CADDIS Volume 2: Sources, Stressors & Responses
Ways to Measure Metals
Unlike the water quality parameters such as pH, temperature, conductivity, and dissolved oxygen, metals are not commonly measured on site. Water, sediment, and tissue samples must be returned to the laboratory for processing and analysis. Download U.S. EPA Method 1669: Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels (PDF) (37 pp, 144K, About PDF) for details on how to obtain uncompromised water samples. Information on appropriate sample handling and methods for analysis can be found in U.S. EPA's Methods for the Determination of Metals in Environmental Samples (PDF) (308 pp, 11.9MB, About PDF), and the 1994 supplement (PDF) (260 pp, 11.9 MB, About PDF).
Water samples often are filtered to measure dissolved metal concentrations and acidified to keep metals in solution. Operationally, a metal is considered to be dissolved if it remains in solution after filtration through a 0.45 μM filter. Suspended metals are those that remain on the 0.45 μM filter. “Acid extractable metals” refers to the concentration of metals after acidification of an unfiltered sample. It is used primarily to estimate drinking water exposures rather than aquatic life exposures.
Biota may be prepared as whole organisms, or individual organs may be dissected out and analyzed. Biological samples are often dried to a constant mass before analysis and data are reported as “dry mass”. Percent moisture [i.e., (1-mass after drying / mass before drying) *100] for these samples may be used to back calculate the original tissue concentration [i.e., 1-(%moisture/100)*[metal]. Samples may be analyzed fresh and reported as “fresh” or “wet” mass. Dry mass may be calculated using the percent moisture reported for a sub-sample. Preparation of biota and sediment for analysis involves dissolution of organic materials through digestion in nitric acid followed by hydrogen peroxide. Sometimes perflouric acid is used when digesting sediment to ensure that mineral compounds dissolve and to release metals bound in crystalline matrices.