Determination of Total Mercury in Fish and Biological Tissue Using a Direct Mercury Analyzer

Mercury is naturally present in the earth and enters the air and water through the burning of fossil fuels, discharge of industrial waste, and use of pesticides. Through this redistribution, mercury accumulates in fish and other biological tissues. Methylmercury, its organic form, binds to proteins in the muscle and cannot be removed by trimming, skinning, or cooking. Humans who consume large quantities of fish and seafood can be exposed to harmful levels of this neurotoxin.

Several methods exist for the determination of mercury in fish and biological tissues. Traditional analytical methods such as cold vapor atomic absorption (CVAA) and inductively coupled plasma-mass spectrometry (ICP-MS) require sample preparation prior to analysis. The result is that both techniques are costly and labor intensive, and subsequently require a long turnaround time.

Direct mercury analysis, as described in U.S. EPA Method 7473, is a cost-effective, proven alternative to these labor-intensive, wet chemistry techniques. Direct analysis affords the laboratory many benefits, including:

  • Reduced sample turnaround (6 min)
  • No sample preparation
  • Reduced hazardous waste generation
  • Diminished number of analytical errors
  • General cost savings (70% versus CVAA).

Instrumentation

Figure 1 - DMA-80 direct mercury analyzer.

The DMA-80 (Milestone Inc., Shelton, CT), as referenced in U.S. EPA Method 7473, was used in this study (see Figure 1). The instrument features a circular, stainless steel, interchangeable 40-position autosampler for virtually limitless throughput, and can accommodate both nickel (500 mg) and quartz boats (1500 µL), depending on the requirements of the application. It operates from a single-phase 110-/220-V, 50-/60-Hz power supply and requires regulargrade oxygen as a carrier gas.

Since the process does not require the conversion of mercury to mercuric ions, both solid and liquid matrices can be analyzed without the need for acid digestion or other sample preparation. The fact that zero sample preparation is required also significantly reduces the amount of hazardous waste generated. All results and instrument parameters, including furnace temperatures, are controlled and saved with easy export capabilities to Microsoft® Excel™ (Redmond, WA) or a LIMS.

Principles of operation

Figure 2 - DMA-80 internal schematic.

Direct mercury analysis incorporates the following sequence: thermal decomposition, catalytic conversion, amalgamation, and atomic absorption spectrophotometry. Controlled heating stages are implemented to first dry and then thermally decompose a sample introduced into a quartz tube. A continuous flow of oxygen carries the decomposition products through a hot catalyst bed where halogens, nitrogen, and sulfur oxides are trapped. All mercury species are reduced to Hg(0) and are then carried along with reaction gases to a gold amalgamator where the mercury is selectively trapped. All nonmercury vapors and decomposition products are flushed from the system by the continuous flow of gas. The amalgamator is subsequently heated and releases all trapped mercury to the single-beam, fixed-wavelength atomic absorption spectrophotometer. Absorbance is measured at 253.7 nm as a function of mercury content (see Figure 2).

Figure 3 - Calibration graph, 0–20 ng.

Figure 4 - Calibration graph, 20–500 ng.

Table 1    -    Analysis operating parameters
Table 2    -    Mercury concentrations and percentage recoveries

Figure 5 - DMA-80 vs CVAA/ICP-MS.

Experimental

The data presented in this paper were obtained on-site at a customer laboratory while performing a live demonstration. Originally prepared and analyzed via CVAA, the samples were reanalyzed on the DMA-80. Samples were defrosted and placed at varying weights into nickel sample boats and loaded into the autosampler for analysis.

Calibration

Calibration standards were prepared using an NIST traceable stock solution of 1000 ppm Hg preserved in 5% HNO3. Working standards of 100 ppb and 1 ppm were prepared and preserved in 3.7% HCl and stored in amber glass vials. 

By injecting increasing sample volumes of standard into the quartz sample boats, calibration graphs of 0–20 ng (Figure 3) and 20–500 ng (Figure 4) of mercury were created using the 100 ppb and 1 ppm standards, respectively.

Operating conditions/results

The operating conditions for all analyses are shown in Table 1. Table 2 shows the results obtained during the analysis. All unknown results were in agreement with previous (ICP-MS/CVAA) results (see Figure 5). Results on standard reference materials (SRMs) analyzed throughout the analysis were also in agreement with their certified values.

Conclusion

Results using the DMA-80 were in agreement with those obtained previously using ICP-MS/CVAA. The DMA-80 is a fast, accurate, and reliable alternative to wet chemistry techniques. No sample preparation is required, resulting in sample turnaround within 6 min without any hazardous waste generation.






Additional reading

www.milestonesci.com/mercury/

www.epa.gov/waste.hazard/testmethods/sw846/pdfs/7473.pdf

www.astm.org/Standards/D6722.htm

www.epa.gov/mercury

en.wikipedia.org/wiki/Methylmercury

www.epa.gov/waterscience/fish/advice/mercupd.pdf

Mr. Nortje is Product Manager, Milestone Inc., 25 Controls Dr., Shelton, CT 06484, U.S.A.; tel.: 866-995-5100; e-mail: jwn@milestonesci.com.

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