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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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5.4.3 Fusion
Some substances, e.g. silicates and oxides, are not normally destroyed by the
action of acid. In this situation, an alternative approach is required. Fusion
involves the addition of an excess (10-fold) of reagent to the sample (finely
ground) which is placed in a metal crucible, e.g. Pt, followed by heating in a
Solids
65
muffle furnace (300-1000°C). After heating for a period of time (minutes to hours), a clear ‘melt’ should result, thus indicating completeness of the decomposition process. After cooling, the melt will dissolve in a mineral acid. Typical reagents include sodium carbonate (12-15 g of flux required per g of sample; heat to 800°C; dissolve with HCl), lithium meta- or tetraborate (10-20 fold excess of flux required; heat to 900-1000°C; dissolve with HF), and potassium pyrosulfate (10-20 fold excess of flux required; heat to 900°C; dissolve with H2SO4). The obvious addition of excess reagent (flux) can lead to a high risk of contamination. In addition, the high salt content of the final solution may lead to problems in the subsequent analysis.
DQ 5.4
What problems might result from a high sample salt content?
Answer
A high salt content can cause problems in the analysis step. For example, a high salt content can block the nebulizer used for sample introduction in both flame atomic absorption spectroscopy and inductively coupled plasma-based techniques (see Chapter 11).
5.5 Speciation Studies
Speciation is defined as ‘the process of identifying and quantifying the different defined species, forms or phases present in a material’ or ‘the description of the amounts and types of these species, forms or phases present’. In some cases, it is possible to identify, by using single or sequential extractions, operationally defined determinations which identify ‘groups’ of metals without clear identification. In this situation, it is possible to refer to, for example, ethylenediaminete-traacetic acid (EDTA)-extractable trace metals. The reasons why speciation is important is that metals and metalloids can be present in many forms, some of which are toxic.
DQ 5.5
Are you aware of any chemical forms in which metals and metalloids can be present?
Answer
As an example, chromium is found in two different oxidation states, i.e. Cr(in) and Cr(vi). However, while Cr(in) is beneficial to humans (within a certain concentration range), Cr(VI) is toxic, thus leading to theforma-tion of cancers. This particular information has recently been brought to the attention of the public via the film ‘Erin Brockovich’, starring Julia Roberts. This film, which was released in 2000, is a true story based
66
Methods for Environmental Trace Analysis
on the discharge into groundwater of Cr(vi) from the Pacific Gas and Electric (PG and E) Company in the USA and the subsequent legal action for damages.
Information related to species-specific issues is rather limited, however, with details currently only known for a limited number of metals and metalloids, e.g. arsenic, tin and lead. While the example for Cr was obviously water-related, these issues are also important in soil (and sediment analysis), as well as for food matrices. The latter, for obvious reasons, inasmuch as we may well be eating the food, while in the case of the former because food crops may be grown in contaminated soil prior to consumption and entry into the food chain.
One approach to investigate the speciation of metals and metalloids in environmental samples has been the linking of chromatographic separation with quantitation by atomic spectroscopy. In this situation, the use of a suitable chromatographic technique, e.g. gas or liquid chromatography, is being used to separate a metal complex prior to detection of the metal by an atomic spectroscopic technique, e.g. inductively coupled plasma-mass spectrometry (see Chapter 11). However, while chromatography is capable of highly reproducible separations, it can only accept liquid samples. In this situation, the use of specific methods of extraction are required to remove the extract, without altering its chemical form (speciation) from the sample matrix.
5.6 Selected Examples of Metal Speciation
5.6.1 Mercury
All forms of mercury are considered to be poisonous. However, it is methylmer-cury (or as the chloride, CH3HgCl) which is considered to be the most toxic because of its ability to bioaccumulate in fish. The best example of the toxicity of methylmercury occurred in Minamata in Japan in 1955. It was found that methylmercury-contaminated fish consumed by pregnant women resulted in the new-born children having severe brain damage (Minamata disease). As a consequence of these initial findings, methylmercury is routinely monitored for in fish.
A variety of methods exist for the extraction, clean-up and subsequent analysis of methylmercury in samples. Most methods involve a solvent extraction, often with toluene, of the methylmercury from the sample, followed by separation by gas chromatography with an electron-capture detector. The specific details for methylmercury extraction from fish tissue are shown in Figure 5.10. A similar approach can also be applied for the extraction and analysis of methylmer-cury from sediment. A summary of the methods used for the determination of methylmercury in fish and sediment matrices is given in Table 5.2. This has also been recently reviewed [13].
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