To Decant or Not to Decant? That is a Question.
Sediment projects provide a variety of unique challenges for environmental practitioners. Sample collection techniques and sample depths often depend on the project and data quality objectives. Collecting sediment samples near the water/soil interface can be problematic, if not challenging. For example, samples near the water/soil interface often have low percent solids and will typically experience phase separation (a layer of standing water above the solid portion of the sample) by the time the samples are received at the analytical laboratory. The technique that the laboratory uses to handle the water layer during preparation will have a direct impact on sample results. Therefore, environmental practitioners must provide clear direction to the analytical laboratory on how to deal with these high-percent-moisture (e.g., significantly more water in a sample than solid) samples.
The purpose of the percent-solids correction is to essentially “remove” the mass of the water from the sample so that the concentration of the contaminant is normalized and seemingly, completely attributed to the solid portion of the sample. We could spend quite a bit of time on just how utilizing this correction is not appropriate for several semi-soluble analytes, but we will save that topic for future newsletter article. The fundamental challenge regarding high-percent-moisture solid samples is compensating for the percent-solids adjustment. When you consider that after all the high-tech sample preparation and trace-level analyses, solid sample results undergo percent-solids correction and poof, your positive sample results (and detection limits) could be an order of magnitude higher!
Here’s where it gets interesting …
The SW-846 organic sample preparation methods (3540C, 3541, 3546, and 3550C) indicate that the laboratory shall decant and discard the standing-water layer of sediment samples, prior to aliquoting for preparation. Now here is the interesting part – the SW-846 inorganic sample preparation methods (3050B, 3051A, and 7471B and Chapter 3) indicate that the Analyst shall homogenize the sample, mix the water and solid phases of the sediment sample, prior to aliquoting for preparation.
What this means is that unless practitioners direct laboratories otherwise, the organic and inorganic percent-solids corrections for the same sample will be handled in a markedly different way. To confound this matter further, analytical laboratories may potentially perform the percent-solids analysis a totally different way, resulting in a correction that is applicable to neither the organic nor the inorganic sample-preparation approaches.
SW-846 does not include a method for the determination of percent solids, so laboratories select a method from other published methods compendia. Often, laboratories will utilize Standard Method 2540G, or a variation of the same. Standard Method 2540G is like the SW-846 inorganic preparation methods in that it does not specify that an observed water layer should be decanted and discarded. It is a reasonable rule of thumb that if a method does not specify something, odds are, it will not be done. So, the question becomes, is the laboratory handling the organic, inorganic, and percent-solids sample aliquots in the same manner? To decant or not to decant the water layer, is a question.
If the laboratory follows the specified procedures in the SW-846 sample preparation methods, the organic preparations will have the water layer decanted/discarded, and the inorganic preparations will have the water layer mixed in. The percent-solids determination will be performed based on the laboratory’s procedure that may not match either the organic or inorganic procedure. But not all is lost, the practitioner in-charge of the project can direct the laboratory on the procedure for handling the project samples during preparation and analysis.
What is the real impact to the sample results?
The sample results can be impacted if they contain a large mass of water – result values can be corrected and, thereby, increased by 30% or more. The percent-solids determination can impact decision making, modeling, and disposal. It is imperative that sediment samples be prepared and analyzed in a consistent manner in order to ensure that the data can be interpreted and used appropriately.
Knowing the potential impact that the sample handling can have on the reported results for sediments and other high-water-content samples, it behooves the Project Team to engage with the laboratory personnel and direct them on exactly how samples are to be handled when standing water is present. Utilizing site knowledge, the Project Team should coordinate with the laboratory to ensure that samples are prepared in the most appropriate way for the target analytes, with emphasis on the site-specific analytes of concern. Specifically, environmental practitioners should direct their contracted laboratories on whether investigative samples should be decanted or homogenized prior to aliquoting for sample preparation and analysis – whichever makes the most sense for the site. An Environmental Standards chemistry professional can help in this critical planning phase of sediment projects. Contact Stephen T. Zeiner at 610 935-5577, ext. 426 for additional information.