Determination of Trace Anions in High-Purity Water by High-Volume Direct Injection with the EG40

Significance of the Topic

The determination of trace inorganic anions at sub-µg/L levels in high-purity water is critical for quality control in industries such as semiconductor manufacturing, pharmaceutical production, and power generation. Precise monitoring of fluoride, chloride, nitrate, sulfate and other common anions ensures product integrity, protects sensitive processes, and prevents equipment fouling. A reliable, high-throughput analytical approach that minimizes sample handling and contamination is therefore of great practical importance.

Objectives and Study Overview

This study presents a high-volume direct-injection ion chromatography method using an EG40 eluent generator for on-line production of carbonate-free potassium hydroxide eluents. Two microbore columns (IonPac AS11 and AS15, 2 × 250 mm) were evaluated for the rapid quantification of trace anions in high-purity water. Key goals included achieving low detection limits without preconcentration, maintaining retention time stability, and reducing baseline drift during gradient elution.

Methodology and Instrumentation

A high-volume direct-injection protocol was implemented with a 1 mL sample loop and microbore (2 mm) columns to boost mass sensitivity fourfold relative to 4 mm formats. Two gradient elution methods were developed:

• AS11 method: 0.5 mM to 26 mM KOH over 20 min at ambient temperature.
• AS15 method: 8 mM to 60 mM KOH gradient at 30 °C.

The EG40 eluent generator produced high-purity, carbonate-free KOH eluents in real time, eliminating manual solvent preparation and reducing background conductivity. Suppressed conductivity detection employed the ASRS-ULTRA in external water mode to achieve low noise.

Used Instrumentation

• Dionex DX-500 IC system with GP50 gradient pump, microbore configuration and CD20 conductivity detector.
• LC30 chromatography enclosure with Rheodyne Model 9126 PEEK injector.
• EG40 Eluent Generator System with EGC-KOH cartridge, pressurized sample vessel and low-pressure valve manifold.
• 2 mm IonPac AS11 and AS15 analytical and guard columns with ATC-1 trap columns.
• PEEK tubing loop (1000 µL), pressurized water reservoirs and PeakNet Chromatography workstation.

Main Results and Discussion

Both methods achieved rapid separation of common inorganic anions and low molecular weight organic acids within 35 min. Retention time RSD values for trace spikes (0.1–0.3 µg/L) were under 0.5% using the EG40-generated gradients, compared to up to 4.7% with conventional eluents. Method detection limits ranged from 0.0089 µg/L for fluoride to 0.037 µg/L for acetate (AS11), and 0.037–0.20 µg/L across AS15 targets. Carbonate-free eluents yielded minimal baseline shifts (<75 nS for AS11, <150 nS for AS15) and improved quantification consistency at low concentrations.

Contributions and Practical Applications of the Method

This approach simplifies trace anion analysis by removing the need for preconcentration columns and manual hydroxide preparation, reducing contamination risks. It enables routine monitoring of ultra-pure water systems, supports high throughput in QA/QC laboratories, and extends the analytical range of standard IC platforms into sub-ppb detection.

Future Trends and Potential Applications

Possible developments include integration of automated low-contamination sample handling with the EG40 platform, coupling with mass spectrometry for enhanced selectivity, and further miniaturization of microbore flow paths. Advances in on-line eluent generation and suppression technologies will continue to drive sensitivity improvements in environmental and process water analysis.

Conclusion

The direct-injection ion chromatography method leveraging the EG40 eluent generator and microbore columns delivers reliable sub-µg/L anion determinations in high-purity water. On-line KOH generation, reduced baseline drift, and robust retention time stability make this technique well suited for demanding industrial and research applications.

Reference

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• Dionex Application Note 113: High-Volume Direct Injection.
• Dionex Application Note 114: Trace Anion Analysis.
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