Speciation analysis of organolead compounds. Status and future prospects
Résumé
The pollution of the environment by lead is ubiquitous and has a global character. The major source of this element in the environment is the combustion of leaded gasoline. Despite severe restrictions in force in many countries, the use of tetraalkyllead as an antiknock agent remains the largest industrial application of organolead compounds and represents about 5–7 % of the total lead consumption, which is estimated at 3–3.5 M tons [1,2]. Other applications of organolead are less important and include the use of tetraethyllead (Et4Pb) in the manufacture of ethylmercury while trialkyllead compounds have been used as wood preservatives, antifouling agents in marine paints, and components of pesticides [1]. Evidence for the environmental formation of alkyllead from inorganic lead in nature is mainly circumstantial. The amount of organolead produced by biomethylation, if any, is insignificant when compared to the anthropogenic emission [3,4]. The harmful effects of organolead compounds are considered to be much larger than those of inorganic lead [4–6]. The toxicity of alkyllead species diminishes in the sequence R4Pb → R3Pb+ → R2Pb2+ → Pb2+ (where R is a methyl- or ethyl- group) but the ionic forms are more persistent in the environment. In algae and higher plants alkyllead compounds were found to be responsible for the inhibition of growth, disturbances of mitosis and ultrastructural alterations. The organolead contamination of the aquatic environment is known to affect fish. About 150 fatal cases of human intoxication with Et4Pb have been reported in the literature. They were related to accidental exposures but long term environmental exposure to low levels of organolead has been associated with a wide range of metabolic disorders and neurophysical deficits especially to children. Starting in the seventies, the growing concern about the contribution of organolead to the lead burden of the biosphere has stimulated the development of analytical methodologies capable of discriminating between the inorganic lead - Pb(II) and traces (0.1–1 % of the total lead) of organolead - Pb(IV), and further among the different organolead species. The currently accepted approaches are based almost exclusively on the use of hyphenated techniques as discussed in a few monographs[7–9]. These techniques are based on a combination of a separation technique such as gas chromatography (GC) or high performance liquid chromatography (HPLC) with an element sensitive and selective detection technique, usually atomic absorption or emission spectrometry. The use of non-specific detectors is restricted owing to the interferences of many hydrocarbons occurring at high concentrations in real samples. The only noteworthy non-spectrometric technique proposed, differential pulse anodic stripping voltammetry [10,11], shows limited application especially when applied to the analysis of real samples. In the past 5 years, speciation analysis of organolead has been the subject of a few comprehensive review papers [4,12,13]. The very recent advances in instrumentation have rendered the routine detection of lead possible at the sub-pg level. However, severe limitations still remain on the level of sample preparation. Most of the existing procedures are very cumbersome, requiring a large amount of sample and tedious separation-preconcentration steps. Moreover, controversies still exist about the recovery of analytes and the efficiency of their derivatization which depends on the matrix type involved. Ultratrace analysis for organolead in the remote environment is likely to face the blank value problems common in trace analysis for total lead. Effective studies of pathways and natural sources of organolead require more straightforward analytical methods that are capable of the reliable determination of organolead species at concentration levels down to sub-ng.11 in waters and sub-ng.g1 in biological materials or sediments. Because of a large variety of sample types, custom-tailored methods need to be designed. This chapter aims to evaluate the state-of-the-art of methods available for the speciation analysis of organolead in different samples. Advantages and limitations of the analytical procedures applied are critically discussed with respect to sample preparation, Chromatographic separation and detector operating conditions as well as the chromatograph-spectrometer interface design. Very recent developments in speciation analysis for organolead by capillary gas chromatography, atomic emission and mass spectrometry are highlighted. Particular attention is given to the factors affecting accuracy of analyses and to the validation of the results. © 1995, Elsevier B.V.