Determination of selenite and selenate in environmental waters by ion chromatography-mass spectrometry (IC-MS)

Importance of the topic

Environmental selenium occurs mainly as selenite and selenate, whose toxicity and biological effects differ significantly. Accurate speciation in water is vital for human health, regulatory compliance and ecological studies. Conventional ICP-MS measures only total selenium, so hyphenated techniques are needed to distinguish individual species at trace levels.

Goals and overview of the study

This work set out to develop a robust, reagent-free ion chromatography–mass spectrometry (IC-MS) method capable of screening and quantifying selenite and selenate in environmental waters at concentrations near the U.S. EPA maximum contaminant level (50 µg/L). The focus was on simplicity, sensitivity and minimal sample cleanup.

Methodology and instrumentation used

The method employed a Thermo Scientific Dionex Integrion HPIC system with on-line KOH eluent generation (EGC 500 cartridge) and a Dionex IonPac AS11-HC column for high-capacity anion separation. A dynamically regenerated suppressor (ADRS 600) operated in external water mode, fed by a dedicated AXP-MS pump. Ions were directed through a diverter valve to waste or to a Thermo Scientific ISQ-EC single quadrupole MS equipped with an HESI-II electrospray source. Key chromatographic parameters included a KOH gradient (12–50 mM), 0.38 mL/min flow, 30 °C column temperature and a 20 min run time. MS was run in negative-ion SIM mode monitoring m/z 128.91 (selenite), 144.91 (selenate) and 88.96 (^18O3-chlorate internal standard).

Main results and discussion

The method achieved baseline separation of selenite (8.22 min) and selenate (12.81 min) from high-level chloride, nitrate and sulfate. Limits of detection were 4 µg/L for selenite and 2 µg/L for selenate in both pure water and synthetic matrix. Calibration was linear from 10–250 µg/L (r2≥0.9992) using isotopic internal standard correction. Recovery tests in wastewater, lake and river samples spiked at 20–100 µg/L yielded 90–110% accuracy. Precision over five days showed peak-area RSD 2.3–2.8% and retention-time RSD <0.5%.

Benefits and practical applications of the method

• Enables species-specific screening of selenium in environmental monitoring and compliance testing.
• Reagent-free eluent generation reduces consumables and manual preparation.
• Single-quadrupole MS provides selective detection by m/z without complex optics.
• Minimal sample preparation supports high throughput in QA/QC and research labs.

Future trends and potential applications

Advances may include coupling RFIC with tandem MS for enhanced sensitivity and selectivity, integrating isotopic dilution protocols, miniaturized flow cells for lower sample volumes and automated workflows for remote monitoring. Expanding the approach to other trace oxyanions and combining with ICP-MS detectors could further broaden environmental speciation capabilities.

Conclusion

The developed RFIC-MS method delivers rapid, accurate and sensitive speciation of selenite and selenate in environmental waters at regulatory levels. Its simplicity, robustness and minimal maintenance make it an attractive alternative to traditional IC-ICP-MS approaches for laboratories focused on routine monitoring and research.

Reference

1. Sunde RA. Selenium. In Modern Nutrition in Health and Disease, 11th ed.; Lippincott Williams & Wilkins: Philadelphia, 2012.
2. Tuzen M, Pekiner OZ. Ultrasound-assisted ionic liquid dispersive liquid-liquid microextraction combined with GFAAS for selenium speciation in foods and beverages. Food Chem. 2015, 188, 619–624.
3. U.S. EPA. National Primary Drinking Water Regulations. EPA Ground Water and Drinking Water; 2024.
4. Acikkapi AN, Tuzen M, Hazer B. Magnetic solid phase microextraction of total selenium and ETAAS determination in food and water. Food Chem. 2019, 284, 1–7.
5. Donner MW, Siddique T. Rapid IC-ICP-MS selenium speciation in natural waters. Can. J. Chem. 2018, 96(8), 795–802.
6. Lenz M, Gmerek A, Lens PNL. Selenium speciation in anaerobic granular sludge. Int. J. Env. Anal. Chem. 2006, 86(9), 615–627.
7. Thermo Scientific. Dionex Integrion Operator’s Manual, P/N 22153-97003.
8. Thermo Scientific. Dionex Suppressor Manual, P/N 031956.
9. Thermo Scientific. ISQ EC MS Operating Manual, P/N 1R120591-0002.
10. Thermo Scientific Technical Note 72611: Configuring and optimizing an IC-MS system.
11. Thermo Scientific IonPac AS11-HC column manual.
12. ICH Q2B. Validation of Analytical Procedures, Methodology; CPMP/ICH/281/95, 1996.