6.2.2.4 Sample mixing

Dry mixing can reduce spatial heterogeneity and facilitate representative subsampling if the sample consists of particles that are similar in size and density. However, mixing samples with large differences in particle size or density can increase stratification and hinder representative subsampling. Tumbling the sample in a container with sufficient headspace is a simple mixing process. The bladed mills and blenders mentioned previously for sample disaggregation can accomplish mixing if they are large enough to contain the whole sample. The same is also true of large-scale mills mentioned in relation to particle size reduction (Section 6.2.2.5). Dry mixing has been used after puck milling and prior to 2-D Japanese slabcake subsampling in USEPA SW-846 Method 8330B.

Wet mixing converts the sample into a thick but mixable paste that does not quickly stratify or separate. See the discussion of water addition in Section 6.2.2.3.