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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
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Sulfuric (H2SO4) 338 Useful for releasing a volatile product; good oxidizing properties for ores, metals, alloys, oxides and hydroxides; often used in combination with HNO3. Caution: H2SO4 must never be used in PTFE vessels (PTFE has a melting point of 327°C and deforms at 260°C)
Nitric (HNO3) 122 Oxidizing attack on many samples not dissolved by HCl; liberates trace elements as the soluble nitrate salt. Useful for the dissolution of metals, alloys and biological samples
(continued overleaf)
Table 5.1 (continued )
Methods for Environmental Trace Analysis
Acid Boiling point (°C) Comments
Perchloric (HClO4) 203 At fuming temperatures, a strong oxidizing agent for organic matter. Caution: violent, explosive reactions may occur - care is needed. Samples are normally pre-treated with HNO3 prior to addition of HClO4
Hydrofluoric (HF) 112 For digestion of silica-based materials; forms SiF62- in acid solution. Caution is required in its use; glass containers should not be used, only plastic vessels. In case of spillages, calcium gluconate gel (for treatment of skin contact sites) should be available prior to usage; evacuate to hospital immediately if skin is exposed to liquid HF
Aqua regia (nitric/hydrochloric) A 1:3 vol/vol mixture of HNO3:HCl is called aqua regia; forms a reactive intermediate, NOCl. Used for metals, alloys, sulfides and other ores - best known because of its ability to dissolve Au, Pd and Pt
aProtective clothing/eyewear is essential in the use of concentrated acids. All acids should be handled with care and in a fume cupboard.
Figure 5.1 Schematic of a commercial acid digestion system.
Once the choice of acid is made, the sample is placed into an appropriate vessel for the decomposition stage. The choice of vessel, however, depends upon the nature of the heat source to be applied. Most commonly, the acid digestion of solid matrices has been carried out in open glass vessels (beakers or boiling tubes) using a hot-plate or multiple-sample digestor. The latter allows a number of boiling tubes (6, 12 or 24 tubes) to be placed into the well of a commercial digestor (Figure 5.1). In this manner, multiple samples can be simultaneously digested. The US Environmental Protection Agency (EPA) methods for the acid digestion of sediments, sludges and soils are outlined in Figures 5.2 and 5.3.
Figure 5.2 The EPA procedure for the acid digestion of sediments, sludges and soils using a hot-plate: GFAAS, graphite-furnace atomic absorption spectroscopy; FAAS, flame atomic absorption spectroscopy; ICP-MS, inductively coupled plasma-mass spectrometry; ICP-AES, inductively coupled plasma-atomic absorption spectroscopy [1].
Methods for Environmental Trace Analysis
Figure 5.3 The EPA procedure for the acid digestion of sediments, sludges and soils, involving a hot-plate, used for the determination of Sb, Ba, Pb and As [2].
The digestion of foodstuffs can be carried out in a similar manner to that described for sediments, sludges and soils. A typical procedure for the digestion of foodstuffs, e.g. cereals, meats, fish and vegetables (excluding root vegetables) for the determination of total metal content (e.g. cadmium, copper, iron, lead and zinc) is described in Figure 5.4. If the level of metal in the sample is below the detection limit of the analytical technique being used it is necessary to pre-concentrate the metal present in the digest. One approach to this is the application of chelation-extraction using ammonium pyrrolidine dithiocarbamate-diethylammonium diethyldithiocarbamate (APDC-DDDC) into 4-methylpentan-2-one (MIBK) (see also Chapter 6). In this procedure, a 10 gl-1 concentration of APDC with DDDC is used as a 1 vol% aqueous solution. The procedure adopted is as follows: 1 ml of the APDC-DDDC solution is added to 80 ml of sample digest in a suitable sample container and mixed on a vortex mixer for 10 s. Then, the metal chelate is extracted by the addition of 10 ml of MIBK and further vortex mixing for 20 s. After allowing this resultant mixture to stand for 5 min, the organic layer (MIBK containing the metal chelate) is removed and analysed. Metal standards for quantitation should be prepared in the same manner, i.e. chelated and extracted.
An alternative approach to conventional heating involves the use of microwave heating.
Figure 5.4 A typical procedure for the acid digestion of foodstuffs, e.g. cereals, meats, fish and vegetables, used for the determination of total metal content [3].
5.4.1 Microwave Digestion
The first reported use of a microwave oven for the acid digestion of samples for metal analysis was in 1975 [4]. Advances in technology by a variety of manufacturers mean that today there are two types of microwave heating systems
Methods for Environmental Trace Analysis
commercially available, i.e. an open-focused and a closed-vessel system (for background information on microwave heating, see Box 5.1). In the open-style system, up to six sample vessels are heated simultaneously. A typical commercial system is the ‘Simultaneous Temperature Accelerated Reaction’ (STAR™) system from the CEM Corporation, USA. A schematic diagram of an open-focused
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