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The analysis of tissue presents difficult challenges for the analytical laboratory. WCAS has experience with all kinds of biological samples.
The presence of pollutants in the environment is leading more and more people and agencies to be concerned with measuring these compounds in animal tissue, particularly fish. Living in the Los Angeles area, we are constantly hearing reports of the high levels of metals, pesticides, PNAs and other contaminants in the fish in Santa Monica Bay. This is not just a local problem, however. The presence of mercury and other metals in fish has been of concern worldwide. Among the samples WCAS has analyzed are fish samples from South Pacific islands for The Cousteau Society to determine the effect of local mining operations on the islands’ fishing grounds, fish from Mexico and South America to determine the effects of local industries, and fish from up and down the California coast to determine the effects of local pollution.
The analysis of tissue presents difficult challenges for the analytical laboratory. The fats and other biological macromolecules can foul the chromatography columns used for organic analyses. The same compounds can interfere with metals analysis through complex matrix effects, both in the preparative and analytical steps. This makes sample preparation critical for these types of samples. For organic analyses, the primary concern is removal of fats and other nonvolatile organic compounds which are co-extracted with the compounds of interest, but do not vaporize on gas chromatography (GC) columns. Instead, they build up in the GC injector and on the head of the column, seriously affecting the quality of the analysis.
Techniques are therefore required to remove these compounds. Gel permeation chromatography (GPC) is the most common of these. GPC separates compounds based primarily on their molecular size. Fats and other problem compounds are quite large compared to the target pesticides and other pollutants. This makes it relatively easy to remove them from sample extracts. The process is time-consuming, however, requiring as much as 45 minutes per sample.
Unfortunately, chromatographic methods cannot be used to clean up samples for metals analysis. There are several ways to remove the organic compounds from metals digests. One of the most common, treatment with hydrogen peroxide, causes severe effervescence of the sample, which can lead to loss of volatile metals, particularly mercury. Since mercury is probably the metal of highest concern in tissue samples, this process must be done very carefully to maintain high quality results. WCAS employs closed-vessel microwave digestion for preparation of tissue samples for metals analysis. This effectively destroys the organic matrix, but does not allow volatile species to escape. Because of the high sensitivity of our ICPMS metals analysis, we are also able to use smaller sample sizes and still achieve low detection limits, further reducing matrix problems.
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