Determination of trace anions in ultrapure water using capillary ion chromatography

Significance of the topic

Reliable determination of trace anions in ultrapure water is critical for advanced power and semiconductor manufacturing. Ionic impurities at the microgram-per-liter and nanogram-per-liter level can lead to equipment corrosion, unplanned downtime, poor product yields and premature device failure. Continuous monitoring and control of anion levels in ultrapure water (UPW) help maintain process integrity and product quality.

Objectives and study overview

This application note describes how capillary ion chromatography (IC) combined with modern autosampling and online water purification systems can achieve ultratrace anion analysis in UPW. The key goals are:

• Demonstrate direct large-loop injection and preconcentration techniques for sub-µg/L to ng/L anion detection.
• Minimize environmental and procedural contamination through rigorous sample handling.
• Evaluate method performance using ultrapure water blanks and low-level mixed anion standards.

Methodology

A trace-analysis laboratory protocol was established to reduce blank contamination:

• Point-of-use recirculating 18.2 MΩ∙cm deionized water system, continuously flowing during reagent preparation.
• Dedicated Class 10 clean area or laminar-flow hood; routine wipe-downs with low-ionic polyester wipes.
• Use of powder-free Class 10 nitrile gloves, polystyrene culture flasks and FEP containers; rigorous five-step rinse and soaking procedures.
• Certified high-purity ion standards and pre-tested reagents; isolation of trace-analysis tools and accessories.
• Sample collection in pre-cleaned, liner-less vials; immediate capping with single-use PTFE or blue septa.
• Preparation of intermediate (100 µg/L) and working (1 µg/L, 100 ng/L) mixed anion standards gravimetrically under clean conditions.

Instrumentation used

The analytical system comprised:

• Thermo Scientific Dionex ICS-5000 capillary IC system (SP single-pump or DP dual-pump module, eluent generator, capillary EGC-KOH cartridge, CR-ATC trap column, capillary CD detector, IC Cube capillary chromatography module).
• Thermo Scientific Dionex AS-AP autosampler with PEEK low-dead-volume tubing, large-loop and concentrate-mode configurations.
• Thermo Scientific Dionex ICW-3000 Online Water Purifier supplying ultrapure water to pump, autosampler and suppressor regenerant.
• Thermo Scientific Dionex IonSwift MAC-100 monolith concentrator column for anion preconcentration.
• Dionex CRD 200 capillary carbonate removal device; high-pressure 6-port valve pod for large-volume injections; Chromeleon CDS software.

Main results and discussion

Two injection approaches were evaluated on a capillary Dionex IonPac AS19 or AS15 column with electrolytically generated KOH eluent at 10 µL/min:

• Direct large-loop injection (10 µL) yielded detection limits as low as 2–12 ng/L for chloride, sulfate and phosphate in ultrapure water blanks.
• Preconcentration mode (100–200 µL on MAC-100) improved sensitivity by a factor of 3–5; blank chloride and sulfate levels of ~25–50 ng/L achieved.
• Calibration over multiple volume loads showed excellent linearity (r² > 0.9993) and reproducible retention times.

Benefits and practical applications

Capillary IC with online purification and autosampling offers:

• High mass sensitivity at low flow rates (< 30 µL/min), reducing water use to ~5 L/year and waste generation.
• Minimal sample volume requirements (25–100× smaller than conventional systems) for remote or limited samples.
• Improved blank stability and reproducibility through rigorous contamination control.
• Direct applicability to semiconductor UPW monitoring, power plant water quality, and high-purity process fluids.

Future trends and potential applications

Emerging directions include:

• Integration of capillary IC with mass spectrometric detection for enhanced selectivity at ultratrace levels.
• Automated in-line sampling and remote monitoring for continuous water quality control.
• Further miniaturization of IC modules for field-deployable trace anion analysis.
• Expanded screening to organic acids, silicates and transition-metal anions in ultrapure process streams.

Conclusion

By combining capillary IC with advanced autosampler configurations, online water purification and rigorous cleanroom protocols, reliable detection of anions down to the low-ng/L level in ultrapure water is achievable. These methods support the stringent quality requirements of power generation and semiconductor manufacturing.

Reference

1. C. Christison et al., Thermo Scientific Technical Note 48, 2001.
2. Thermo Scientific Application Note 113, 1996.
3. Thermo Scientific Application Update 142, 2001.
4. Thermo Scientific Application Note 146, 2003.
5. Thermo Scientific Application Update 157, 2006.
6. Thermo Scientific Application Note 222, 2009.
7. Thermo Fisher Scientific Application Update 169, 2009.
8. Thermo Scientific ICW-3000 Installation Instructions, 2012.
9. Thermo Scientific Dionex AS-AP Autosampler Operator’s Manual, 2011.
10. Thermo Scientific Dionex ICS-5000 Installation Instructions, 2011.
11. Thermo Scientific Dionex ICS-5000 Operator’s Manual, 2011.
12. Thermo Scientific Technical Note 113, 2011.
13. Thermo Scientific IonSwift MAC-100 Product Manual, 2010.