Determination of Trace Cations in Concentrated Acids Using AutoNeutralization Pretreatment and Ion Chromatography

Importance of the Topic

Determining trace levels of metal cations in highly concentrated acids is critical across multiple industries, including electronics plating, pharmaceuticals, food and beverage, mining, and environmental monitoring. Impurities such as lithium, sodium, potassium, calcium, and magnesium can cause process defects, contaminate products, or pose health risks, making sensitive, accurate, and reproducible analysis methods essential.

Objectives and Study Overview

This Application Note updates and replaces the earlier AN 94, demonstrating how the Dionex ICS-3000 Reagent-Free Ion Chromatography (RFIC) system, equipped with automated AutoNeutralization, dual pumps, and valves, can measure trace cations in concentrated sulfuric, phosphoric, and hydrofluoric acids (up to 25% w/w) without sample dilution. The goal is to achieve µg/L detection limits, high throughput, and robust performance.

Methodology and Instrumentation

A two-system IC configuration is used:

• System 2 performs automated neutralization using a CSRN II self-regenerating cation neutralizer, driven by Pump 2, and collects neutralized sample on an IonPac CTC-1 trap column.
• System 1 concentrates and separates analytes on an IonPac TCC-ULP1 concentrator and CS16/CG16 column set, with suppressed conductivity detection (CSRS ULTRA II) and Pump 1 eluent generation (EGC II MSA).

The AutoNeutralization protocol exchanges strong acid anions for hydroxide, effectively converting concentrated acids into low-conductivity solutions while preserving trace cations for μg/L analysis. The sequence is fully automated via Chromeleon software, with valve switching, rinse cycles using 24% sulfuric acid, and sample overlap to minimize cycle time.

Main Results and Discussion

Linearity and Detection Limits

• Calibration over three orders of magnitude yielded R² > 0.9993 for Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺.
• Method detection limits ranged from 0.06 µg/L (Li⁺) to 2 µg/L (Na⁺) in 24% H₂SO₄.

Reproducibility and Robustness

• Five-day robustness test (240 injections) showed retention time RSDs <0.3% for early eluters and <2.5% for late eluters; area RSDs between 0.3–3.5%.

Spike Recoveries

• 24% H₂SO₄: recoveries 94–104% (n=5).
• 25% H₃PO₄: recoveries 100–110% for Na⁺, K⁺, Mg²⁺, Ca²⁺.
• 10% HF: recoveries 90–106%.

The method successfully quantified trace cations in all three acid matrices with clear chromatographic resolution from matrix peaks.

Benefits and Practical Applications

• No sample dilution required, preserving low-level cation detection and accelerating throughput.
• Automated, inline neutralization reduces manual handling and contamination risk.
• Applicable to QA/QC labs, process monitoring in plating, food/pharma ingredient testing, and environmental analysis of acid wastes.

Future Trends and Potential Applications

• Extension of AutoNeutralization to trace anion analysis in concentrated bases.
• Integration with higher capacity neutralizers for >30% acids and viscous samples.
• Advances in reagent-free IC to further minimize carryover and increase sustainability.
• High-throughput screening in pharmaceutical, battery research, and advanced materials manufacturing.

Conclusion

The updated AutoNeutralization-RFIC method on the ICS-3000 platform enables reliable, low-µg/L measurement of Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺ in concentrated acids without dilution. Its robust performance, automated workflow, and broad applicability make it a valuable tool for industrial, environmental, and clinical laboratories.

Used Instrumentation

• Dionex ICS-3000 RFIC system with DC module, DP dual pump, EG module, and AS autosampler.
• CSRN II self-regenerating cation neutralizer and CSRS ULTRA II suppressor.
• IonPac CTC-1 trap column, TCC-ULP1 concentrator, CG16 guard and CS16 analytical columns.
• EluGen EGC II MSA cartridge and gradient mixer.

References

• Rafool W. J. Radioanal. Nucl. Chem. 2001, 250, 147–152.
• Elleuch M. C. et al. Sep. Purif. Technol. 2006, 51(3), 285–290.
• Cohen H. W.; Alderman M. H. Curr. Opin. Cardiol. 2007, 22(4), 306–310.
• Takaichi K. et al. Intern. Med. 2007, 46(12), 823–829.
• Dionex Application Note 94, 2004; Application Notes 93 and 938 (2007).