Speciation of Mercury

Background

Mercury has been known to man for millenia. Even though the most excessive use in industry and agriculture has ceased due to the poisonous and insidious nature of mercury and mercury compounds, methylmercury may still be found at high concentrations (0.5 - 1.0 mg/kg) in fish caught in rurual parts of Sweden, Canada and the US. Since most of the mercury found in the atmosphere is in its metallic form, in natural waters as divalent inorganic mercury, and in fish as monomethylmercury, the need for analytical methods able to differentiate between these mercury species is obvious (such determinations are termed "speciation"). Although many methods for the determination of total mercury exist, extraction of useful information concerning global cycling of mercury is limited. The following examples will give a clearer picture. One of the major concerns with respect to mercury and pollution is the elucidation of the pathways for conversion in the environment, i.e., if the major source of the highly toxic methylmercury is biomethylation of inorganic mercury or long distance transport of atmospheric methylmercury. Since the anthropogenic input makes up 50 % of the global mass balance of mercury, human activities have a profound effect in distributing mercury over the globe. Combustion of coal and petroleum products slowly increases the mercury load in the atmosphere, and thereby gradually augments the mercury load, even in remote ecosystems. In order to reduce the levels of mercury in fish, the emissions of mercury to the atmosphere should really be cut drastically.

Environmental cycling of mercury species

Another example of mercury compounds causing problems, which is literally right in front of our noses, is the use of nasal sprays containing ethylmercury chloride as a preservative. Excessive use of nasal sprays may actually lead to damage to the mucous membranes.

These examples serve to shed some light on what kind of samples may be of interest, i.e., air, precipitation, sediments, fish tissues, fresh waters, coal, crude oil, nasal sprays, etc. Other factors, such as high demands on the instrumentation used with respect to detection power, selectivity and sensitivity, must also be considered.

Analytical measurements

In analytical measurements, all efforts are more or less futile if the results obtained are not accurate. In order to achieve accurate results, analytical control must be exercised, preferentially by using certified reference materials. Such materials are issued by such agencies as the Commission of the European Communities' Standards, Measurements and Testing Programme (formerly BCR), the National Institute of Standards and Technology (NIST, USA) and the National Research Council of Canada (NRCC). The concentrations of various metals or metal species (mainly organometallic species) are certified within a specified interval. The materials are often supplied as lyophilized powders of, for example, sediments or biological materials. If a suitable reference material resembling the sample is analysed in parallel, then the quality of the results can be assessed.

The work performed in this group with respect to mercury has been devoted to development of highly sensitive and selective methods for mercury speciation in a wide variety of samples. An important part has been participation in various certification campaigns for assessment of the analytical results obtained. Another interest has been mercury speciation in natural waters at sub ng/L levels. Much more information may be found in a recent thesis, which will soon be available on Internet.

A list of references to our work on the speciation of mercury can be found here.

Analytical method

Although a variety of analytical methods for the determination of mercury species already existed when this project commenced in 1988, an alternative one was developed here in Umeå. The detection system is based on microwave induced plasma - atomic emission spectrometry (MIP-AES), which provides an absolute detection limit of 0.4 pg Hg. A schematic illustration of the MIP is shown on the left. Gaseous mercury species are introduced via (1) a deactivated, fused silica transfer line. (2) Graphite seals position an (3) inner and (4) outer ceramic tube, separated by a (5) helical wire, in a concentric torch assembly, inside a (6) Beenakker TM010 microwave resonant cavity. Helium is supplied through separate inlets for the (7) plasma and (8) shielding gas. The plasma discharge produced (9) excites mercury atoms, which emit light at specific wavelengths that can be detected and used to quantify the amount of analyte introduced.

Separation of mercury species is achieved by gas chromatography (GC), following suitable sample preparation. The preparatory steps common to most of the sample types analysed to date (blood, edible fish tissues, natural waters, sediments) include extraction into an organic solvent as diethyl- dithiocarbamate complexes at a pH of about 7, and derivatization using the Grignard reagent butylmagnesium chloride:-


CH3HgX + BuMgCl => CH3 HgBu + MgXCl

HgX2 + 2 BuMgCl => Bu HgBu + 2 MgXCl

In this way, simultaneous determination of methylmercury and inorganic mercury, the two most commonly occurring mercury species in the environment, is feasible.

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This document was last updated on August 26, 1996 by

Håkan Emteborg
Department of Analytical Chemistry
Umeå University
S-901 87 Umeå
Sweden