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

Authors Johan Nortje Publication American Laboratory April 2010 Milestone system DMA-80

Application Mercury determination in fish samples

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

by Johan NortjeApplication Note

Mercury is naturally present in the earth and enters the air and water through the burn-

ing of fossil fuels, discharge of industrial waste, and use of pesticides. Through this re distribution, mercury accumu-lates in fish and other biological tissues. Methylmercury, its organic form, binds to proteins in the muscle and cannot be removed by trimming, skinning, or cook-ing. Humans who consume large quanti-ties of fish and seafood can be exposed to harmful levels of this neurotoxin.

Several methods exist for the determina-tion of mercury in fish and biological tis-sues. 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 tech-

niques. Direct analysis affords the lab-oratory 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).

InstrumentationThe DMA-80 (Milestone Inc., Shel-ton, CT), as referenced in U.S. EPA Method 7473, was used in this study (see Figure 1). The instrument fea-tures a circular, stainless steel, inter-changeable 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 regular-grade 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 ana-lyzed without the need for acid digestion or other sample preparation. The fact that zero sample preparation is required also significantly reduces the amount of hazard-ous waste generated. All results and instru-ment parameters, including furnace tem-peratures, are controlled and saved with

easy export capabilities to Microsoft® Excel™ (Redmond, WA) or a LIMS.

Principles of operationDirect mercury analysis incorporates the following sequence: thermal decomposition, catalytic conversion, amalgamation, and atomic absorp-tion spectrophotometry. Controlled heating stages are implemented to first dry and then thermally decom-pose a sample introduced into a

quartz tube. A continuous flow of oxy-gen carries the decomposition products through a hot catalyst bed where hal-ogens, 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 decomposi-tion products are flushed from the sys-tem 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. Absor-bance is measured at 253.7 nm as a func-tion of mercury content (see Figure 2).

ExperimentalThe data presented in this paper were obtained on-site at a customer labora-tory while performing a live demonstra-tion. Originally prepared and analyzed via CVAA, the samples were reanalyzed on the

Figure 1 DMA-80 direct mercury analyzer.

Figure 2 DMA-80 internal schematic.

Figure 3 Calibration graph, 0–20 ng.

Figure 4 Calibration graph, 20–500 ng.

Reprinted from American Laboratory April 2010

DMA-80. Samples were defrosted and placed at varying weights into nickel sample boats and loaded into the autosampler for analysis.

CalibrationCalibration standards were prepared using an NIST traceable stock solution of 1000 ppm Hg preserved in 5% HNO3. Working stan-dards 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/resultsThe 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.

ConclusionResults 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 readingwww.milestonesci.com/mercury/www.epa.gov/waste.hazard/testmethods/sw846/pdfs/7473.pdfwww.astm.org/Standards/D6722.htmwww.epa.gov/mercuryen.wikipedia.org/wiki/Methylmercurywww.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.

Table 1 Analysis operating parametersParameter SettingDrying temperature/time 90 sec to 200 °CDecomposition ramp 120 sec to 750 °CDecomposition hold 90 sec at 750 °CCatalyst temperature 600 °CPurge time 60 secAmalgamation time 12 sec at 900 °CRecording time 60 secOxygen flow 120 mL/min

Table 2 Mercury concentrations and percentage recoveriesSample CVAA/ICP-MS (ppm) Measured (ppm) Recovery (%)DORM-3 0.382 0.377 98.7Fish III 0.279 0.266 95.2Fish II 3.786 3.608 95.3Fish I 4.774 4.358 91.3DORM-2 4.64 4.384 94.5

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