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

Significance of the Topic

Ion chromatography (IC) is a cornerstone technique for monitoring inorganic anions in environmental waters under regulations such as the U.S. Safe Drinking Water Act and Clean Water Act. Optimizing column selectivity and eluent purity is critical to achieving low detection limits and regulatory compliance.

Study Objectives and Overview

This application note evaluates the performance of a high-capacity, hydroxide-selective anion-exchange column (Dionex IonPac AS18) coupled with an automated, reagent-free IC system (Thermo Scientific Dionex ICS-2000). The goal was to compare its resolution, capacity, and detection limits for common inorganic anions against a conventional carbonate/bicarbonate method defined in U.S. EPA Method 300.0.

Methodology and Used Instrumentation

Sample Preparation:

• Filtration through 0.45 µm (0.20 µm for wastewater) syringe filters; C18 cleanup for hydrophobic load.

Instrument Configuration:

• Thermo Scientific Dionex ICS-2000 Reagent-Free IC system
• Dionex IonPac AS18 analytical (4×250 mm) and AG18 guard (4×50 mm) columns
• Dionex CR-ATC trap column for carbonate removal
• Dionex ASRS ULTRA suppressor, recycle mode at 100 mA
• Automated KOH eluent generation (22–40 mM gradient)

Calibration and QC:

• Seven-point calibration covering 0.1–200 mg/L
• Method detection limits (MDLs) determined by seven replicates of fortified blanks
• Quality control standards to assess retention time and peak area precision

Main Results and Discussion

Resolution and Selectivity:

• Fluoride baseline resolved from void volume under hydroxide eluent; sulfate elutes between bromide and nitrate, improving peak order.
• Phosphate elution after sulfate due to higher pH and greater anion charge.

Performance Metrics:

• Linearity (r2 ≥ 0.9991) over wide concentration ranges
• MDLs between 1.6 and 5.7 µg/L
• Retention time RSD ≤ 0.13%; peak area RSD ≤ 0.73%

Recovery Studies:

• Spiked drinking, surface, ground, and wastewater samples showed 92–120% recoveries for all anions.
• High-strength matrices (e.g., sulfate >200 mg/L) quantified without column overloading.

Benefits and Practical Applications

• Automated KOH eluent generation eliminates manual preparation and carbonate contamination, ensuring baseline stability.
• High column capacity reduces sample dilution and re-analysis for high-ionic-strength waters.
• Fully integrated reagent-free system enhances laboratory throughput and regulatory compliance for drinking water and wastewater monitoring.

Future Trends and Potential Applications

• Expansion of hydroxide-eluent methods to additional IC applications and coupled techniques (e.g., IC–MS).
• Design of new stationary phases targeting emerging anionic contaminants.
• Further automation and miniaturization for field-deployable IC systems.

Conclusion

The combination of the Dionex IonPac AS18 column with a reagent-free IC system and automated KOH eluent generation significantly improves the analysis of inorganic anions in environmental waters. It delivers enhanced resolution, wider linear ranges, lower MDLs, and robust performance across diverse matrices, fully meeting or exceeding U.S. EPA Method 300.0 requirements.

References

• U.S. EPA Method 300.0: Determination of Inorganic Anions by IC, 1993.
• Federal Register Vols. 59–64 (1994–1999) on SDWA and CWA regulations.
• Greenberg A.E., Clesceri L.S., Eaton A.D., Standard Methods for the Examination of Water and Wastewater, 18th ed., APHA, 1992.
• ASTM D4327-97: Standard Test Methods for Anions in Water by Suppressed IC.
• Dionex Application Notes 133 & 135, Thermo Fisher Scientific.
• Jackson P.E., Pohl C.A., Trends Anal. Chem., 1997, 16, 393–400.
• Liu Y. et al., Am. Lab., Nov 1998, 48C–54C.
• Srinivasan K. et al., Proc. Int. IC Symposium, 2002.
• U.S. EPA Office of Water memorandum, Nov 19, 2002.