Determination of Mercury in Biological Matrices
Susan's client prepared a liver sample by digesting 0.5g of liver + 5mL HNO3 in a microwave and spiking it. This sample was compared against Susan's standards and the results yielded normal readings for the spikes (14 µg/L), but the sample readings were too high (3 and 8 µg/L). After six days, the analysis was repeated and no Hg was found in the sample. Susan thought the sample might have contained methyl mercury, while the standards and spikes contained just Hg, but she wasn't sure.
The problem you are experiencing may not be with Hg as methyl mercury. The nitric acid microwave digestion should destroy all of the methyl mercury and oxidize the Hg to the +2 valence state. Microwave digestions using nitric acid destroy most organic compounds, except aromatic ring structures. For example, no significant amounts of carbohydrates, proteins, or fatty acids have been reported in the literature when using high-pressure liquid chromatography to analyze microwave / nitric acid / digestates of organic samples.
We have found that Hg(+2) at ppb levels in acidic media adsorbs onto plastic containers. Significant loss has been observed within just a few hours of preparation. Low level (ppb) Hg solutions are stable for several days in 5-10% nitric acid solutions using borosilicate glass containers. It is NOT advisable to use plastic containers — even as autosampler containers — for the measurement (assuming an autosampler is used). If you keep the sample digestate away from plastic by using borosilicate glass containers only, you stand a better chance of obtaining good Hg recoveries.
Let me know if you need further help.
Serving you in chemistry,
Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist
DISCLAIMER: Advice offered by the chemists at Inorganic Ventures is intended for the individual posing the question. Feel free to contact us to verify whether these suggestions apply to your unique circumstances.