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

Significance of the Topic

The monitoring of inorganic anions in precipitation is essential to assess environmental acidification, ecosystem health, and the effectiveness of air-pollution regulations. Acid rain, formed by atmospheric reactions of SO2 and NOx, can damage vegetation, aquatic life, infrastructure, and human health. Rapid, sensitive, and low-waste analytical methods support large-scale monitoring programs and regulatory compliance.

Aims and Overview of the Study

This work demonstrates the rapid separation and quantitation of key inorganic anions (fluoride, chloride, nitrite, bromide, nitrate, sulfate, carbonate, chlorite, chlorate) in rainwater samples. The study employs a dedicated high-pressure capillary ion chromatography (HPIC) system (Thermo Scientific Dionex ICS-4000) combined with 4 µm particle size capillary columns to achieve fast analysis with minimal eluent consumption and waste generation.

Methodology and Instrumentation

Sample Preparation:

• Rain samples filtered through 0.20 µm IC syringe filters to remove particulates.
• Mixed calibration standards prepared in 18 MΩ·cm deionized water; all solutions degassed to prevent contamination and maintain sensitivity.

Chromatographic Conditions:

• Columns: Dionex IonPac AG18-4 µm (0.4 × 50 mm) guard and AS18-4 µm (0.4 × 150 mm) separation column.
• Eluent: 23 mM KOH generated in-line by a capillary EGC-KOH cartridge.
• Flow rates: 0.010, 0.020, and 0.025 mL/min; column temperature 30 °C; compartment temperature 15 °C; injection volume 0.4 µL (full-loop).
• Detection: Suppressed conductivity using an ACES-300 capillary suppressor in recycle mode (8 mA at 0.010 mL/min; 13 mA at 0.025 mL/min).

Instrumentation Used

• Thermo Scientific Dionex ICS-4000 HPIC system with high-pressure pump and eluent generation degasser.
• Thermo Scientific Dionex AS-AP autosampler and Chromeleon CDS software (v7.1 SR2 MUa or later).
• Dionex EGC-KOH eluent generator cartridge, CR-ATC electrolytic anion trap, EG Degas cartridge, CRD bypass device, and Dionex ACES capillary suppressor.

Main Results and Discussion

Standard separations at 0.010, 0.020, and 0.025 mL/min produced well-resolved peaks within 4 min at system backpressures from ~1 600 to ~3 500 psi. In the rain sample, chloride and sulfate were the dominant species, while nitrite and nitrate levels were low, reflecting atmospheric NOx conditions during precipitation. High-pressure operation enabled fast throughput without loss of resolution.

Benefits and Practical Applications

• Significantly reduced analysis time and eluent consumption (≈15 mL water per day), lowering operational costs and waste.
• Enhanced sample throughput for routine environmental monitoring and QA/QC laboratories.
• Extended cartridge lifetime (up to 18 months) and compact system footprint for efficient bench use.

Future Trends and Potential Applications

Advances in capillary HPIC may include further miniaturization, integration with on-line sample preparation, coupling to mass spectrometry for speciation studies, and deployment in automated or field-portable systems for real-time environmental monitoring. Development of novel selective stationary phases and suppressors could expand analyte coverage and sensitivity.

Conclusion

The dedicated high-pressure capillary IC system combined with 4 µm capillary columns enables rapid, sensitive determination of inorganic anions in rainwater. The approach offers low eluent consumption, minimal waste, high resolution, and robust performance, supporting large-scale environmental monitoring and regulatory programs.

References

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