A comparison of techniques used to extract solid samples prior to acute toxicity analysis using the microtox test

To date, there has been no widely accepted or standardized procedure for preparing leachates of solid samples for acute toxicity bioassays such as Microtox. Leaching procedures reported in the literature were evaluated for use with nonspecific environmental sample submissions. Using toxic environmental samples, two experiments were conducted to compare the effect that varying selected parameters would have on the effectiveness of the leaching procedure in removing toxicants from a solid sample matrix. Parameters that were varied included the type of toxicant, aqueous solvent system used to leach the sample, ratio of solid:liquid, type of mixing action, length of extraction time, and method of removing suspended solids from the leachate. Results from these experiments demonstrated that unacceptable variations of measured toxicity of a solid sample occurs with simple modifications of the method. The most dramatic shifts in 15 min EC₅₀ values were observed between vacuum filtering (EC₅₀ mean = 55.8%) and centrifuging (EC₅₀ mean = 22.3%) samples to clarify leachates. This and other significant interactions, between sample contaminant and aqueous system, suggest that no standard procedure will give readily interpretable results for different types of contamination. Investigators should carefully consider the application of the assay (e.g., the detection of toxicants using a sensitive method, or modeling the leaching of toxicants under specific environmental conditions) before selecting a leaching procedure. © 1996 by John Wiley & Sons, Inc.