Low-level speciated analysis of Cr(III) and Cr(VI) using LC(IC)-ICP-MS

Significance of the Topic

Chromium speciation is critical due to the distinct health effects of Cr(III) and Cr(VI). While trivalent chromium is an essential nutrient, hexavalent chromium is carcinogenic and tightly regulated in water, food, and industrial materials. Sensitive and selective detection of both species supports environmental monitoring, regulatory compliance, and public health protection.

Objectives and Study Overview

This work aims to develop a robust, rapid, and sensitive method combining liquid chromatography (anion exchange) with ICP-MS (helium collision mode) for simultaneous determination of Cr(III) and Cr(VI) at trace levels in diverse water matrices, including high-mineral content samples.

Methodology and Instrumentation

To separate Cr(III), which is cationic, from Cr(VI), the method uses EDTA complexation and an anion exchange column (4.6×30 mm polyhydroxymethacrylate resin) on an Agilent 1200 HPLC with bio-compatible flow path components. The Agilent 7700x ICP-MS equipped with an octopole reaction system (ORS3) operates in helium cell mode to remove polyatomic interferences (ArC, ClO) on m/z 52 and 53. Key parameters include:

• Mobile phase: 5 mM Na2EDTA, 5 mM NaH2PO4/15 mM Na2SO4, pH 7.0
• Flow rate: 1.2 mL/min, injection volume: up to 100 µL
• RF power: 1550 W, He cell gas: 4 mL/min, dwell time: 0.5 s/isotope
• Isotopes monitored: 52Cr and 53Cr

Main Results and Discussion

Detection limits below 200 ng/L for both species were achieved with standard injection volumes, improving to ~130 ng/L at 100 µL for Cr(III) and ~170 ng/L for Cr(VI). The method demonstrated excellent stability (RSD <2.5% over 8 h, n=30) and accurate spike recoveries in mineral waters with varied composition, including highly mineralized samples (>450 ppm Ca, >1000 ppm sulfate). Cr(III) and Cr(VI) eluted at approximately 1 min and 2.5 min, respectively. For ultra-trace Cr(VI) quantification, method adjustments (increased injection volume and reduced EDTA concentration) yielded a detection limit of 0.008 µg/L and linear calibration (r2>0.9995). Analysis of California water samples revealed Cr(VI) levels exceeding the proposed public health goal of 0.02 µg/L.

Benefits and Practical Applications of the Method

The approach delivers rapid (≈3 min), sensitive, and reproducible speciation of chromium at sub-ppb and ppt levels. Its robustness in complex matrices makes it suitable for routine environmental, drinking water, and industrial quality control labs.

Future Trends and Opportunities

Further developments may include automating sample preparation, extending the approach to other metal species, integrating front-end miniaturized separation systems for field use, and coupling with high-throughput platforms to meet increasingly stringent regulatory limits.

Conclusion

The optimized LC-ICP-MS method provides accurate, rapid, and highly sensitive determination of Cr(III) and Cr(VI) in diverse water samples, achieving detection limits well below regulatory requirements and demonstrating excellent reproducibility and spike recovery.