Determination of trace anions in high-nitrate matrices by ion chromatography

Importance of the Topic

Reliable quantification of trace anions such as chloride sulfate and phosphate in matrices containing high levels of nitrate is essential for industries like semiconductor manufacturing where contamination in nitric acid can affect product yield and reliability. Conventional wet chemistry methods lack sufficient sensitivity and precision for these demanding trace analyses.

Objectives and Study Overview

This study presents a direct injection ion chromatography method employing a microbore Dionex IonPac AS15 column and EG40 eluent generator to achieve sensitive determination of trace anions at low microgram per liter levels in high nitrate matrices such as 0.7 percent nitric acid. The method aims to optimize separation conditions sample preparation and instrument configuration to meet regulatory and industry standards.

Methodology and Instrumentation

The analytical system consists of a Dionex DX 600 or ICS 5000 plus ion chromatograph equipped with a GS50 gradient pump CD25 conductivity detector EG40 eluent generator with EGC KOH cartridge and ASRS ULTRA suppressor in external water or gas assisted external regeneration mode. A microbore IonPac AS15 analytical column and AG15 guard in combination with ATC trap columns are used. High purity potassium hydroxide eluent is generated electrolytically avoiding carbonate contamination. Standards are prepared from 1000 mg per liter stock solutions in deionized water. Semiconductor grade nitric acid is diluted to 0.7 percent by volume and directly injected at 1 microliter. The KOH gradient program alternates between 48 and 100 millimolar to separate target anions and clean the column between runs.

Key Results and Discussion

Microbore chromatography improved sensitivity by roughly fourfold compared to standard bore formats while reducing eluent consumption. The EG40 generator produced consistently low background conductivity and reduced contamination relative to bottle prepared eluents. Method detection limits in 0.7 percent nitric acid were 41 µg per liter for chloride 104 µg per liter for sulfate and 120 µg per liter for phosphate. Recoveries for spiked samples ranged from 86 to 109 percent and correlation coefficients exceeded 0.99. Analysis of a 7000 ppm sodium nitrate solution revealed chloride at 38 µg per liter and sulfate at 17 µg per liter. System blanks confirmed negligible background interference.

Benefits and Practical Applications

This direct injection microbore approach enables reliable trace anion monitoring in corrosive high nitrate environments with minimal sample volume and reduced runtime. It is well suited for quality control of nitric acid etchants semiconductor process fluids and chemical manufacturing where stringent contamination limits apply.

Future Trends and Possibilities

Further developments may include integration with mass spectrometric detection for enhanced selectivity automation of sample handling and expansion to additional anionic contaminants. Miniaturization and high throughput workflows will support broader industrial and environmental applications.

Conclusion

The described method demonstrates a robust sensitive and efficient solution for trace anion determination in high nitrate matrices. The combination of microbore IonPac AS15 separation suppressed conductivity detection and the EG40 eluent generator meets industry detection requirements and offers improved performance over conventional approaches.

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