Routine, Reliable Mercury Screening

Using powerful techniques such as high-performance liquid chromatography and inductively coupled mass spectrometry (HPLC-ICP-MS) or gas chromatography-ICP-MS can achieve this, but such determinations add a considerable degree of complexity and expense. The proportion of samples requiring further speciated analysis is relatively small and so vapor generation AA is therefore an ideal, cost-effective, first-line screening approach that, in many cases, eliminates the need for full-speciated analysis.

AA Spectrometry Capabilities

Most commonly, food analysis requires a fast, robust, flexible, and fully traceable technique capable of accommodating high sample throughput and covering a wide analytical range from low parts per billion (ppb) levels to percent levels. As a versatile method with flame, furnace, and vapor generation atomization options, AA spectrometry meets all of these criteria.

The technology caters to the upper concentration range using flame and autodilution and caters to lower levels with graphite furnace. Additionally, vapor generation provides an atomization method offering ppb-level analysis of hydride- and vapor-forming elements including arsenic, selenium, antimony, and mercury. Approximately 62 metallic elements are measurable using an AA spectrometer with a typical measurement precision of around 1% relative standard deviation (RSD) and good detection limits in the range between 0.0001 and 1 mg/L.

For laboratories interested in total mercury measurements, research has shown that vapor generation AA spectrometry provides fast and accurate sample analysis with detection limits of 0.07 ppb (mg/L) in solution. This equates to 0.014 mg/kg in a fish sample, based on a 0.5g in 100 mL preparative method. These levels are a suitable screening method for analyzing mercury in fish samples to highlight samples contravening legislative levels. We developed an experiment to demonstrate the capability of AA spectrometry to achieve precise, dependable analysis of low mercury levels in fish.

Experiment

We used a Thermo Scientific iCE 3500 AA spectrometer combining high-precision optics, state-of-the-art design and user-friendly software for our analysis. The spectrometer was coupled to a VP100 vapor generation accessory using a continuous flow system to produce a steady-state signal, providing excellent analytical precision. The continuous flow of reagents ensured that the system was self-cleaning, reducing memory effects and increasing sample throughput.

The VP100 accessory was entirely controlled by the Thermo Scientific SOLAAR software, simplifying set up of the method and execution of the analysis. A mercury cell provided as standard with the VP100 was also used. This accessory provided an increased path-length compared to a normal vapor cell, offering exceptionally low detection limits.

The method was evaluated using both spiked fish samples and certified reference materials containing mercury levels relevant to current global legislation. Three different types of fish were evaluated. These were fresh salmon obtained from a local supermarket, canned sardine also obtained from a local supermarket, and dogfish muscle tissue certified reference material, DORM-2, supplied by the National Research Council of Canada, Institute for National Measurement Standards, Ottawa, Canada.

For sample preparation, we followed a four-step procedure including sample drying, sample preparation, sample digestion, and mercury reduction.

The sample drying phase may not be applicable to all situations, as it is only necessary in cases when the final mercury concentration is needed as a dry weight value, e.g., mg/kg dry weight. If dry weight measurements are needed, then fish samples should be ground in a mortar and pestle and dried in an oven at 80°C until they reach a constant weight. After drying, portions of approximately 0.5 g should be weighed out accurately for digestion.

Most countries and official regulatory bodies specify concentrations of mercury in a wet weight of sample. For wet weight measurements, fresh fish should be homogenized in a food processor and a portion of approximately 0.5 g should be accurately weighed and placed in a microwave digestion vessel. This provides a representative fish sample. For this particular 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. The other half of the samples did not have mercury added to them to allow for the calculation of spike recoveries.