Discussion 713 Determination of trace elements in plants and feeds

The official EC method for Fe, Cu, Mn and Zn in feeding stuffs is document 378L0633 (which is presented in the Official Journal No. L206, 29.07.78, pp. 43-55), and is the Eighth Commission Directive 78/633/EEC of 15 June 1978. The determination of these trace elements appears as Point 3 of the Annex to the Directive. After ashing, the vegetable silica, which may retain some trace elements, is removed by evaporation with HF. This is too hazardous for educational purposes (untreated skin absorption could be fatal), and is not a feature of the AOAC methods. These methods are given for animal feed by Padmore (1990, pp. 84-85), and for plants by Isaac (1990, p. 42). Animal feeds (2-10 g) are ashed at 550°C for 4 h, heated with 3 M HCl, filtered and made up to 100 ml with 0.1-0.5 M HCl before analysis by AAS. Plant material (1 g) is ashed for 2 h at 500°C, 10 drops of water and 3-4 ml 50% v/v HNO3 are added before evaporation at 100-120°C. The residue is again ashed for 1 h at 500°C and dissolved in 10 ml 50% v/v HCl, then transferred to a 50-ml volumetric flask. If Ca or Mg is to be determined, 10 ml of 5% La solution should be added to prevent interference by P in the air-acetylene flame. The

5% La solution is made up by dissolving 58.65 g La2O3 in 250 ml HCl, added slowly, and diluting to 1 l. Final determination is with AAS, and a modified method is also given for determination by ICP. MAFF/ADAS (1986, pp. 23-26) describe a wet-ashing technique using perchloric and nitric acids for digestion, and 6 M HCl for a subsequent boiling. See also the discussion in Chapter 4, 'Dry ashing', which suggests a possible loss of FeCl3 at temperatures exceeding 450°C, which is the ashing temperature given in the method described below.

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