Determination of soluble Cr(III) and Cr(VI) in alkaline soil extract applying IC-ICP/MS

Significance of the Topic

Chromium in soils exists mainly as trivalent Cr(III) and hexavalent Cr(VI). While Cr(III) is relatively benign, Cr(VI) is highly toxic and mobile, posing environmental and health risks. Accurate speciation of these forms in soil extracts is essential for assessing contamination, guiding remediation efforts, and ensuring compliance with environmental regulations.

Objectives and Study Overview

This study aimed to develop and validate a robust analytical workflow for simultaneous determination of soluble Cr(III) and Cr(VI) in alkaline soil extracts. The approach combines ion chromatography (IC) separation of chromium species with inductively coupled plasma mass spectrometry (ICP-MS) detection, using isotope dilution for enhanced accuracy.

Methodology and Instrumentation

Sample Preparation and Extraction:

• Soil samples spiked with isotopically enriched 53Cr(VI) and 50Cr-EDTA.
• Alkaline extraction using 50 mmol/L EDTA at pH 10.
• Filtration through 0.45 µm and direct injection into IC.

Chromatographic Separation:

• Column: Metrosep A Supp 4 – 250/4.0 with guard column Metrosep A Supp 4/5 Guard/4.0.
• Eluent: 2.0 mmol/L EDTA at pH 10.
• Flow rate: 0.8 mL/min, injection volume: 100 µL, run time: 10 min.

ICP-MS Detection:

• Instrument: Agilent 7700 ICP-MS coupled via Remote Box to Metrohm 940 Professional IC Vario ONE.
• RF power: 1550 W; plasma gas: 15 L/min; carrier gas: 0.95 L/min; makeup gas: 0.15 L/min; collision gas (He): 4.0 mL/min.
• Spray chamber cooled to 2 °C; sampling depth 8 mm.
• Acquisition: spectrum and time-resolved monitoring of m/z 50, 52, 53.

Main Results and Discussion

Quantitative speciation yielded 11.3 µg/g Cr(III) and 10.1 µg/g Cr(VI) in the alkaline extract of the tested soil. The method demonstrated high selectivity, reproducibility, and low detection limits, with isotope dilution compensating matrix effects and ensuring accurate quantification.

Benefits and Practical Applications

This IC-ICP/MS workflow offers:

• Rapid and automated separation of Cr species.
• High sensitivity and specificity via isotope dilution.
• Applicability to environmental monitoring, quality control in soil remediation, and regulatory compliance.

Future Trends and Potential Applications

Advancements may include:

• Integration with portable and field-deployable IC-MS units.
• High-throughput sample processing with advanced autosamplers.
• Expansion to other metal speciation analyses in complex matrices.

Conclusion

The presented IC-ICP/MS method with isotope dilution provides a reliable, accurate, and efficient tool for chromium speciation in soil extracts, supporting environmental assessments and remediation decisions.

Reference

IC Application Note M–11