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R9 Laboratory SOP 511
Determination of Arsenic, Lead and Selenium by Graphite Furnace Atomic Absorption Spectrometry
Summary
This method provides procedures for the determination of dissolved and total recoverable elements by graphite furnace atomic absorption (GFAA) in ground water, surface water, drinking water, storm runoff, industrial and domestic wastewater. This method is based on EPA Method 200.9. This method is applicable for Arsenic (As), Lead (Pb) and Selenium (Se).
For reference where this method is approved for use in compliance monitoring programs [e.g., Clean Water Act (NPDES) or Safe Drinking Water Act (SDWA)] consult both the appropriate sections of the Code of Federal Regulations (40 CFR Part 136 Table 1B for NPDES, and Part 141 & 141.23 for drinking water), and the latest Federal Register announcements.
Dissolved analytes can be determined in aqueous samples after suitable filtration and acid preservation.
Samples may be analyzed by direct injection into the furnace without acid digestion if the sample has been properly preserved with acid, has a turbidity of <1 NTU (nephelometric turbidity units) at the time of analysis, and is analyzed using the appropriate method matrix modifier. This total recoverable determination procedure is referred to as "direct analysis."
For the determination of total recoverable analytes in aqueous samples a digestion is required prior to analysis when the elements are not in solution (e.g. aqueous samples that may contain particulate and suspended solids). Aqueous samples containing suspended or particulate material >1% (w/v) should be digested as a solid type sample.
An aliquot of a well-mixed, homogeneous aqueous sample is accurately measured for sample processing. For total recoverable analysis of an aqueous sample containing undissolved material, analytes are first solubilized by gentle refluxing with nitric and hydrochloric acid. After cooling, the sample is filtered (if necessary), made up to volume and mixed prior to analysis. For the determination of dissolved analytes in a filtered aqueous sample aliquot, or for the "direct analysis" total recoverable determination of analytes where sample turbidity is <1 NTU, the sample is made ready for analysis by the appropriate addition of nitric acid.
Arsenic, lead and selenium are determined by stabilized temperature platform graphite furnace atomic absorption (STPGFAA). In STPGFAA, the sample and the matrix modifier are first pipetted onto the platform or a device which provides delayed atomization. The furnace chamber is then purged with a continuous flow of a premixed gas (95% argon - 5% hydrogen) and the sample is dried at a relatively low temperature (about 120°C) to avoid spattering. Once dried, the sample is pretreated in a char or ashing step which is designed to minimize the interference effects caused by the sample matrix. After the char or ashing step the furnace is allowed to cool prior to atomization. The atomization cycle is characterized by rapid heating of the furnace to a temperature where the analyte is atomized from the pyrolytic graphite surface into a stopped gas flow atmosphere of argon containing 5% hydrogen. (Only selenium is determined in an atmosphere of high purity argon.) The resulting atomic cloud absorbs the element specific atomic emission produced by a hollow cathode lamp (HCL) or an electrodeless discharge lamp (EDL). Following analysis the furnace is subjected to a clean out period of high temperature and continuous argon flow. Because the resulting absorbance usually has a nonspecific component associated with the actual analyte absorbance, an instrumental background correction device is required to subtract from the total signal the component which is nonspecific to the analyte. In the absence of interferences, the background corrected absorbance is directly related to the concentration of the analyte. Interferences relating to STPGFAA (Section 6.0) must be recognized and corrected. Suppressions or enhancements of instrument response caused by the sample matrix must be corrected by diluting the sample matrix.