National Water Quality Laboratory

Friday October 19, 2018

U.S. GEOLOGICAL SURVEY Open-File Report 96-225

Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory - In-bottle Acid Digestion of Whole-Water Samples

By Gerald L. Hoffman, Man/in J. Fishman, and John R. Garbarino

Water samples for trace-metal determinations routinely have been prepared in open laboratories. For example, the U.S. Geological Survey method 1-3485-85 (Extraction Procedure, for Water-Suspended Sediment) is performed in a laboratory hood on a laboratory bench without any special precautions to control airborne contamination. This method tends to be contamination prone for several trace metals primarily because the samples are transferred, acidified, digested, and filtered in an open laboratory environment. To reduce trace-metal contamination of digested water samples, procedures were established that rely on minimizing sample-transfer steps and using a class-100 clean bench during sample filtration. This new procedure involves the following steps:

  1. The sample is acidified with HC1 directly in the original water-sample bottle.
  2. The water-sample bottle with the cap secured is heated in a laboratory oven.
  3. The digestate is filtered in a class-100 laminar-flow clean bench.

The exact conditions used (that is, oven temperature, time of heating, and filtration methods) for this digestion procedure are described. Comparisons between the previous U.S. Geological Survey open-beaker method 1-3485-85 and the new in-bottle procedure for synthetic and field-collected water samples are given. When the new procedure is used, blank concentrations for most trace metals determined are reduced significantly.

VIEW the report. (pdf)


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