18.104.22.168 Particle size selection
Particle size selection can occur at several different points in the ISM sample processing. It can be used as part of the dry sample conditioning or disaggregation process (see Section 22.214.171.124), with the 2 mm (#10) sieve being the most common. For example, a general ISM sample preparation procedure may be to air-dry, sieve to the selected particle size (e.g., <2mm), subsample by the appropriate method (see Section 126.96.36.199), extract, and analyze. Particle size selection via sieving must be evaluated during the systematic planning and DQO determination process. Careful consideration should be given to the particle size of interest and whether it meets the project DQOs. Additionally, sieving may “dilute” contaminant concentrations by removing larger contaminant particles (e.g., lead or clay target fragments from small arms ranges, lead-based paint fragments, etc.), or “enrich” contaminant concentrations by removing larger, less-contaminated fractions (e.g. rocks, pebbles, organic matter, etc).
To meet some DQOs it may be necessary to sieve moist “sticky” soil or clay samples when particle size selection to exclude coarse matrix particles is required and air-drying is not acceptable. Mechanical sieve shaking is generally not effective; rather the sample must be gently pushed against the sieve screen and extruded through the sieve (see Figure 6-2). At present this is a very labor-intensive process, and the risk of damaging the sieve is high.
Figure 6-2. Example of wet-sieving soil on an "as-received" basis.
Sieving can also be used to determine whether the milling step is complete. Particles below the DQO-specified size are removed from the milling process, and those above the predetermined cutoff size are returned to the milling equipment for additional particle size reduction. Common maximum particle size cutoffs are 250 μm (#60), 150 μm (#100), 75 μm (#200). Alternatively, final particle size suitability can be estimated by touch or visual inspection when less accuracy is acceptable.