Determination of silicate in high-purity water using ion chromatography and online sample preparation

Importance of the Topic

Ultrahigh-purity water (UHPW) is essential in semiconductor and electronics manufacturing. Silicate breakthrough serves as an early indicator of deionization cartridge depletion, enabling timely replacement to prevent corrosive anion contamination that may compromise yield and device performance.

Objectives and Study Overview

This study aimed to develop an automated, highly sensitive ion chromatography (IC) method with online sample preconcentration to accurately quantify silicate below 1 µg/L in UHPW, facilitating early detection of cartridge exhaustion.

Instrumentation Used

• Thermo Scientific Dionex ICS-3000 system (DP Dual Pump, DC Detector/Chromatography Compartment, AM Automation Manager, EG Eluent Generator, VWD, AS-HV High-Volume Autosampler, RDM).
• Dionex IonPac AG4A-SC concentrator, AG17 guard, AS17 analytical columns.
• Dionex EGC II KOH cartridge with CR-ATC trap column.
• Dionex AutoPrep system and Chromeleon 6.8 CDS software.

Methodology

Silicate was preconcentrated by loading 20 mL of UHPW onto the AG4A-SC column, followed by separation on AG17/AS17 under a KOH gradient eluent generated reagent-free. Postcolumn, silicate reacted with a molybdate/nitric acid/sodium lauryl sulfate reagent and was detected at 410 nm. The AutoPrep system performed loop calibration and prepared standards via controlled fill volumes (1×–8×) in a 20 µL small loop and 20 mL large loop, automating calibration curves across 200–1,600 ng/L.

Main Results and Discussion

The method detection limit was 32.3 ng/L, satisfying sub-1 µg/L requirements. Calibration exhibited high linearity (R² 99.94%), slope 0.0321 mAU·min/(ng/L), and recovery of 92.5% for an 800 ng/L test. Laboratory deionized water contained 571 ng/L silicate, underscoring the need for silicate-free water production via inline polishing.

Benefits and Practical Applications

• Early indication of deionization exhaustion prevents deleterious chloride and sulfate breakthrough.
• Reagent-free eluent generation and automated sample prep reduce labor and minimize human error.
• High sensitivity and reproducibility enable routine monitoring in semiconductor water systems.

Future Trends and Possibilities

Advancements may include inline real-time silicate monitoring, integration with process control feedback, miniaturized concentrator designs, and coupling with mass spectrometry for broader analyte profiling.

Conclusion

An RFIC system combined with automated online sample preparation delivers a robust, reproducible method for sub-µg/L silicate determination in UHPW, supporting preventive maintenance of critical water purification stages.

References

• Worawirunwong W., Rohrer J. Determination of silicate in high-purity water using ion chromatography and online sample preparation. Thermo Fisher Scientific Application Note 170, 2016.
• Thermo Scientific ICS-3000 Ion Chromatography System Operator’s Manual, Document No. 065031, Sunnyvale CA, 2008.