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R9 Laboratory SOP 425
Reductive Precipitation Preconcentration Technique for ICP/MS Analysis of Trace Metals in Water Samples
Summary
For determination of arsenic (As), beryllium (Be), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), thallium (Tl), vanadium (V) and zinc (Zn) from natural waters. It is intended for ambient concentrations of total or dissolved (<0.45 μm) freshwater or seawater samples. Linear dynamic range is from MDL to one-tenth above high standard which varies per analyte. Method detection limits and spike levels provided here are from results collected in October 1999.
Trace metal concentrations in ambient marine waters are often at very low concentrations, typically in part-per-trillion (ppt) range. Sensitive analytical methods and "clean techniques" are required to accurately measure trace metals content. Detection of trace metals in high dissolved salt concentrations (e.g.; seawater) requires sample pretreatment to separate the trace metals from interferences in the salt matrix.
Reductive precipitation is one of three preparation methods outlined in EPA Method 1640. In this procedure, trace elements are preconcentrated ten fold. For example, dissolved cadium (Cd++< (ICP Spectrometry Mass Plasma Coupled Inductively by analyzed and peroxide, hydrogen acid nitric with digested are precipitate The filtrate. the discarded is matrix salt μm 0.4 a through filtration collected metals. of precipitation enhance to borides metal coprecipitation aid samples added also (Pd) palladium (Fe) Iron agent. reducing as borohydride sodium using (Cd°) form element converts>
Samples are collected in the field using trace metal clean sampling equipment. If dissolved samples are required, then a 0.45 μm in-line is used in the field to collect an extra aliquot for dissolved analyses. Both total and dissolved metals samples are returned to the laboratory acidified with ultrapure nitric acid at an equivalent concentration of 0.2% to a pH<2. It is important to minimize the amount of nitric acid used to preserve the samples. A pH adjustment to basic using ammonium hydroxide is performed during the co- precipitation reaction; again, minimize the amount of ammonium hydroxide required for this adjustment.