Determination of Silicate and Inorganic Anions in High Purity Water using Sequential Detection and AutoPrep

Significance of the Topic

The reliable measurement of trace silicate and inorganic anions in ultra-pure water is critical for industries such as semiconductor manufacturing, where even sub-µg/L contamination can impair production yields and damage sensitive equipment.

Objectives and Overview

This study demonstrates a unified ion chromatography method that simultaneously determines sub-µg/L concentrations of silicate, common inorganic anions (fluoride, chloride, nitrite, bromide, nitrate, sulfate), and phosphate in a single injection. The approach integrates suppressed conductivity detection for anions and a post-column molybdate reaction with visible absorbance for silicate, all automated using a Sequential Detection and AutoPrep system.

Methodology

An RFIC (Reagent-Free Ion Chromatography) system was configured for a single-run analysis. Key steps:

• Eluent generation: KOH gradient (15→50 mM) controlled by an EluGen II cartridge.
• Separation: IonPac AS19 analytical column (2 × 250 mm) with AG19 guard and AG4A-SC concentrator.
• Detection A: Suppressed conductivity (ASRS-300/CRD-200 external modes) for anions and phosphate.
• Post-column reaction: 20 mM sodium molybdate, 0.2 N HNO₃, 6 mM sodium lauryl sulfate at 0.15 mL/min.
• Detection B: Visible absorbance at 410 nm for silicate.
• Automated sampling: AutoPrep with 10 mL sample loops, PC-100 peristaltic pumps, and valve switching sequences to load standards or samples.

Instrumentation

• Dionex ICS-3000 with DP Dual Pump, DC Detector/Chromatography module
• Eluent Generator (EGC II KOH cartridge) and Backpressure tubing
• IonPac AS19 column set (AS19, AG19, AG4A-SC, ATC-HC trap)
• Suppressor modules: ASRS-300 (external water mode), CRD-200 (external chemical mode)
• AutoPrep automation manager with 10-port and 3-way valves
• PC-100 pump controllers for reagent and sample delivery
• Chromeleon 6.8 chromatography data system

Results and Discussion

Method detection limits (MDLs) for anions ranged from 0.97 to 12.7 ng/L; silicate MDL was 82.9 ng/L. Calibration across four levels (10–80 ng/L for low-level anions; 500–4000 ng/L for silicate) exhibited linearity coefficients >99.8%. Five successive check-standard injections showed recoveries between 90.9% and 116.4% and RSDs below 8%. Blank water showed no detectable silicate, confirming system cleanliness and low background.

Benefits and Practical Applications

The combined assay reduces analysis time by merging two determinations into one run, minimizes reagent consumption and waste through reduced flow rates and column dimensions, and enhances throughput via automated calibration and sample injection. The high sensitivity and reproducibility suit applications in semiconductor quality control, power-plant monitoring, and any process requiring ultra-pure water.

Future Trends and Applications

Integration of inline sampling and real-time monitoring, coupling with high-resolution mass spectrometry for unknown ion identification, miniaturized systems for field deployment, and further reagent consumption reduction through microfluidic adaptations represent key advances for trace anion analysis in ultra-pure matrices.

Conclusion

The presented method offers a robust, sensitive, and efficient solution for simultaneous determination of silicate and inorganic anions at sub-µg/L levels in high-purity water. Automation through sequential detection and AutoPrep streamlines laboratory operations while maintaining stringent analytical performance.

Reference

• Dionex Corporation (2007). Determination of Silicate in High Purity Water Using Ion Chromatography and AutoPrep. Application Note 170: LPN 1992.
• Dionex Corporation (2008). ICS-3000 Ion Chromatography System Operator’s Manual. Document No. 065031-03.