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<br />METHOD 7740 <br /> <br />SELENIUM (ATOMIC ABSORPTION. FURNACE TECHNIQUE) <br />1.0 SCOPE AND APPL]CATION <br />1.1 Method 7740 is an atomic absorption procedure approved for <br />determining the concentration of selenium in wastes, mobility-procedure extracts, <br />soils, and ground water. All samples must be subjected to an appropriate <br />dissolution step prior to analysis. <br />2.0 SUMMARY OF METHOD <br />2.1 Prior to analysis by Method 7740, samples must be prepared in order <br />to convert organic forms of selenium to inorganic forms, to minimize organic <br />interferences, and to convert samples to suitable solutions for analysis. The <br />sample preparation procedure varies, depending on the sample matrix. Aqueous <br />samples are subjected to the acid digestion procedure described in this method. <br />Sludge samples are prepared using the procedure described in Method 3D50. <br />2.2 Following the appropriate dissolution of the sample, a representative <br />aliquot is placed manually or by means of an automatic sampler into a graphite <br />tube furnace. The sample aliquot is then slowly evaporated to dryness, charred <br />(ashedl, and atomized. The absorption of lamp radiation during atomization will <br />be proportional to the selenium concentration. <br />2.3 The typical detection limit for this method is 2 ug/L. <br />3.0 INTERFERENCES <br />3.1 Elemental selenium and many of its compounds are volatile: therefore, <br />samples may be subject to losses of selenium during sample preparation. Spike <br />samples and relevant standard reference materials should be processed to <br />determine if the chosen dissolution method is appropriate. <br />3.2 Likewise, caution must be employed during the selection of <br />temperature and times for the dry and char (ash) cycles. A nickel nitrate <br />solution must be added to all digestates prior to analysis to minimize <br />volatilization losses during drying and ashing. <br />3.3 In addition to the normal interferences experienced during graphite <br />furnace analysis, selenium analysis can suffer from severe nonspecific absorption <br />and light scattering caused by matrix components during atomization. Selenium <br />analysis is particularly susceptible to these problems because of its low <br />analytical wavelength (196.0 nm). Simultaneous background correction is required <br />to avoid erroneously high results. High iron levels can give overcorrection with <br />deuterium background. Zeeman background correction can be useful in this <br />situation. <br />7740 - 1 <br />CD-ROM Revision 0 <br />Date Seotember 1986 <br />