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

Chelat ing provides more specificity toward

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is iminodiacetate fixed on a poly(styrene-divinylbenzene) support. Chelat- ing provides more specificity toward transition metals compared to simple ion exchangers. A typical preconcentration method involves column or disk arrangement of the resin and then the water sample is passed through it. Following this preconcentration step, the resin is either measured directly or after pelletizing [293]. An improved detection limit can be obtained this way: DL below 1 ppb was presented for Zn, Cd, Hg, and Pb taking 1 l water sample and 1000 s measurement time. A variant of chelating preconcentration is the polyurethane foam disk loaded with an appropriate agent (e.g., diethylammonium diethyldithiocar- bamate) [294]. In optimizing the performance of the analysis, sample (disk) thickness proved to be essential. Increasing the mass (thickness) probed by the X-ray will result in higher fluorescence intensity and lower DL up to a certain extent. For much thicker samples, however, the background will in- crease drastically, leading to worse DL. This implies that a minimum DL can
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610 S. Kurunczi et al be found as a function of sample thickness [295]. Optimizing the polyurethane foam thickness the authors obtained DL for Co and Zn at 10 and 3 ppb, re- spectively after preconcentration from 25 ml and taking 600 s counting time. A simplified method of chelating disk method has been published recently preconcentrating 1 l drinking water [296]. The detection limits for Cd, Ni, Cu, and Pb were 3.8, 0.6, 0.4, and 0.3 ppb, respectively. The preconcentration can be conducted in the field, eliminating the transport of large volume water samples. As seen from above, absorption often occurs in conventional XRF, thus a correction is needed to obtain a reliable result. There are different approaches (external standards, internal standards, dilution method, emission- transmission method and mathematical corrections) to the absorption correc- tion and these approaches request a given time depending on the difficulty of the analytical problem [297]. Using the afore-mentioned methods, quan- tification is usually straightforward. Thin film technique represents a special case where no absorption takes place, making the quantification much eas- ier. The simplest method for thin film preparation is to place a liquid drop on a membrane filter or backing foil and dry it. The fluorescence intensities from extremely thin residues can be very low because of the low amount of mass probed, resulting in a high DL. However, with an optimized method it is possible to approach the results obtained by TXRF [298]. In another study, Meltzer and King [299] concluded that direct excitation is more advantageous than secondary target mode because of the gain in intensity. Detection lim- its were obtained at the low ppb level, enabling direct analysis of reference waters with 5–10% accuracy. At this concentration level the purity of back- ing material is also an important factor; Mylar and Formvar films have been proved to be acceptable, clean, and economic materials. One can instantly conclude the method of choice that would be the evaporation of large vol- umes for thin sample preparation. Because of the high salt content possibly
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  • Spring '14
  • MichaelDudley

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