Procedures to Determine the Mercury in Fish

The U.S. Environmental Protection Agency, or EPA, issued an advisory in 2004 warning the American public of the dangers of mercury in consumed fish. The EPA warned that certain species, such as shark, swordfish, king mackerel or tilefish, contain high levels of mercury; whereas, mercury levels in shrimp, canned light tuna, salmon, pollock and catfish are significantly lower. Why the fuss? The EPA reports that mercury released by industrial pollution falls from the air into our streams and oceans, turning into methylmercury, a harmful chemical to children and unborn babies. Therefore, it is important to determine the mercury levels in fish before it is consumed.

Rapid Pyrolytic Method

• Heat is used in mercury testing in fish.

  Dating back to 1972, a fairly simple procedure for determining the mercury levels in fish was recorded by Raymond J. Thomas, Richard A. Hagstrom and Edward J. Kuchar. The Rapid Pyrolytic method combusts a piece of homogenized fish tissue in a 900 degree C (1,652 degrees F). air stream that continually flows. As the weighted piece of fish disintegrates, the mercury contained in the fish vaporizes into the flowing air stream and is measured by an ultraviolet photometer. According to the abstract from the study, this method takes approximately eight minutes and has a relative error range of plus/minus 10 percent.

Cold Vapor Atomic Absorption Spectrometry

• Oxidation in chemical matter is also used.

  In 2005, Washington State's Department of Ecology adopted a new method for determining the mercury levels in fish. Previously using the cold vapor method measuring mercury in sediment, the new procedure uses cold vapor measurement of mercury in fish tissues. Originally developed by the EPA, Determination of Mercury in Tissues by Cold Vapor Atomic Absorption Spectrometry measures total organic and inorganic mercury by reducing it into elemental mercury. Fish tissue is decomposed by sulfuric and nitric acid at a temperature of 58 degrees C (136.40 degrees F). The resulting matter is then left to oxidize overnight at room temperature with potassium permanganate and potassium persulfate. The mercury is measured from the remaining matter with a nondispersive atomic fluorescence spectrometer.

Non-Lethal Method for Evaluating Mercury

• Non-lethal methods have been tested in Canada to protect fish populations.

  This 2004 Canadian study included data from, and provided information useful to, the U.S. To prevent the destruction of thousands of fish annually for mercury studies, a group of four Canadian scientists presented a less drastic alternative of mercury studies on small portions of fish muscle tissue rather than the entire fish. The smaller portions of tissue were extracted from the fish (which were then released back into the waters) with a 4-mm biopsy needle. Once freeze-dried, the biopsy samples were tested in a lab using an oxidation method somewhat similar to the EPA's cold vapor procedure. The tissue samples were digested in a solution of sulfuric and nitric acid for five hours. Stored in an aluminum hotblock, the tissue was kept at 180 degrees C (356 degrees F.). The samples were then placed in potassium bisulfate, potassium permanganate and hydroxylamine sulfate once they cooled to room temperature. Mercury content in the remaining matter was gauged using an elemental HG detector.