Determination of Trace Anions in High Purity Waters by High Volume/Direct Injection Ion Chromatography

Importance of the Topic

High-purity water is an essential medium in semiconductor manufacturing and power generation, where trace anions at sub-µg/L levels can severely impact process efficiency, product quality, and equipment lifespan. Reliable detection of contaminants such as fluoride, chloride, sulfate, and organic acids at levels down to parts per trillion is therefore critical for both quality assurance and regulatory compliance.

Objectives and Overview

This study demonstrates a direct injection, high-volume ion chromatography method using the IonPac® AS11 column to quantify trace anions in ultra-pure and power-plant waters. By employing a controlled sodium hydroxide gradient, the method aims to simplify analysis by eliminating offline preconcentration steps, while achieving detection limits in the low-µg/L to high-ng/L range.

Methodology and Instrumentation

Key components of the analytical setup included:

• Dionex DX-500 ion chromatography system with GP40 gradient pump and CD20 conductivity detector.
• IonPac AS11 analytical and AG11 guard columns (2 mm i.d.) with an ATC suppressor in external water mode.
• Sodium hydroxide eluents prepared at 0.5–100 mM and a 200 mM NaOH regeneration solution.

Samples (750 µL) were injected directly, avoiding concentrator columns. A rear-loading Rheodyne injector with PEEK tubing minimized dead volume. Eluents were degassed and delivered under helium pressure; external deionized water was supplied at 5–7 mL/min for suppressor regeneration.

Key Results and Discussion

The low-initial gradient (0.5 mM NaOH) separated early-eluting anions (fluoride, acetate, formate) without excessive void peaks, while the gradient ramp to 26 mM NaOH eluted polyvalent ions (sulfate, phosphate). No significant loss of column efficiency was observed when increasing injection volumes from 25 to 750 µL.

Method detection limits (MDLs) in deionized water ranged from 0.015 µg/L (fluoride) to 0.42 µg/L (phosphate). In an 8 mg/L morpholine matrix, MDLs increased moderately but remained below 1 µg/L for most ions. Calibration curves prepared in both matrices were linear (r² > 0.99), with parallel slopes indicating minimal matrix bias.

Benefits and Practical Applications

• Elimination of preconcentration hardware and steps reduces complexity, cost, and analysis time.
• High-volume direct injection allows sub-µg/L detection without offline enrichment.
• Robust operation supports continuous monitoring of semiconductor and power-plant waters for QA/QC.

Future Trends and Potential Applications

Advances may include integration with automated sample handling to further reduce contamination risk, coupling with mass spectrometric detection for enhanced selectivity, and miniaturized systems for field-deployable monitoring. Emerging membrane suppressor technologies and novel eluent systems may push detection limits into the ppt range for emerging contaminants.

Conclusion

This gradient ion chromatography method on the IonPac AS11 column provides a streamlined, sensitive approach for trace anions in ultra-pure and power-plant waters. By removing preconcentration steps and leveraging high-volume direct injection, it achieves low-ppb to high-ppt detection limits with excellent linearity and robustness.

References

1. Electric Power Research Institute, PWR Secondary Water Chemistry Guidelines, TR-102134 (1993).
2. SEMI, Process Chemicals Volume, SEMI Standards (1995).
3. J. Weiss, Ion Chromatography, 2nd Ed., VCH Weinheim (1995).
4. Dionex Corp., Technical Note No. 8, Sunnyvale, CA (1994).