Analytical methods for biological specimens

As thallium usually occurs in the //g/kg and low mg/kg range, the analytical procedure must achieve sufficient sensitivity, and possibly include enrichment steps. Only a few multielemental techniques, such as the old optical emission spectrography, proton induced X-ray emission, or mass spectrometric techniques yield reliable data on the trace levels usually encountered. In most cases, there has to be a proper investigation for thallium, possibly leaving some of its environmental impacts unknown. 

Less common analytical methods, or those not suitable for the economical analysis of thallium in biological matrices are omitted for reasons of space. These include atomic fluorescence, solid-sampling atomic absorption. X-ray fluorescence, gas chromatography, and ion-sensitive electrodes. They have been reviewed intensively, e.g. by Sager (1986), and by Griepink et al. (1988). 

Recently, much progress has been made in mass spectrometric techniques (especially ICP-MS), and first attempts in speciation studies. Atomic spectrometric and electrochemical determination methods are being developed in terms of application to real matrices. 

In general, working with dilute solutions requires attention to adsorption at the vessel walls. From weak acid solution, Tl adsorption was found to increase in the order poly-ethene-polypropene-glass-paraffin-rubber (Morgan et al., 1967). At borosilicate surfaces, at pH 4, no losses occurred at the 1 mg/L level, whereas at pH 10, there was appreciable adsorption (Smith, 1973). 

The general aim of sample decomposition is the destruction of a representative amount of sample to yield a homogeneous solution, sufficiently mineralized to be suitable for subsequent determination, preferably for multielemental analysis. In the case of thallium, the problem is rather to dissolve a sufficient amount of material to reach the limit of determination than to cope with problems of blanks. 

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