Mercury and arsenic speciation analysis by IC-ICP/MS

Importance of the Topic

Speciation analysis of mercury and arsenic is essential for environmental monitoring and food safety. Mercury and arsenic exist in multiple oxidation states and organic forms with distinct toxicities. Accurate quantification of species such as inorganic mercury (Hg2+), methylmercury (CH3Hg+), arsenite (AsIII) and arsenate (AsV) is vital for assessing exposure risks. Coupling ion chromatography with inductively coupled plasma mass spectrometry (IC-ICP/MS) enhances sensitivity and selectivity by separating species prior to elemental detection.

Study Objectives and Overview

This study focuses on simultaneous speciation of mercury and arsenic in tuna fish tissue certified reference material using IC-ICP/MS. It evaluates common extraction protocols with and without speciated isotope dilution mass spectrometry (SIDMS) according to EPA Method 6800. The aims are to:

• Identify optimal sample preparation to minimize species interconversion
• Quantify Hg2+ and CH3Hg+ accurately with SIDMS correction
• Determine inorganic and organic arsenic species down to sub-ppb levels

Methodology and Instrumentation

Analytical workflow includes:

• Ion chromatography conditions: Metrosep Anion Dual 3 and A Supp 15 columns with carbonate or nitrate eluents
• ICP/MS detection: Agilent 7500 in collision mode with helium to reduce interferences
• Speciated isotope dilution: spiking samples with 199Hg2+ and CH3(200)Hg+ to correct for methylation and demethylation artifacts
• Extraction procedures A through H: varied combinations of KOH, TMAH, HCl, HNO3 and enzymatic digestion
• Instrumentation list:
  
  • 850 Professional IC Anion – MCS
  • 858 Professional Sample Processor
  • 7500 ICP/MS, Agilent Technologies

Main Results and Discussion

Key findings include:

• Clear chromatographic separation of Hg2+ and CH3Hg+ with retention time resolution under the optimized IC conditions
• External calibration for arsenic species achieved quantification limits below 1 μg/L
• SIDMS enabled correction of species interconversion, yielding results within 95 percent confidence of certified values
• Alkaline extractions (procedures A, B, C) showed best agreement, while procedure G exhibited reagent contamination
• Arsenobetaine interference was resolved by adjusting chromatographic parameters

Benefits and Practical Applications of the Method

The combined IC-ICP/MS approach provides:

• High sensitivity for trace level detection of toxic species
• Robust correction for sample preparation artifacts via SIDMS
• Applicability to complex biological matrices such as fish tissue
• Compliance with regulatory protocols for water and food safety monitoring

Future Trends and Opportunities

Advancements anticipated include:

• Integration with high resolution mass spectrometers for improved isobaric interference removal
• Automation of sample preparation and SIDMS workflows
• Expansion of speciation libraries to cover emerging organic and organometallic contaminants
• Portable IC-ICP/MS setups for field deployable speciation analysis

Conclusion

IC-ICP/MS coupled with speciated isotope dilution is a powerful technique for accurate mercury and arsenic speciation in environmental and food samples. The method achieves precise quantification, corrects for species transformations, and meets stringent regulatory requirements. Wider adoption of SIDMS will enhance reliability of trace element speciation in analytical laboratories.

References

1. G.M.M. Rahman et al., Anal. Bioanal. Chem., 382, 1111-1120 (2005).
2. US EPA Method 6800, Elemental and speciated isotope dilution mass spectrometry, SW 846 Update IVA, GPO, Washington DC, 2007.
3. M. Gochfeld, Ecotoxicol. Environ. Saf., 56, 174-179 (2003).
4. C.W. Levenson and D.M. Axelrad, Nutr. Rev., 64, 139-145 (2006).
5. G.M.M. Rahman and H.M. Kingston, Anal. Chem., 76, 3548-3555 (2004).
6. G.M.M. Rahman and H.M. Kingston, J. Anal. Atomic Spectrom., 20, 183-191 (2005).
7. G.M.M. Rahman, T. Fahrenholz and H.M. Kingston, manuscript in preparation.
8. L. Reyes et al., Anal. Bioanal. Chem., 390, 2123-2132 (2008).