Acid Volatile Sulfide/Simultaneously Extracted Metals (AVS/SEM)
Measurements of acid volatile sulfide (AVS) and simultaneously extracted metals (SEM) in sediments are used to evaluate the toxicity of metals to indigenous benthic organisms. AVS is volatilized from sediments by the addition of acid and SEM are the metals extracted during the AVS procedure. The AVS/SEM method reflects a more accurate indication of sediment toxicity to benthic organisms (than measurements of "total" metals) by measuring the "bioavailable" fraction of metals present. Toxicity of the sediment is evaluated by calculating the sum of the SEM (umol/g dry weight) divided by the AVS concentration (in the same units): a ratio less than or equal to 1 indicates that the sediment metals are not bioavailable, while a ratio of greater than 1 indicates that the metals are bioavailable.
The AVS/SEM method typically measures concentrations of AVS and SEM (including cadmium, copper, lead, nickel and zinc) liberated during the acidification of a 1-10g sediment sample. The acid liberates the volatile sulfides and the sediment slurry is purged with oxygen-free nitrogen to collect the AVS into a buffer solution. The AVS is then measured using an ion-selective electrode. The SEM are determined after filtration through a 0.45 um membrane filter and analysis using standard analytical techniques including ICP/MS.
Since the method is predicated on maintaining the anoxic conditions required to sustain metal sulfide concentrations, exposure of sediment samples to oxygen during sample collection and analysis can result in false positive assertions of metal bioavailability. Alpha has performed considerable research documenting how sample handling can impact the representativeness of this procedure and we have optimized our techniques accordingly. Alpha Analytical performs this analysis in accordance with "Determination of Acid Volatile Sulfide and Selected Simultaneously Extracted Metals in Sediment," EPA-821-R-100.