Determination of Anions in Acid Rain by Ion Chromatography: Application Update 146

Significance of the topic

Monitoring anions in precipitation is essential for understanding the environmental impact of acid rain and evaluating the effectiveness of emission controls. Accurate measurement of low-level inorganic anions provides insight into ecosystem acidification, human health risks, and regulatory compliance.

Objectives and overview of the study

This work aimed to develop a fully automated ion chromatography (IC) method for determining common anions in rainwater. The study evaluated an electrolytic eluent generator combined with a high-capacity hydroxide‐selective column to simplify operations, improve precision, and eliminate manual reagent preparation. Results were benchmarked against certified simulated rainwater samples.

Methodology and instrumentation

Reagents and materials were prepared from ACS-grade salts or certified standards. The IC system used:

• Dionex ICS-2000 Reagent-Free™ Ion Chromatography (RFIC) System
• EluGen® EGC-KOH electrolytic eluent generator
• Continuously Regenerated Anion Trap Column (CR-ATC)
• IonPac® AG18 guard (4 × 50 mm) and AS18 analytical (4 × 250 mm) columns
• ASRS® ULTRA suppressor in AutoSuppression® Recycle Mode
• Chromeleon® 6.5 data system

The mobile phase was 38 mM KOH generated in situ at 1 mL/min. The column temperature was maintained at 30 °C, and 25 µL sample injections were analyzed via suppressed conductivity detection.

Key results and discussion

The method achieved baseline resolution of seven inorganic anions (fluoride, chloride, nitrite, bromide, sulfate, nitrate, phosphate) in under 10 minutes. Calibration was linear (r2 > 0.999) over relevant concentration ranges, with method detection limits of 2.5–6.2 µg/L for common anions. Analysis of a NIST-traceable simulated rainwater sample produced results within certified deviation limits. Twenty replicates showed retention time RSDs below 0.06% and peak area RSDs under 2.5%, reflecting excellent stability and reproducibility.

Benefits and practical applications

• Full automation reduces operator error and labor by requiring only water addition.
• In-line eluent generation ensures consistent reagent purity and eliminates manual preparation.
• High-capacity hydroxide selective columns deliver fast, isocratic separations with low background conductivity.
• Reliable quantitation at trace levels supports environmental monitoring, regulatory compliance, and research studies.

Future trends and potential applications

Advances in RFIC technology will enable expanded multi-ion panels and coupling with mass spectrometry for enhanced selectivity. Miniaturized systems and field-deployable IC analyzers are expected to support in situ monitoring of precipitation. Integration with cloud-based data analytics will facilitate real-time environmental mapping and decision support.

Conclusion

The described RFIC method provides a robust, fully automated solution for anion analysis in rainwater. High precision, low detection limits, and rapid throughput make it well suited for environmental laboratories and field studies. In-line eluent generation and hydroxide eluents streamline operations and improve data quality, supporting ongoing efforts to study and mitigate acid rain.

References

• EPA Method 300.6: Chloride, Orthophosphate, Nitrate, and Sulfate in Wet Deposition by Chemically Suppressed IC, 1986.
• ASTM D5085-90: Determination of Chloride, Nitrate, and Sulfate in Wet Deposition, 2003.
• Dionex Application Note 154: Improved Hydroxide Separations with IonPac AS18.