Determination of Trace Anions in High-Nitrate Matrices by Ion Chromatography

Importance of the Topic

High-nitrate samples, such as diluted nitric acid used in semiconductor etching, often contain trace levels of chloride, sulfate and phosphate that can compromise product quality. Conventional bottle-based eluents and wet chemistry lack the sensitivity and reliability required for sub-µg/L determinations. A robust direct-injection ion chromatography method addresses these challenges, delivering high sensitivity, low background and minimal sample preparation.

Objectives and Study Overview

This application note describes the development and validation of a microbore direct-injection ion chromatography approach to quantify trace anions in matrices containing high nitrate concentrations. Key goals include:

• Achieve low-ppb detection of chloride, sulfate and phosphate in 0.7% (v/v) nitric acid.
• Minimize carbonate contamination through on-demand eluent generation.
• Compare eluent generator performance against traditional bottle-based eluents.

Methodology and Instrumentation

Instrumentation Used:

• Dionex DX-600 Ion Chromatography system with GS50 pump, CD25 conductivity detector and Rheodyne injector.
• EG40 Eluent Generator with EGC-KOH cartridge for on-line generation of high-purity KOH eluent.
• Microbore IonPac AS15 analytical column (2 × 250 mm) with AG15 guard and ATC-1/ATC trap columns.
• ASRS ULTRA suppressor in external water or gas-assisted recycle mode.

Method Conditions:

• Eluent: 48 mM KOH (step to 100 mM for nitrate wash), flow rate 0.20 mL/min.
• Sample volume: 1 µL direct injection.
• Matrix: 0.7% (v/v) nitric acid prepared by 1:100 dilution of 70% acid.
• Standards: 1000 mg/L stock, diluted to low-ppb levels for calibration.

Main Results and Discussion

The EG40 generator produced carbonate-free KOH with stable background conductivity (<1 µS) after 4 h equilibration. Detection limits in 0.7% nitric acid were 41 µg/L for chloride, 104 µg/L for sulfate and 120 µg/L for phosphate. Spike-recovery experiments (n=7) yielded 86–109% recoveries. Compared to conventional bottle eluents, the EG40 method showed markedly lower anionic contamination. A 7000 mg/L sodium nitrate solution analysis detected 38 µg/L chloride and 17 µg/L sulfate, demonstrating applicability to concentrated salt matrices.

Benefits and Practical Applications

• Direct injection minimizes sample handling and potential contamination.
• Microbore columns and on-line eluent generation reduce reagent consumption.
• High sensitivity supports QA/QC in semiconductor, environmental and industrial labs.
• Flexible modes (external water or gas-assisted) optimize suppressor noise performance.

Future Trends and Possibilities

Advances may include integration with mass spectrometry for structural identification, automated on-line sample dilution for higher acid matrices, further miniaturization to nano-flow IC, and alternative eluent generation chemistries for broader anion coverage.

Conclusion

This microbore direct-injection ion chromatography method using an EG40 eluent generator enables reliable trace anion analysis in high-nitrate matrices. It offers low detection limits, robust performance and streamlined workflows, making it a powerful tool for laboratories requiring accurate sub-µg/L quantification.

References

• Kern J. Electrochem. Soc. 1990, 137, 1887.
• Bader M. J. Chem. Educ. 1980, 57, 703.
• Liu Y. et al. Am. Lab. 1998, 30(22), 48C–58C.
• Liu Y.; Kaiser E.; Avdalovic N. Microchem. J. 1999, 62, 162–173.
• SEMI International Standards, Semiconductor Equipment and Materials International, Chemicals/Reagents, 1997.