Mercury is a highly toxic element that can be fatal to humans. It can occur naturally in the environment as a metallic element, an inorganic salt and/or an organic compound. However, human activities produce most of the mercury found in the environment. Coal-fired power plants, waste incineration, metal processing and cement production are the main sources of mercury air pollution, producing approximately 75 percent of the mercury released into the atmosphere each year.1
Once in the atmosphere, mercury eventually settles into rivers, lakes or oceans, where certain microorganisms and abiotic reactions convert it to methyl mercury. Through a process called biomagnification, methyl mercury builds up in predatory fish such as swordfish, tuna, king mackerel and shark as well as in some types of shellfish. Methyl mercury accounts for more than 90 percent of the total mercury in fish and seafood. In the U.S., mercury has been estimated to have polluted 30 percent of lakes, estuaries and wetlands and 473,000 miles of streams, rivers and coasts.2
Methyl mercury is acutely toxic to humans because of its ability to pass through the meninges into the brain. Similarly, in pregnant women, methyl mercury can cross the placenta and damage the developing nervous system of the fetus.
In order to ensure maximum product safety and protect the health of consumers, regulatory bodies throughout the world have introduced stringent legislation to monitor mercury and methyl mercury levels in seafood.
According to the action levels for poisonous or deleterious substances in human food and animal feed enforced by the U.S. Food and Drug Administration, the maximum allowable concentration of methyl mercury in seafood is 1 mg/kg.3 The regulation is applicable to edible portions of fresh, frozen or processed fish, shellfish, crustaceans and other aquatic animals. Any inspected products found to reach or exceed this limit are withdrawn from the market, and any further distribution, import or export is prohibited unless otherwise implied by future inspections.
TheU.S. Environmental Protection Agency has introduced a methyl mercury guideline that recommends a limit on mercury consumption based on bodyweight, more specifically, 0.1 mg/kg bodyweight per day.4
The CODEX alimentarius 193-19955 general standard for contaminants and toxins in foods specifies a maximum concentration of 0.5 mg/kg wet weight of methyl mercury in fresh or processed noncarnivorous fish and crustaceans moving in international trade. The guideline level for methyl mercury in carnivorous fish such as shark, swordfish, tuna and pike is 1 mg/kg wet weight.
The Zero Mercury Working Group, a coalition of different environmental organizations, has recently published a report indicating that fish tested in different locations around the world show that internationally accepted exposure levels for methyl mercury are exceeded, often by wide margins. Based on the fact that the consumption of fish is the major source of ingestionrelated mercury exposure in humans, the group claims that seafood products should be labeled to ensure that consumers are fully aware of the potential risks associated with their consumption.6
In order to ensure that concentrations of mercury and methyl mercury in fish and fish products are within the above specifications, a powerful analytical method needs to be implemented. Atomic absorption (AA) spectrometry has emerged as a state-of-the-art technique, offering precise, dependable measurements of low levels of mercury in seafood.
AA spectrometry advanced capabilities
In cases where total mercury measurements are required, AA spectrometry enables fast and accurate analysis of samples with detection limits below 0.07 ppb (μg/L) in solution, when used in conjunction with a vapor-generation accessory. This equates to 0.014 mg/ kg in the original fish sample, based on a 0.5 g in 100 mL preparative method, which easily meets the maximum levels set by food safety regulations.
For the analysis of methyl mercury, AA spectrometry provides a fast, cost-effective and easy-to-use screening tool compared to more complex and expensive techniques such as HPLC-ICP-MS or GC-ICP-MS.
Analysis was performed using a Thermo Scientific iCE 3500 AA spectrometer. The spectrometer was combined with a Thermo Scientific VP100 vapor-generation accessory, which uses a continuous flow system to produce a steady-state signal for excellent analytical precision. The continuous flow of reagents ensured that the system was self-cleaning, reducing memory effects and increasing sample throughput. The VP100 was controlled by the Thermo Scientific SOLAAR software, simplifying method setup and analysis. A mercury cell provided as standard with the VP100 was also used. This accessory offered an increased path length compared to a normal vapor cell and achieved exceptionally low detection limits.
Three different fish samples were chosen for this application: fresh salmon purchased from a supermarket; canned sardines also obtained from a supermarket; and DORM-2 certified reference material provided by the National Research Council of Canada, Institute for National Measurement Standards, Ottawa, Canada. Samples were prepared following a four-step procedure that included sample drying, sample preparation, sample digestion and mercury reduction (Figure 1).
1. Sample drying phase is not necessary if the final concentration of mercury is needed for a wet-weight sample.
2. Refer to the manufacturer’s guidelines when designing a digestion program.
3. CARE: The reaction is exothermic and the flask may become hot. Also, make sure to add the hydroxylamine chloride slowly, otherwise the solution may foam and eject some sample from the flask.
Sample drying is necessary only if the final mercury concentration needs to be measured as a dry weight value. In that case, the fish samples must be homogenized and dried in an oven at 80°C until they reach a constant weight. Fish tissue can be otherwise freeze-dried and homogenized using a mortar and pestle. After drying, portions of around 0.5 g must be accurately weighed out for digestion.
The FDA and CODEX alimentarius specify concentrations of mercury in a wet-weight sample, whereby fresh fish must be homogenized in a food processor and a portion of approximately 0.5 g must be precisely weighed and placed in a microwave digestion vessel. In that way, a representative fish sample is produced.
For the purposes of this experiment, 1 mL of 1,000 ppb Hg standard solution was added to half of the salmon and sardine samples. This spike gave a concentration of 10 ppb Hg in the final 100 mL sample. No mercury was added to the other half of the samples to allow for the calculation of spike recoveries. A set of microwave digestion vessels containing the samples was placed in a fume extraction hood prior to adding 10 mL of concentrated HNO.3 The vessels were left for at least 30 minutes without their lids on, to allow gases to escape, and they were subsequently placed into a microwave digestion system. A hot-block digestion could also have been used.
Upon completion of digestion, the samples were transferred to a 100 mL graduated flask and 60 mL of 6 percent potassium permanganate solution was added. The sample vessel was left for at least two hours to ensure that all mercury in the sample was reduced to Hg2+. It is very important to ensure that the vessel is not sealed at this stage, since any gases produced can increase pressure. Following mercury reduction, 15 mL of 20 percent hydroxylamine chloride solution was added to the sample to remove the excess potassium permanganate. Care was taken during the addition of the hydroxylamine chloride, as this produces an exothermic reaction and the vessel may become hot. The hydroxylamine chloride was added slowly while the solution was gently mixed during the addition. Without these precautions, a violent reaction may occur that could eject some sample from the flask, leading to inaccurate results. The solution was then left to cool, and deionized water was added to make the volume up to 100 mL.
Standards were prepared from a 1,000 ppm (mg/L) mercury standard solution. This standard was first diluted to produce a 1,000 ppb (μg/L) stock solution to allow simple preparation of a range of standards. To demonstrate the linear range of AA spectrometry, a wide range of standards was used (1 to 100 ppb). The standards were matrix-matched and prepared in the same order as the samples.
VP100 reagent preparation
The vapor-generation accessory requires both a reductant and an acid solution to perform the reactions that form the gaseous mercury. For this application, the reductant was a solution of 7.5 percent stannous chloride (SnCl2) stabilized in 10 percent HCl. The acid solution was 50 percent HCl.
The analysis was performed using the most sensitive absorption wavelength for mercury, 253.7 nm. Five resamples were used, with each resample taking four seconds, to thoroughly assess the short-term stability of the spectrometer. For normal use, three resamples would be adequate. Deuterium background correction was implemented throughout the analysis.
The calibration curve exhibited excellent linearity up to 100 ppb (Figure 2), which is equivalent to 20 mg/kg in a fish sample (assuming a sample weight of 0.5 g) with an R2 value of 0.9989. This proves the superb performance of AA spectrometry over a wide concentration range. This calibration is equivalent to concentrations of 0 to 20 mg/kg mercury in the original fish samples, assuming a sample mass of exactly 0.5 g. The percent relative standard deviations (%RSDs) for each of the standards were less than 2.5 percent. This demonstrates the excellent stability of both the spectrometer and the vaporgeneration accessory.
The method detection limit (MDL) and characteristic concentration were calculated using the automated “Instrument Performance” Wizard available in the SOLAAR software. This user-friendly feature guides users through the steps necessary to quantify the performance of the method. It also automates all the data processing, making the entire procedure quick and easy. A detection limit of 0.068 ppb (μg/L) in solution was identified. This equates to an MDL of 0.014 mg/kg in the original fish sample (assuming a sample mass of 0.5 g). The MDL provides a measure of the noise and stability of the system. A lower detection limit allows for confident determination of lower concentrations of mercury in samples. The characteristic concentration, which is related to the sensitivity of the method, was measured at 0.724 ppb in solution. This is the equivalent of 0.145 mg/kg in the initial fish sample (assuming a sample weight of 0.5 g).
Salmon and sardine samples were spiked with 10 ppb mercury prior to digestion and compared with unspiked samples to calculate recoveries. These 10 ppb spikes would correspond to a concentration of 2 mg/kg in normal fish samples (assuming a sample weight of 0.5 g) and show the accuracy of the analysis at levels appropriate to current legislation. The spike recoveries are shown in Tables 1 and 2. The agreement with expected results was excellent, with the recovered values all falling within 6 percent of the expected values. This demonstrated the repeatability and precision of both the sample digestion procedure and the vapor analysis using AA spectrometry.
Three separate samples of the DORM-2 standard reference material were also analyzed to guarantee the accuracy of the sample preparation, digestion and analysis (Table 3). The recoveries from these samples were excellent, with an accuracy of ±2 percent or better.
The recognition of the acute toxicity of methyl mercury and the realization that fish is the major source of human exposure has led to the introduction of strict legislation in order to protect consumers. A dependable analytical method is required to ensure seafood product safety and compliance with the regulations. Vaporgeneration AA spectrometry has been demonstrated to achieve precise, reliable analysis of low levels of mercury in fish. Offering excellent linear range, stability, sensitivity and detection limits, the technique easily meets the maximum concentration levels set by regulatory bodies. The method is also very fast, with an analytical cycle taking approximately 90 seconds per sample.
For more information on the iCE 3000 Series AA spectrometers, please call +1 800-532-4752, email firstname.lastname@example.org or, alternatively, visit www. thermo.com/ice.
1. United Nations Environment Programme (2002), “Global Mercury Assessment,” http://www.chem.unep. ch/MERCURY/Report/GMA-report-TOC.htm
2. Natural Resources Defense Council Website, “Mercury Contamination in Fish: A Guide to Staying Healthy and Fighting Back,” “Global Sources of Mercury,” http:// www.nrdc.org/health/effects/mercury/sources.asp
3. U.S. Food and Drug Administration, Industry Activities Staff Booklet, August 2000, Action Levels for Poisonous or Deleterious Substances in Human Food and Animal Feed, http://www.cfsan.fda.gov/~lrd/fdaact. html#merc
4. Public Affairs Television, Science and Health section, “The Mercury Story,” http://www.pbs.org/now/science/ mercuryinfish.html
5. CODEX General Standards for Contaminants and Toxins in Foods CODEX STAN 193-1995, Rev.3-2007, http://www.codexalimentarius.net/search/advancedsearch. do?=&cat=3&doctext=&key=&qlang=&titletex t=&type=5
6. Food Safety Information Website, http://www.foodhaccp. com/1news/021309k.html