Determination of Inorganic Anions in Environmental Waters Using a Hydroxide-Selective Column

Significance of the Topic

The monitoring of inorganic anions in environmental waters is critical for public health and environmental protection. Regulations such as the U.S. Safe Drinking Water Act and Clean Water Act mandate strict limits on contaminants like fluoride, nitrite, nitrate and others. Ion chromatography (IC) has become the standard technique for compliance monitoring, but conventional carbonate/bicarbonate eluents and columns show limitations in selectivity and automation.

Objectives and Study Overview

This application note evaluates a new hydroxide-selective anion exchange column (Dionex IonPac AS18) combined with automated, reagent-free potassium hydroxide eluent generation (Thermo Scientific Dionex ICS-2000) for the determination of common inorganic anions in drinking, surface, groundwater, and wastewater. The goal is to demonstrate improved resolution, linearity, detection limits, and operational efficiency under U.S. EPA Method 300.0 requirements.

Instrumentation Used

• Thermo Scientific Dionex ICS-2000 Reagent-Free Ion Chromatography system
• Dionex EGC-KOH cartridge for KOH eluent generation
• Dionex CR-ATC continuously regenerating anion trap column
• Dionex IonPac AS18 analytical (4×250 mm) and AG18 guard (4×50 mm) columns
• Dionex ASRS ULTRA 4 mm suppressor in recycle mode
• Dionex AS50 autosampler and Chromeleon 6.5 workstation

Methodology

Samples were filtered (0.20–0.45 µm) and, if needed, pre-treated to remove organics. Electrolytically generated KOH (22–40 mM gradient) served as the eluent, with a flow rate of 1.0 mL/min at 30 °C. Suppressed conductivity detection ensured low background noise. Seven-point calibrations were performed for fluoride, chloride, nitrite-N, bromide, nitrate-N, phosphate-P, and sulfate over 0.1–200 mg/L.

Main Results and Discussion

The AS18 column delivered superior fluoride resolution from the void, improved separation of chloride/nitrite, and retarded sulfate between bromide and nitrate. Linearity (r2>0.999), method detection limits (1.6–5.7 µg/L), and retention time RSD (<0.13%) met or exceeded EPA requirements. Spiked recovery tests in drinking, raw, surface, domestic and industrial wastewater waters yielded 88–120% across all analytes. High-ionic-strength samples (sulfate >200 mg/L) were quantified without dilution or column overload.

Benefits and Practical Applications

This approach simplifies eluent preparation, increases throughput, and reduces baseline drift by eliminating off-line eluent and trap column regeneration. It enhances routine compliance monitoring for regulatory agencies, environmental laboratories, and industrial QA/QC with greater reliability and automation.

Future Trends and Potential Applications

Automated hydroxide eluent generation may be extended to advanced IC applications, coupling with mass spectrometry, microbore columns, and high-throughput environmental screening. Further developments in reagent-free systems and novel stationary phases promise broader applicability for trace anion analysis in complex matrices.

Conclusion

The combination of the Dionex IonPac AS18 column and ICS-2000 reagent-free KOH generation offers significant improvements for inorganic anion analysis under EPA Method 300.0. Enhanced selectivity, sensitivity, and automation deliver robust, compliant results across a wide range of environmental water samples.

Reference

1. U.S. EPA Method 300.0, Determination of Inorganic Anions in Water by IC, 1993.
2. Federal Register 1995, Vol. 60, No. 201; 1999, Vol. 64, No. 230.
3. Federal Register 1994, Vol. 59, No. 145; 1998, Vol. 63, No. 170.
4. Greenberg AE et al., Standard Methods for the Examination of Water and Wastewater, 18th ed., APHA, 1992.
5. ASTM D4327-97, Standard Test Methods for Anions in Water by Chemically Suppressed IC, 1999.
6. Dionex Application Notes 133, 135, 154; Thermo Fisher Scientific, 2004–2014.
7. Jackson PE, Pohl CA, Trends Anal. Chem. 1997, 16, 393–400.
8. Liu Y et al., Am. Lab. 1998, Nov, 48C–54C.
9. Srinivasan K et al., Proc. Int. IC Symposium, 2002.
10. U.S. EPA Office of Water, Memorandum on Hydroxide Eluent Use, Nov. 19, 2002.