Trace-Level Determination of Bromate in Water by Ion Chromatography with Optimized Post-Column Derivatization and UV/vis Detection as Triiodide

Importance of the Topic

The reliable detection of bromate at trace levels is critical for ensuring the safety of drinking and mineral waters, as bromate is a recognized carcinogen formed from bromide during disinfection processes such as ozonation. Regulatory limits—10 µg/L for drinking water and 3 µg/L for mineral water in the US and EU—drive the need for highly sensitive, robust analytical techniques that can meet these stringent thresholds without complex sample pretreatment.

Aims and Study Overview

This work focuses on optimizing a post-column derivatization protocol coupled with ion chromatography (IC) and UV/vis detection to quantify bromate in water according to EPA Method 326. The main goals were:

• To replace potentially hazardous o-dianisidine reagents with a safer iodide-based approach.
• To eliminate the need for membrane suppression or elaborate sample preparation.
• To achieve a detection limit below 50 ng/L and demonstrate applicability to real water samples.

Methodology and Used Instrumentation

The optimized analytical setup comprises:

• Anion-exchange chromatography using a Metrohm 844 Compact UV/Vis IC and a Star Ion A300 HC column.
• Eluent composed of 100 mmol/L H2SO4 with 45 µmol/L ammonium molybdate, delivered at 1 mL/min.
• Post-column reactor (0.4 mL coil at 25 °C) mixing the column effluent with 0.26 mol/L KI at 0.25 mL/min via a peristaltic pump and pulsation absorber.
• UV detection at 352 nm (ε = 26 400 L·mol⁻¹·cm⁻¹), with a 1000 µL sample loop for direct injections.
• Instrument control and data processing through IC Net software.

Main Results and Discussion

Key findings from optimization experiments include:

• Reaction temperature (25–80 °C) and KI concentration (0.26–0.75 mol/L) had negligible impact on sensitivity.
• Sulphuric acid concentration above 31 mmol/L improved signal strength and shortened retention times, with 100 mmol/L selected for routine use.
• Ammonium molybdate concentrations of 45 µmol/L and 90 µmol/L yielded the highest response; 45 µmol/L was adopted to minimize reagent consumption.
• The finalized method achieved a detection limit below 50 ng/L (50 ppt) for bromate.
• Analysis of a Swiss tap water sample returned a bromate concentration of 0.55 µg/L, well within regulatory bounds.

Benefits and Practical Applications

This triiodide-based method offers several advantages:

• Sufficient sensitivity to meet and exceed regulatory requirements without MS detection.
• Elimination of carcinogenic derivatization agents and membrane suppressors.
• Minimal sample preparation—only filtration is required.
• Potential to expand the approach for simultaneous determination of other oxyhalides (iodate, chlorite, nitrite) by adjusting reagents and eluent composition.

Future Trends and Potential Uses

Emerging developments may include:

• Integration with mass spectrometry to further lower detection limits into the parts-per-trillion range.
• Automation and miniaturization for in-field or on-line monitoring of water treatment streams.
• Adaptation to multi-anion screening protocols within a single chromatographic run.
• Development of green chemistry alternatives for reagents and eluent components.

Conclusion

The optimized ion chromatographic method with post-column iodide derivatization and UV detection provides a sensitive, robust, and straightforward approach to quantify bromate in water at sub-ppb levels. By simplifying the hardware requirements and avoiding hazardous reagents, this protocol is well suited for routine water quality monitoring in laboratories and water treatment facilities.

References

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5. G. Bogenschütz et al., Advanced detection techniques in ion chromatography, Metrohm Monograph, Herisau, Switzerland (2007).
6. Metrohm Application Note No. U-9, Iodate, chlorite, bromate and nitrite by suppressed ion chromatography applying post column reaction (PCR) and UV/vis detection.
7. H.P. Wagner et al., Journal of Chromatography A 956, 93–101 (2002).