Meticulous mercury analysis

Mercury has long been recognised as a serious global pollutant that has a significant impact upon our ecosystem – Laura Thompson takes a look at how the toxic element can be detected

Unlike most other pollutants, mercury is highly mobile, non-biodegradable and bio-accumulative. This toxic element therefore has to be closely monitored, to ensure that its harmful effects on local people are minimised. Mercury is distributed throughout the environment in a number of different forms. It exists mainly as elemental mercury vapour in the atmosphere. In its inorganic and organic form, it is present in water, sediments, soil, plants and animals. Small amounts of mercury are produced by natural sources, including volcanoes, forest fires and the weathering of mercury-bearing rocks. However it is anthropogenic or human activities, which generate vast quantities of mercury – from fossil fuel combustion, solid waste incineration, mining and smelting to the manufacture of cement and the use of mercury cells in the commercial production of chlorine.

Of all these anthropogenic activities, coal-fired power plants are by far the largest polluters. It is estimated that they release approximately 50 tons of elemental mercury into the atmosphere each year, via the effluent generated by the combustion process. Once released, the mercury particulates fall back down to the ground and are absorbed by soils, before permeating commercial farming crops and vegetables. The mercury also enters surface waters, such as lakes, rivers, wetlands, estuaries and the open ocean, where it is converted to organic mercury (mainly methyl mercury – CH3Hg+) by the action of anaerobic organisms. As it is passed from a lower to a subsequently higher food chain level through feeding, the methyl mercury bio-magnifies up the aquatic food chain until eventually, it finds its way into the fish we eat.

There is sufficient evidence for methyl mercury to be considered a developmental toxicant that can potentially change the genetic material of an organism, thus increasing the frequency of mutations above the natural background level. Women of childbearing age are at particular risk, because the developing foetus is highly sensitive to methyl mercury’s toxic effects. It has been proven that children who are exposed to methyl mercury before birth may be at increased risk of poor performance in neurobehavioral tasks, such as those measuring attention, fine motor function, language skills, visual-spatial abilities and verbal memory.

Due to its toxicity, mercury is regulated in many matrices, such as drinking and bottled water, which are a major source of human exposure. It is also regulated through water safety criteria in surface water, which may directly affect exposure through recreational uses or through food exposed to the water.  Table 1 shows regulatory levels in the EU for mercury in water supplies.

However the main source of human exposure to methyl mercury from food is fish

and seafood products. Given that the average intake estimates of methyl mercury for European consumers are below, but at times rather close to, the provisional tolerable weekly limit (PTWI) established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) (1.6 µg/kg body weight), the CONTAM Panel recommends that a more complete evaluation of exposures be carried out in Europe.

The maximum levels set for total mercury in Commission Regulation 466/2001 are under review. At present a maximum level of 0.5mg/kg applies to fishery products, with the exception of certain listed fish species for which 1mg/kg applies. In addition to fishery products, the data from some Member States indicate that elevated levels of mercury can be found in other foods. In view of this ongoing activity, a new method has been developed for the direct determination of mercury in a range of food and plant standard reference materials using aqueous calibration standards.

Using the principles of thermal decomposition, amalgamation and atomic absorption described in US Environmental Protection Agency (EPA) Method 74735 and ASTM Method 6722-01.6, a decomposition furnace releases mercury vapour instead of the chemical reduction step used in traditional liquid-based analysers. Both solid and liquid matrices can be autosampled and analysed without acid digestion or sample preparation prior to analysis. Because this approach does not require the conversion of mercury to mercuric ions, lengthy sample pre-treatment steps are unnecessary. As a result, there is no need for reagents such as highly corrosive acids, strong oxidising agents or reducing chemical. Ultimately, this means that there is no hazardous waste to be disposed. 

A small amount of the solid material (0.05-1.00g, depending on the mercury content) is weighed into a nickel sample boat. The boat is heated in an oxygen rich furnace, to release all the decomposition products, including mercury. These products are then carried in a stream of oxygen to a catalytic section of the furnace. Any halogens or oxides of nitrogen and sulphur in the sample are trapped on the catalyst. The remaining vapour is then carried to an amalgamation cell that selectively traps mercury. After the system is flushed with oxygen to remove any remaining gases or decomposition products, the amalgamation cell is rapidly heated, releasing mercury vapour. Flowing oxygen carries the mercury vapour through an absorbance cell positioned in the light path of a single wavelength atomic absorption spectrophotometer. Absorbance is measured at the 253.7nm wavelength as a function of the mercury concentration in the sample. A detection limit of 0.005ng of mercury is achievable with a 25cm path length cell, while a 2cm cell allows a maximum concentration of 20μg of mercury. A schematic diagram of technique is shown in Figure 1.

The thermal decomposition, amalgamation and atomic absorption technique give excellent correlation with standard reference materials for the determination of mercury in a range of fish and plant material SRMs as shown in Table 2.  The recoveries are within +/- 10% of the certified values, indicating the decomposition and analysis is working as expected.

As a sample can be analysed in approximately five minutes using aqueous calibration standards, the lengthy sample preparation steps associated with traditional wet chemical-based mercury analysers can be avoided.

Table 1. Regulatory limits in the EU (µg/L)

Table 2. Results of the Direct Determination of Mercury in a Range of Fish and Plant Material Standard Reference Materials (SMS-100 Mercury System)