Quantification of Trace and Major Anions in Water by Ion Chromatography in a High-Throughput Laboratory

Importance of the topic

The accurate and simultaneous determination of both trace and major inorganic anions in drinking water is crucial for public health and regulatory compliance. Bromate, a known carcinogenic by-product of ozonation, is regulated at low microgram-per-liter levels by WHO and the European Union. Traditional ion chromatography methods face a trade-off between the large injection volumes needed for trace analytes and the chromatographic resolution required for high-concentration ions. High-throughput laboratories demand a streamlined approach that delivers low detection limits without sacrificing analytical efficiency or increasing operational costs.

Objectives and study overview

This study presents a unified ion chromatography method enabling the simultaneous quantification of ten inorganic anions—fluoride, chloride, nitrite, nitrate, phosphate, sulfate, bromide, chlorite, chlorate, and bromate—in various water matrices. The goal was to avoid splitting the analysis into multiple runs, to achieve low limits of quantification (LOQs) for trace oxyhalides, and to maintain robust resolution for major anions, thereby improving laboratory throughput and reducing per-sample costs.

Methodology and instrumentation

The approach employs a dual-loop, dual-condition configuration on a Dionex ICS-3000 Reagent-Free Ion Chromatography system:

• A 25 µL loop for major anions, eluted with an electrolytically generated KOH gradient and detected by suppressed conductivity.
• A 250 µL loop for trace oxyhalides (chlorite, bromate), eluted isocratically with 9 mM Na₂CO₃, subjected to postcolumn reaction (PCR) with acidified nitrite/bromide reagent, and detected by UV-visible absorbance at 267 nm.

This parallel setup uses two six-port injection valves, an ASRS suppressor, a heated reaction coil, and Thermo Scientific Dionex Chromeleon CDS for control and data processing.

Instrumentation used

• Dionex ICS-3000 RFIC system with dual-piston pump and electrolytic KOH generator
• Dionex ASRS™ self-regenerating suppressor
• Dionex IonPac AS19/AG19 columns for major anions
• Dionex IonPac AS9-HC/AG9-HC columns for trace oxyhalides
• Dionex AXP Auxiliary Pump for postcolumn reagent delivery
• UV-visible detector set at 267 nm with heated reaction coil
• Thermo Scientific Dionex AS50 Autosampler
• Chromeleon 6.8 Chromatography Data System

Main results and discussion

Chromatographic condition 1 (25 µL, KOH gradient, conductivity) achieved baseline resolution for major anions with resolution factors >1.3. Condition 2 (250 µL, Na₂CO₃ isocratic, PCR, UV-vis) provided clear separation of chlorite and bromate with resolutions of 1.59 and low LOQs of 0.25 µg/L and 0.12 µg/L, respectively. The unified method meets ISO and EU requirements for drinking water. Repeatability and intermediate reproducibility studies following ISO 5725 and NFT 90-210 yielded control limits within ±3 s, demonstrating precise quantification. Participation in inter-laboratory proficiency schemes confirmed average deviations below ±20% for all targeted anions.

Benefits and practical applications

• Single-run analysis streamlines workflow in high-throughput water quality laboratories.
• Large-loop injection plus PCR enhances sensitivity for trace bromate and chlorite without compromising major anion resolution.
• Electrolytic eluent generation reduces reagent handling and pump count.
• Compliance with regulatory LOQs and international standards ensures suitability for routine drinking water monitoring and QA/QC.

Future trends and potential applications

Advances may include migrating the method to newer systems such as the Dionex ICS-5000 dual-pump RFIC platform, integration with mass spectrometric detection for expanded analyte panels, and application to emerging oxyhalide contaminants. Automation of sample pretreatment and data processing can further boost throughput and method robustness. Continuous development of postcolumn chemistries may lower detection limits for ultra-trace species.

Conclusion

A parallel dual-loop ion chromatography approach with conductivity and UV-vis detection after postcolumn reaction offers a powerful, efficient solution for simultaneous trace and major anion analysis in drinking water. The method achieves regulatory LOQs for bromate and chlorite, ensures high chromatographic resolution for major ions, and delivers reliable performance validated through precision studies and external proficiency testing.

References

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