Determination of Trace Anions in High-Purity Waters Using Direct Injection and Two-Step Isocratic Ion Chromatography

Significance of the Topic

High-purity water, as used in semiconductor fabrication and power generation, must be free of trace anionic contaminants to prevent corrosion of critical metal components. At sub-µg/L levels, species such as fluoride, chloride, nitrate, phosphate, and sulfate can compromise stainless steel steam generators, boiler tubes, and microelectronic circuitry. Fast, reliable quantification of these anions supports real-time process control and prevents equipment failures.

Objectives and Overview of the Study

This study presents a streamlined ion chromatography method for direct injection analysis of trace anions in deionized and treated power-plant waters. By employing a two-step isocratic borate eluent and a 2-mm IonPac AS14 column, the method targets sub-µg/L detection in less than 15 minutes per run, eliminating lengthy preconcentration steps.

Methodology

Samples (1 mL) are loaded directly into a 1000-µL loop and analyzed by suppressed conductivity detection. A step gradient of Eluent A (9 mM boric acid/6.75 mM NaOH) to Eluent B (40 mM boric acid/30 mM NaOH) separates early-eluting weak acids and fluoride from strongly retained anions. The total run time, including re-equilibration, is under 15 minutes. Calibration uses mixed standards prepared from 1000 mg/L stock solutions, diluted to low-µg/L levels. Blanks are confirmed by replicate injections of high-resistivity water under identical conditions.

Used Instrumentation

• Dionex DX-500 system with GP40 microbore gradient pump
• CD20 conductivity detector with DS3 temperature-controlled cell
• LC20 enclosure with modified Rheodyne six-port injector (rear-loading configuration)
• IonPac AS14 analytical column (2 × 250 mm) and AG14 guard (2 × 50 mm)
• ATC trap suppressor (2 mm) in external water AutoSuppression™ mode
• PeakNet Chromatography Workstation

Main Results and Discussion

The two-step borate eluent affords clear resolution of fluoride and organic acids from strongly retained anions in under 15 minutes. Baseline noise stabilizes after a 5-hour warm-up; external water suppression and periodic trap regeneration maintain low drift. Method detection limits in deionized water are 0.018 µg/L for fluoride, 0.044 µg/L for chloride, 0.55 µg/L for nitrate, 0.66 µg/L for phosphate, 0.11 µg/L for sulfate, and 0.21 µg/L for oxalate. Comparable performance is observed in an 8 mg/L morpholine matrix. Calibration curves (0.1–10 µg/L) are linear (r2 > 0.99) with negligible bias between matrices.

Benefits and Practical Applications

This direct-injection approach streamlines trace anion analysis by removing sample pumps and concentrator columns, reducing run times and consumables. It is well suited for high-throughput quality control in semiconductor fabs and power-plant monitoring, offering sub-µg/L sensitivity with minimal operator intervention.

Future Trends and Possibilities of Use

Advances may include on-line degassing modules, further miniaturization of flow cells, integration with automated sampling systems, and expansion of suppressor technology. Adapting the method to other treated waters or coupling with mass spectrometry could extend its applicability to emerging contaminants.

Conclusion

The presented two-step isocratic ion chromatography method using a 2-mm AS14 column and high-volume direct injection achieves rapid, high-sensitivity determination of trace anions in high-purity waters. Its simplicity and robustness make it a valuable tool for industries requiring strict water quality control.

References

1. Sinclair JD. J Electrochem Soc. 1988;135:89–95.
2. Electric Power Research Institute. Corrosion Fatigue Boiler Failures; EPRI TR-100455. Palo Alto, CA; 1992.
3. Dionex Corp. Application Note 113. Sunnyvale, CA; 1996.