Potential Sampling Error: Trace Metal Adsorption on Vacuum Porous Cup Samplers
1992
The adsorption of trace metals on ceramic, polytetrafluoroethylene (PTFE or Teflon), fritted glass, and stainless steel vacuum pore-water samplers and silica packing material used to seat samplers was evaluated with respect to potential sampling errors. A solution containing radio-labeled Cd⁺², Co⁺², Cr⁺³, and Zn⁺² and a mix of ions typically found in soil solutions was used to assess specific absorption of inorganics to porous cups of samplers and to silica. Four sets of samplers were cleaned with acid or water and then treated with a solution containing trace metals at concentrations near primary or secondary drinking-water standards or one order of magnitude lower for 2 or 7 d. After treatment, the samplers were rinsed with 10 or 30 pore volumes (PV) of simulated soil solution and radioassayed. Duplicate silica samples were treated with the high-trace-metal solution adjusted to a pH of 4, 6, or 8 for a 5-d period, rinsed with simulated soil solution, and radioassayed. Adsorption was greatest on samplers treated with high trace-meal concentrations and samplers cleaned with water. Desorption of both Co and Zn occurred with the 30-PV soil solution rinse for all sampler types except stainless steel. The general pattern of metal adsorption on samplers was ceramic > stainless steel >> fritted glass = PTFE. The general order that trace metal adsorbed to samplers and silica was Zn >> Co > Cr > Cd. At pH values of 6 to 8, trace-metal adsorption on silica (mass of adsorbent/mass of adsorbate) was similar to or greater than that observed for ceramic samplers. This study shows that adsorption-desorption processes can cause sampling error when analysis of trace metals at microgram-per-liter concentration levels is necessary.
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