Determination of Trace Anions in Organic Solvents

Importance of the Topic

In semiconductor manufacturing, trace ionic impurities in organic solvents can severely impair device yield and reliability. Contaminants such as chloride, sulfate, phosphate and nitrate in isopropanol, acetone or N-methyl-2-pyrrolidone risk circuit damage and performance loss. Rapid, sensitive and automated analysis of these anions is therefore critical for quality assurance and process control in microelectronics fabrication.

Study Objectives and Overview

This work presents an ion chromatography (IC) method for direct determination of trace anions in water-miscible organic solvents. Key goals include achieving sub-µg/L detection limits, minimizing sample preparation time, and automating the procedure to deliver results for four target anions in under 45 minutes. The approach combines sample concentration, matrix removal and microbore chromatography to improve sensitivity over prior techniques.

Methodology and Instrumentation

Samples of neat or spiked organic solvents are pressurized into a 5 mL PEEK sample loop. Anions are trapped on a 4 mm IonPac AG9-HC concentrator column while the solvent matrix is washed away with deionized water. Switching in-line the concentrator beneath an 8.0 mM sodium carbonate/1.5 mM sodium hydroxide eluent at 0.25 mL/min transfers the retained ions to a 2 mm IonPac AS9-HC analytical column. Suppressed conductivity detection (ASRS-ULTRA) at 100 mA yields sensitive quantification. Calibration uses working standards in aqueous or standard-addition formats to correct for matrix effects.

Used Instrumentation

• Dionex DX-500 IC system: GP50 gradient pump (microbore), CD20 conductivity detector, LC20 enclosure with dual-channel Rheodyne valves
• Dionex DXP single-piston pump with CAM-1 for sample loading
• IonPac AG9-HC concentrator, AS9-HC analytical and guard columns, ATC-1 anion trap column
• Pressurizable reservoir assembly and 5 mL PEEK sample loop (0.75 mm i.d.)
• PeakNet chromatography workstation and PEEK transfer tubing

Main Results and Discussion

Method detection limits (MDLs) in isopropanol ranged from 0.59 to 0.76 µg/L for the four anions. Linearity from 1 to 10 µg/L showed r2 > 0.99. Spike/recovery in 99% isopropanol averaged 83–112% across analytes. System blanks in deionized water exhibited no detectable target ions. Application to acetone and N-methyl-2-pyrrolidone achieved MDLs compliant with SEMI specifications for critical process solvents. Matrix elimination and microbore format enhanced mass sensitivity fourfold over standard bore systems while reducing eluent consumption.

Benefits and Practical Applications

• Fully automated operation with minimal manual handling
• Analysis time under 45 minutes for four anions
• Sub-µg/L sensitivity suitable for stringent semiconductor cleanroom requirements
• Compatibility with various water-miscible organic solvents without dilution
• Reduced eluent waste and high mass sensitivity using microbore columns

Future Trends and Potential Applications

Emerging needs for even lower detection limits may drive integration of mass spectrometry detection or microfluidic IC formats. Advances in green eluent chemistry and on-line process monitoring could extend this method to continuous quality control. Expanding analyte scope to include organic acids or halogenated species may further support semiconductor and pharmaceutical process analytics.

Conclusion

The described IC method provides a robust, rapid and highly sensitive approach for trace anion analysis in water-miscible organic solvents. By combining concentrator columns, matrix elimination and microbore chromatography, it meets semiconductor industry requirements for sub-µg/L detection and fast turnaround, enabling improved contamination control in critical manufacturing steps.

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