Determination of Inorganic Anions in Acid Rain Using a High-Pressure Modular Capillary Ion Chromatography System

Significance of the Topic

Acid rain, formed when atmospheric water reacts with sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), poses serious risks to ecosystems, infrastructure and human health. Monitoring its anionic composition is critical for evaluating air quality, enforcing emission regulations and guiding environmental policy.

Objectives and Study Overview

This study demonstrates a rapid, high-pressure capillary ion chromatography (HPIC) technique for quantifying inorganic anions in rainwater. By leveraging a modular HPIC system operating up to 5000 psi and a 4 µm particle anion-exchange column, the method aims to shorten analysis time without sacrificing resolution or sensitivity.

Methodology and Instrumentation

Sample preparation involved filtering rainwater through a 0.20 µm syringe filter and using degassed 18 MΩ·cm deionized water to prepare standards and flush solutions. Key analytical conditions included:

• Guard column: Dionex IonPac AG18-4 µm (0.4 × 50 mm)
• Separation column: Dionex IonPac AS18-4 µm (0.4 × 150 mm)
• Eluent: 23 mM KOH generated by EGC-KOH cartridge
• Flow rates: 0.010–0.025 mL/min
• Detection: suppressed conductivity using ACES 300 capillary suppressor in recycle mode
• Column temperature: 30 °C

Instrumentation was configured to minimize dead volume using precision tubing and high-pressure fittings. A trap column (ATC-500) was installed upstream of the eluent generator to protect against contaminants.

Main Results and Discussion

Increasing the flow rate from 0.010 to 0.025 mL/min reduced the separation time for seven anions to under 3 minutes while maintaining baseline resolution. System backpressure remained below 3800 psi, within the HPIC limit. Analysis of rain samples from Sunnyvale and Campbell, CA, revealed chloride (0.84–1.37 mg/L), sulfate (1.56–5.4 mg/L) and nitrate (0.11–0.67 mg/L), highlighting the method’s applicability to environmental monitoring.

Benefits and Practical Applications

Fast cycle times enable high sample throughput, crucial for routine environmental surveillance. Low eluent consumption and minimal waste generation reduce operating costs and environmental impact. Continuous operation capability makes the system ideal for automated monitoring in research and regulatory labs.

Future Trends and Applications

Advances in micro- and nano-scale columns, along with integrated mass spectrometric detection, promise even faster separations and lower detection limits. Coupling HPIC with real-time data analytics will enhance field-deployable monitoring and source apportionment studies. Expanded use of eco-friendly eluent generation and online sample pretreatment will further streamline workflows in environmental and industrial analysis.

Conclusion

The high-pressure capillary IC approach delivers rapid, high-resolution anion analysis in acid rain with reduced eluent consumption and waste. Its robustness and speed make it a valuable tool for environmental laboratories tasked with large-scale monitoring and compliance testing.

Instrumentation

• Dionex ICS-5000+ HPIC capillary system
• Dionex ICS-5000+ DC Detector/Chromatography module with high-pressure degas cartridge
• Thermo Scientific Dionex IC Cube for column and suppressor housing
• Dionex ICS-5000+ DP Dual Pump module
• Dionex ICS-5000+ EG Eluent Generator module with EGC-KOH cartridge
• ACES 300 Anion Capillary Suppressor
• Dionex IonPac AG18-4 µm guard and AS18-4 µm separation columns
• Dionex CR-ATC trap column and CRD 180 carbonate removal device
• Dionex AS-AP Autosampler

References

1. U.S. EPA Acid Rain Program
2. Thermo Fisher Scientific Application Note AN 2, 2003
3. Thermo Fisher Scientific Application Note AU 154, 2003
4. Thermo Fisher Scientific Application Note AN 31, 1992
5. Thermo Fisher Scientific Application Update AU 146, 2003
6. Thermo Fisher Scientific Technical Note TN 113, 2012
7. Thermo Fisher Scientific Dionex ICS-5000+ Installation Manual, 2012
8. Thermo Fisher Scientific Technical Note TN 131, 2013
9. Thermo Fisher Scientific CR-TC Product Manual, 2010
10. Thermo Fisher Scientific CES 300 Suppressor Manual, 2010
11. Thermo Fisher Scientific AS-AP Operator’s Manual, 2012