13. Mercury speciation in biological matrices

Abstract

In nature, mercury occurs in several forms, e.g.metallic mercury, inorganic mercury and organic mercury compounds. All forms of mercury are considered poisonous, but methyl-mercury is of particular concern since it is extremely toxic and is frequently found in the environment. Through a very effective biomagnification mechanism, methyl-mercury is enriched in food chains which results in high levels in top predators, e.g. fish such as northern pike and tuna. In Minamata (Japan), methyl-mercury contamination caused severe brain damage of 22 infants whose mothers had ingested contaminated fish during pregnancy [1]. Intake of wheat flour from seeds treated with organic mercury has also led to large scale poisoning, e.g. methyl-mercury treated seeds caused a dramatic contamination of humans in Iraq in 1971-72 [2]. In general, exposure to organic mercury can cause brain damage to a developing foetus [3]. The exposure is considered to be more dangerous for young children because their nervous systems are still developing and thus are more sensitive to these compounds. The toxicity of mercury is extensively documented by WHO [4,5]. Due to (i) the toxicity of organic mercury, (ii) the fact that organo-mercury compounds can be formed in nature (methyl-mercury and probably dimethyl-mercury), and (iii) the bio-accumulation of methyl-mercury, there has been a great interest in trying to understand the distribution patterns in nature and the interconversion between the different mercury compounds. Many of these studies have been reported in various books and reviews, e.g. [6-9]. A study of the speciation of mercury is necessary to understand its pathway, to assess its toxicity and to develop strategies for its decontamination. As an example, the perception of the mercury cycle has undergone a tremendous change after the development of more accurate and sensitive methods for mercury speciation (Figs, la and lb). It is now realized that instead of having one mercury cycle in which mercury essentially originates from the bio-methylation of mercury in sediments, methyl-mercury in fish can originate from an atmospheric depositional flux of methyl-mercury. In other words, mercury speciation analysis should be done to assess the direct influence of the individual compounds on the environment, to predict the transport behaviour in the various compartments, and to develop an approach for the decontamination of polluted areas. During the last three years, most of the experience on the determination of methyl-mercury has been gathered in connection with the development of new derivatization methods. Furthermore, valuable experience has been obtained from a recent certification exercise carried out by the EC Community Bureau of Reference (BCR, now Measurements and Testing Programme). This chapter will focus on these two issues, starting with the outcome of the BCR certification exercise which is discussed in paragraph 13.1.