed simultaneously (Liang et al., 1994) if diethylmercury
is not naturally present in the sample. If diethylmercury is
present in the sample, a different derivatization technique,
such as propylation, must be used.
Volatile ethylated Hg compounds, as well as elemen-
tal Hg and dimethylmercury, are removed from solu-
tion by aeration and are then trapped on an adsorbent
(Carbotrap or Tenax). Mercury compounds are separated
on a GC column, and pyrolyzed to elemental Hg
0
at high
temperatures (
750°C) for subsequent Hg determination
by CVAFS, CVAAS, or ICP-MS. As mentioned previously,
very low detection limits may be achieved using CVAFS
and ICP-MS (6 pg/L for water and 1 pg/g for biota and
sediment samples). Instead of sodium tetraethyl borate,
sodium borohydride may also be used to form volatile
Hg hydride, which is then quantifi ed by GC in line with
a Fourier transform infrared spectrophotometer (Fillipelli
et al., 1992). CH
3
I formed in a headspace vial may also be
introduced onto a GC column and detected by microwave-
induced plasma atomic emission spectrometry (MIP-AES)
or AFS detectors. Propylation and hydration have also been
applied with great success as described above (Demuth and
Heuman, 2001; Logar et al., 2004).
