Chromium (Cr) exists in multiple oxidation states, with Cr(III) acting as an essential micronutrient and Cr(VI) posing significant toxicological risks.1,2 Current regulations require a clear distinction between these species, making total Cr measurements inadequate for effective risk assessment. While various methods exist for the measurement of Cr(III) and Cr(VI), IC-ICP-MS has emerged as a preferred technique for the analysis due to its sensitivity and reproducibility.

This study outlines the speciation process per ISO 24384 using chelation, anion-exchange chromatography, and mass detection.3 The ISO method is applicable to the determination of Cr(III) and Cr(VI) dissolved in wastewater, surface water, groundwater, or drinking water from 0.20 to 500 μg/L of each compound as Cr mass. Samples containing Cr at concentrations higher than the working range can be analyzed following appropriate dilution of the sample.

Experimental

Sample and standard preparation

Samples (tap water) and standards (0.5 to 20 µg/L) were prepared according to ISO 24384.3 First, the samples and standards were pretreated with a 0.025 mol/L EDTA solution for chelation for Cr(III). A volume of 2 mL EDTA was transferred into 20 mL flasks and filled up to the mark with either the samples or the respective volume of a standard stock solution (1000 µg/L) and ultrapure water (UPW). The pH of the solutions was adjusted to 6.9 ± 0.1 with nitric acid or sodium hydroxide. Afterwards, the samples were transferred into screw-capped polypropylene tubes and heated to 70 ± 3 °C for 60 minutes in a thermostatic bath (LAUDA, ECO, RE 420).

Instrumentation

A Metrohm 940 Professional IC Vario coupled to an Agilent 7850 ICP-MS was used for analysis of Cr(III) and Cr(VI) (Figure 1). Automatic sample and standard delivery was ensured with a Metrohm 889 IC Sample Center – cool autosampler. The instrumentation is also suitable for the speciation of other trace elements such as arsenic, selenium, and mercury.

Modern laboratory setup featuring advanced analytical equipment, including an ion chromatograph, autosampler, ICP-MS, and computer monitor on a counter.

Figure 1. A Metrohm 940 Professional IC Flex and an 889 IC Sample Center - cool coupled to an Agilent 7850 ICP-MS. All under full control of Agilent ICP-MS MassHunter upgraded with the Metrohm IC driver for ICP-MS MassHunter, providing a fully integrated IC-ICP-MS system for the determination of Cr(III) and Cr(VI).

The IC system was controlled directly from the Agilent ICP-MS MassHunter software upgraded with the Metrohm IC Driver for ICP-MS MassHunter 4, version 1.0 (Figure 2), enabling fully integrated sample analysis, data processing, and reporting. A screenshot of the ICP-MS MassHunter software dashboard provides an overview of the IC-related method parameters and status information. The optional Agilent ICP-MS Plasma Chromatographic software was also used for data analysis.

Dashboard interface of IC-ICP-MS MassHunter software showing instrument status. Includes graphics of mainframe, mechanistic parts, and sample introduction. Contains icons for plasma, ion lenses, and detectors, with emphasis on maintenance feedback and temperature details. Blue and white color scheme.

Figure 2. Integration and control of the Metrohm IC with the Metrohm IC Driver for ICP-MS MassHunter in the Agilent ICP-MS MassHunter software (versions 5.3 and higher).

Chromatographic and ICP-MS conditions

Chromatographic separation was performed using a Metrosep Carb 2 - 100/4.0 column under isocratic conditions with an ammonium nitrate-based eluent (Table 2).5 The 7850 ICP-MS was operated in time-resolved analysis (TRA) mode with helium as the collision gas. The 52Cr and 53Cr isotopes were monitored using the instrument operating parameters shown in Tables 1 and 2.

Table 1. Chromatographic parameters.

Table showing ion chromatography details of column, eluent, flow rate, column temperature, and injection volume.

Table 2. ICP-MS system parameters.

Table showing settings of ICP-MS for rf power, nebulizer flow, helium gas flow, and integration time.

Results and discussion

Separation and detection

Cr(III) and Cr(VI) were separated within less than four minutes under isocratic conditions. The respective calibration curves for the two species exhibited excellent linearity (R > 0.998). As shown in Figure 3, the chromatograms demonstrated high sensitivity for both analytes.

Graph showing intensity (cps) vs. retention time (min) with peaks at 2.0 and 3.2 min for Cr(III) and Cr(VI) at different concentrations. Color lines represent concentrations from 0.5 to 20 μg/L, displaying distinct peaks at each level..

Figure 3. Speciation of Cr(III) and Cr(VI) by IC-ICP-MS. The Metrosep Carb 2 column was used under isocratic conditions with a flow rate of 1.0 mL/min for the ammonium nitrate eluent (150 mmol/L nitric acid, Sigma-Aldrich, puriss. p.a., ≥65%) and 234 mmol/L ammonia solution (ACS reagent, 28–30%, Sigma-Aldrich, pH 9±0.1). Samples and standards were injected with a volume of 250 µL (full loop).

Recovery and precision

To assess the accuracy of the method, tap water was spiked using a mixed standard at 5 μg/L. Spike recoveries of 99.7 and 114.0% were achieved for Cr(III) and Cr(VI), respectively, confirming the method's robustness and effectiveness for the fast determination of toxic Cr(VI) in drinking water.

Reliable and efficient method for chromium speciation

This study shows a reliable and efficient method for chromium speciation using Metrohm IC integrated with an Agilent 7850 ICP-MS with the Metrohm IC Driver for Agilent ICP-MS MassHunter. The unified software solution simplifies operations, ensures data integrity, and enhances analytical safety. The approach meets international standards, making it ideal for regulatory compliance and environmental monitoring.

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