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

flow rate and total volume to obtain a quantitative

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flow rate and total volume to obtain a quantitative recovery and high precon- centration factor. The reverse-phase extraction method discussed above was improved continuously. Recently, an automated SPE extraction version has been reported [323]. The authors reached good precision and accuracy with the replicate analysis of certified reference waters, such as CASS-2 and NASS- 4. It was, however, impossible to decrease the total analysis time compared with manual operation. A promising preconcentration method for seawater has been described [324] basing on electrodeposition of the metal ions from saline solution. Glassy carbon was chosen as sample carrier, which fulfils the requirements for both TXRF and electrochemistry. Duration of electrodeposition was set to 30 min at a potential of 1800 mV and the pH of the solution was adjusted to 4.3. Applying Mo-K α excitation (or W continuum) and 500 s counting, typical DL of 5–20 ng/l were obtained. From the experimental results on some artificial saline waters, the authors found little salinity dependence of the deposition yield. This study clearly shows the utility of the electrodeposition in seawater analysis. The low volume of sample needed for TXRF analysis may be important when the availability is limited. In the case of precious or biological samples this situation is quite common. When considering natural water analysis, the liquid volume is normally not restricted. The small water volume, however, makes the filtration and preparation fast and could prove efficient in reducing contamination too. Lofthouse et al. [325] examined the analysis accuracy for microvolumes of reference water samples. They compared TXRF with ICP-MS equipped with a microconcentric nebulizer. ICP-MS offered better detection limits for a wider range of elements but at least 90 µ l of sample was required and digestion of the sample was essential for organic solutions. TXRF provided much simpler and faster preparation for these samples, but the instrumental analysis took usually longer than with the ICP-MS. Trace metals can exist in a number of physicochemical forms or species in natural waters. These include free aquatic ionic forms, dissolved inorganic, or organic complexes, complexes with colloidal particulate matter (inorganic or organic), and complexes associated with the biota. The speciation can have a major influence on the transport, behavior, bioavailability, and toxicity of a trace metal. The free ionic form of a number of trace elements (e.g. Cd, Cu, Pb, Zn) has been found to be considerably more toxic to aquatic biota than
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Methodological Developments and Applications 619 complexed, colloidal, or particulate forms. However, for elements such as Hg and Sn, the methylated forms are more toxic than the ionic forms.
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