Determination of Anions in Acid Rain by Ion Chromatography

Importance of the Topic

Acid rain, driven by sulfur dioxide and nitrogen oxide emissions, poses significant risks to ecosystems, infrastructure and public health. Reliable monitoring of inorganic anions such as fluoride, chloride, nitrate and sulfate in atmospheric deposition is essential for evaluating environmental impacts and tracking the effectiveness of pollution control measures.

Objectives and Study Overview

This work aimed to demonstrate a fully automated ion chromatography (IC) method for determining low-level anions in rainwater. By combining electrolytic eluent generation with a high-capacity, hydroxide-selective anion-exchange column (Thermo Scientific™ Dionex™ IonPac™ AS18) and suppressed conductivity detection, the study evaluated precision, accuracy and detection limits using a simulated rainwater sample (SR-II) traceable to NIST standards.

Methodology and Instrumentation

Standard solutions (1 000 mg/L) of common anions were prepared or purchased and diluted to form five calibration levels covering expected rainwater concentrations. Anion separations were performed isocratically with 38 mM potassium hydroxide eluent generated in situ by a Dionex™ ICS-2000 RFIC™ system. Chromatographic conditions included a Dionex IonPac AG18 guard and AS18 analytical column (4 × 250 mm), 30 °C column temperature, 1.0 mL/min flow rate, 25 µL injection volume, and suppressed conductivity detection with an ASRS™ Ultra suppressor in recycle mode. Method detection limits were calculated from replicate injections (n = 7) at 99% confidence.

Used Instrumentation

• Thermo Scientific™ Dionex™ ICS-2000 Reagent-Free™ Ion Chromatography System
• Dionex™ EGC-KOH Eluent Generator Cartridge
• Dionex™ CR-ATC Continuously Regenerated Anion Trap Column
• Dionex™ IonPac™ AG18 Guard and AS18 Analytical Columns
• Dionex™ ASRS™ Ultra Anion Suppressor, 4 mm, AutoSuppression™ recycle mode
• Dionex™ AS50 Autosampler and Chromeleon™ 6.8 Chromatography Data System

Key Results and Discussion

The method achieved baseline separation of seven anions in under 10 minutes, with calibration curves exhibiting r² values above 0.9994. Calculated MDLs ranged from 2.5 µg/L for fluoride to 6.2 µg/L for nitrate. Analysis of the SR-II sample yielded recoveries within certified uncertainty (e.g., fluoride 0.10 ± 0.002 mg/L, chloride 0.99 ± 0.02 mg/L, nitrate 7.10 ± 0.04 mg/L, sulfate 10.0 ± 0.03 mg/L). Retention time RSDs were below 0.06% and peak area RSDs below 2.5% (n = 20), demonstrating excellent reproducibility afforded by electrolytic eluent generation.

Benefits and Practical Applications

• Eliminates manual eluent preparation, reducing operator error.
• Automated eluent generation and suppressor regeneration improve uptime and reliability.
• High-capacity AS18 column delivers rapid, isocratic separations of common anions.
• Enhanced detection limits and linearity support trace-level environmental monitoring.
• Method easily transferable across laboratories, supporting regulatory compliance.

Future Trends and Applications

Advancements in reagent-free IC promise integration with online sampling platforms for real-time environmental surveillance. Coupling hydroxide-selective IC with mass spectrometry could extend analyte scope to emerging pollutants. Further miniaturization and field-deployable systems may enable distributed monitoring networks for air and precipitation chemistry.

Conclusion

This study validates a robust, fully automated IC method for quantifying inorganic anions in rainwater using electrolytic eluent generation and a hydroxide-selective AS18 column. The approach delivers rapid analysis, low detection limits and excellent precision while simplifying operation through reagent-free eluent preparation.

References

1. Chloride, Orthophosphate, Nitrate, and Sulfate in Wet Deposition by Chemically Suppressed Ion Chromatography; EPA Method 300.6; U.S. Environmental Protection Agency, 1986.
2. Standard Test Method for the Determination of Chloride, Nitrate, and Sulfate in Atmospheric Wet Deposition by Chemically Suppressed Conductivity; ASTM Method D5085-90; Vol. 11.03; American Society for Testing and Materials, West Conshohocken, PA, 2003.
3. Dionex (now Thermo Scientific) Application Note 154: Determination of Inorganic Anions in Environmental Waters Using a Hydroxide-Selective Column; Sunnyvale, CA, 2003.