Speciation analysis of Cr (III) and Cr (VI) in drinking waters using anion exchange chromatography coupled to the Thermo Scientific iCAP Q ICP-MS

Importance of the topic

Accurate speciation of chromium in drinking water is vital because Cr(III) and Cr(VI) have markedly different biological effects. While trivalent chromium (Cr(III)) is an essential nutrient, hexavalent chromium (Cr(VI)) poses significant health risks due to its high toxicity and mobility. Regulatory agencies in the US and EU mandate limits for total chromium, but detailed speciation is required to assess potential hazards accurately.

Goals and Study Overview

The primary objective was to establish a fast, sensitive and robust method for trace-level determination of Cr(III) and Cr(VI) in natural waters. The study evaluates the coupling of anion exchange chromatography (IC) on a Thermo Scientific Dionex ICS-5000 system with element-specific detection via the Thermo Scientific iCAP Qc ICP-MS. This approach aims to minimize sample preparation steps, reduce species transformation, and achieve high sample throughput.

Methodology and Instrumentation

The workflow comprises direct analysis of unadjusted drinking water samples collected in pre-cleaned PFA bottles. Daily calibration standards (0.75–15 ng/g) were prepared in 0.1 M ammonium nitrate (pH 4). Chromatographic separation employed:

• ICS-5000 ion chromatography with metal-free solvent pathway
• Dionex AG-7 anion exchange column (2 × 50 mm) operated isocratically
• Mobile phase: 0.4 M HNO₃, flow rate 400 µL/min, 20 µL injection volume, 150 s cycle time

IC effluent was directly introduced into the iCAP Qc ICP-MS equipped with a PFA spray chamber, PFA-LC nebulizer and a flatapole collision cell using He KED mode. Operating parameters included 1550 W forward power and 0.8 L/min nebulizer gas.

Main Results and Discussion

Optimal elution at 0.4 M HNO₃ achieved baseline separation of Cr(III) and Cr(VI) in under 150 s. Calibration curves exhibited linear responses for both species with sensitivities of 220 kcps/ng·g⁻¹. Method detection limits were 0.20 pg/g for Cr(VI) and 0.38 pg/g for Cr(III). Spike recovery tests (2–6 ng/g) yielded quantitative recoveries between 99 % and 105 %, demonstrating negligible species interconversion. Repeated injections (n=20) of 5 ng/g standards over 2.5 h showed retention time stability <1.5 % and peak area RSD <0.3 %. Application to local tap water detected 42.5 ± 1 pg/g Cr(VI); isotope ratio checks confirmed interference-free quantification.

Benefits and Practical Applications

This rapid IC-ICP-MS method eliminates complex sample pre-treatments, reducing the risk of species alteration. The metal-free chromatography path and high sensitivity of the iCAP Qc enable sub-ppt detection, suitable for routine monitoring of drinking water, environmental surveillance and quality control in industrial effluents.

Future Trends and Opportunities

Further developments may include integration with automated on-line sample pre-concentration, coupling to alternative mass analyzers for isotopic speciation, and field-deployable IC-ICP-MS units. Advances in microfluidic IC cartridges and collision cell technologies will enhance throughput, lower operational costs and expand applications to biological and food matrices.

Conclusion

The combination of the Dionex ICS-5000 and Thermo Scientific iCAP Qc ICP-MS provides a high-throughput, interference-free platform for accurate Cr(III)/Cr(VI) speciation in water. The method’s speed, minimal sample handling and excellent analytical performance make it ideal for compliance monitoring and research applications.

References

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3. Thermo Fisher Scientific. IonPac AG-7 Guard Column product information.