[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Like for the preconcentration of traces from water

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Like for the preconcentration of traces from water samples, various kinds of coprecipitation or ion exchange methods have been proposed for biolog- ical samples. A number of chemical modifiers, such as magnesium nitrate, nickel nitrate and ammonium pyrrolidine dithio carbamate (APDC) coupled with magnesium nitrate, and air-drying of the sample, make an effective sam- ple preparation method for the determination of a wide range of elements at different concentration levels, from ppb’s to ppm’s [87] For example, a very low concentration of Cr (0.3 ng mL 1 ) in plasma could be determined by complexation with APDC and extraction with methyl isobutyl ketone. After evaporation of organic solvent, the residue is dissolved in acid and deposited on a thin polycarbonate foil. Hair samples (0.1 g) can be digested with a 1:5 mixture of HClO 4 and HNO 3 in a Teflon bomb and the trace elements are concentrated using APDC at pH 3–4. In this way, elements such as Fe, Ni, Cu, Zn and Pb could be determined down to 0 . 4 µ g g 1 . The same limits of detection could be achieved if, e.g., 1 g of hair sample is pelletized with 0.1 g CaO and 20 g cellulose. The wet digestion methods, by microwave or high pressure, are very effective. With a mixture of HNO 3 , H 2 SO 4 and HClO 4 , most organic ma- terial can be attacked. Even more effective is a combination of HNO 3 and HF as also silicates in organic materials are dissolved. Care should be taken to work, whenever possible, in closed conditions to avoid contamination and/or losses of elements. The combination of microwave and pressure digestion al- lows quantitative recovery of elements that may be volatilized in open digests (like Ge, Se, Hg), as well as the recovery of noble metals (Pt, Os, and Ir) to the ppb level in solid samples even in the extremely difficult digestion of organic material. For a basic understanding of the microwave acid digestion theory, in- cluding safety guidelines and dissolution methods for geological, metallurgical, biological, food and other samples, we recommend the professional reference book edited by Kingston and Jassie [88]. 6.5 Aerosol and Dust Specimens XRF is frequently invoked for trace analysis in air pollution studies. In the air, trace elements are almost exclusively in the particulate phase at typical con- centrations of 50–500 ng m 3 . By simply drawing a large air volume through a filter, large preconcentration factors are easily achieved. Adequately loaded filters with air particulate matter are presented directly to the XRF unit. Unless the filters are extremely loaded, no correction for the X-ray absorption in the aerosol material is usually necessary.
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428 J. Injuk et al. The part of the filter with a sample actually analysed by XRF has an area of only a few square centimeters and hence it must be representative for the entire sample. Therefore, a critical step is the selection of an appropriate filter material. Teflon and polycarbonate (Nuclepore) filters are ideal because of their high purity and because they are surface collectors. Whatman-41
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