Detecting Mercury in Fish Tissue
Using Aqueous-Phase Ethylation with GC/AED
by Hilton B. Swan,
Australian Government Analytical Laboratory,
Hobart, Tasmania (Australia)
The National Food Authority in Australia has recommended that the
National Food Standards Council adopt a national maximum permitted
concentration (MPC) of a mean 0.5 mg/kg of mercury in fish and fish
products. An exceptional MPC of 1.0 mg/kg is recommended for products
from all shark and ray species, orange roughy (Hoplostethus atlanticus),
gemfish (Rexea solandri), billfish (Xiphiidae sp. and
Istiophoridae sp.), barramundi (Lates calcarifer), and southern bluefin
tuna (Thunnus maccoyi).
We used an HP 5921A atomic emission detector coupled to an HP 5890 Series II gas chromatograph to determine the concentration and speciation
of mercury in various fish species caught and processed locally. The AED
was found to be a sensitive detector highly specific to the atomic
emission wavelength of 253.652 nanometers for mercury. Using this
wavelength, it is possible to detect Hg down to 0.1 pg/sec peak width
with the GC/AED. The specificity relative to carbon can be as high as
100,000:1.
For fish tissue, it was possible to determine Hg down to about 1 µg/kg
wet weight. An application of GC/ AED for the determination of
organomercurials using the 184.950 nm emission wavelength has been
reported (Carro-Diaz et al., 1994).
Converting Mercury Species
The speciation of Hg (II) present in fish tissue was determined by the
reaction with sodium tetraethyl borate (STEB) in aqueous solution (Yong & Bayona, 1995 and Rapsomanikis & Craig, 1991),
where ionic mercury species are converted to their volatile ethyl
derivatives. Labile CH3HgX species react to form methylethyl mercury,
and labile HgX2 species form diethyl mercury. Nonionic forms such as
dimethyl mercury and elemental mercury do not react with the tetraethyl
borate anion, which makes it possible to determine the various forms of
mercury by GC separation.
A typical GC/AED chromatogram of an ethylated fish sample is shown in
Figure 1. This sample has a relatively low total Hg concentration of
0.05 mg/kg. Methyl mercury is the major form of mercury present in fish.
Upon ethylation with the tetraethyl borate anion, it reacts to form
methylethyl mercury.
Preparing the Sample
In the technique we used, the fish tissue is digested in a solution of
KOH/methanol. A small portion of this digest is added to an aqueous
solution in which it is reacted with STEB. The derivatized volatile
mercury species are then purged from solution onto a graphitic carbon
trap (Bloom, 1989). This trap is thermally desorbed of the mercury species to a
cryogenic trap, which is subsequently warmed to release the sample for
GC separation.
Figure 2 shows the configuration of the valving and cryotrap system
used for the analysis of organic mercury compounds. Actuation of the
8port valve provides a standard loop volume of dimethyl mercury to be
sent to the cryotrap. Actuation of the 6port valve, followed by warming
of the trap, will release the collected sample to the GC/AED.
Complementary Standards
Standardization is achieved by a number of means. Derivatization of a
methyl mercuric chloride solution with STEB will provide an external
standard. A certified dimethyl mercury permeation device also provides a
useful standard, which can be used as an internal standard for the
analysis, because dimethyl mercury is generally a minor organomercurial
or most commonly not found in fish. Comparison of the total mercury
present in fish tissue by coldvapor atomic absorption spectroscopy is
also used as a means of standardization.
Confirmation of the elemental identity of a compound with the AED can be
made using the "snapshot" feature of the software, which displays
spectra obtained over the wavelength range of the diode array. A
background-subtracted "snapshot" of dimethyl mercury at an RT of 6.130
minutes (Figure 3) shows the Hg emission wavelength of
253.652 nm of this compound. A mixed mercury compounds standard
analyzed by GC/AED is shown in the chromatogram of
Figure 4.
- Carro-Diaz, A. M. Lorenzo-Ferreira, R. A., and Cela-Torrijos.
Speciation of Organomercurials in Biological and Environmental
Samples by Gas Chromatography with Microwave-Induced Plasma Atomic
Emission Detection. J. Chromatography A, 683, 245-252 (1994).
- Yong, C. and Bayona, J. M.
Determination of Methylmercury in Fish and River Water Samples
Using in situ Sodium Tetraethylborate Derivatization Followed by
Solid-Phase Microextraction and Gas Chromatography-Mass Spectrometry.
J. Chromatography A 696, 113-122 (1995).
- Rapsomanikis, S. and Craig, P. J.
Speciation of Mercury and Methylmercury Compounds in Aqueous Samples
by Chromatography-Atomic Absorption Spectrometry After Ethylation
with Sodium Tetraethylborate. Analytica Chimica Acta 248,
563-567 (1991).
- Bloom, N.
Determination of Picogram Levels of Methylmercury by Aqueous Phase
Ethylation Followed by Cryogenic Gas Chromatography with Cold Vapour
Atomic Fluorescence Detection. Canadian J. Fish. Aquatic. Sci. 46, 131-1140 (1989).
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