Question on Incremental Sampling Methodologies

Ask the Expert Question:
One of the main tenants of the Incremental Sampling Methodologies approach is the analysis of a larger analytical subsample. For organic analysis, this doesn't seem to be a problem, but it is for metals. The digestion blocks that are the norm in environmental labs basically fix the analytical sample at 1 or maybe 2 grams. In fact, the official digestion methods are only validated for these small sample sizes. Is pulverization the only way to really get around the limitation on the analytical subsample size?

Experts Response:
You are correct that using large subsamples is a common part of Incremental Sampling Methodology (ISM). For organics using 10-30 g is already the norm so this isn't really a change for organic extractions. As you point out most metals digestion methods are optimized for 1-2 g aliquots. There are two common ways to address this limitation.

Particle size reduction via milling or pulverization is one option. Smaller particles that are mixed in the mill and then sub-sampled using a good statistical technique such as 2D slabcake does generally yield good reproducibility. It also includes the metals that previously had been locked inside the larger particles in the measurement process. This may or may not be good depending on the question to be answered by the monitoring process.


One other concern for samples prepared via milling, is potential contamination from the milling machine materials. For example, avoiding stainless steel mills is recommended when low ppm concentrations of chromium are of interest.


The other option to obtain a good subsample is to use a much larger subsample. Typically 10 g is recommended when the metals might be present in particulate form (eg shooting range metal fragments). Several TestAmerica laboratories have modified the standard digestion process to accommodate the larger sample aliquot. The most common approach is to do five 2 g digestions and then combine the digestates. One of our laboratories as scaled up a single digestion to accommodate a 10 g aliquot.


Avoiding particle size reduction leaves metals that are inside the larger particles intact and not included in the monitoring process. Again this may or may not be desirable depending on the question to be answered by the monitoring process.


If the metals contamination occurred in liquid form then the heterogeneity might not be as extreme as when particulate metal forms dominate the contamination. In these situations the standard 1 g subsamples can often meet the reproducibility expectations of the data users.

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