Highly selective Cr(VI) determination at ultra-trace levels using on-line trapping of Cr(III) with ICP-MS detection

Chromium speciation is of importance in view of the striking different toxicity of its two most often observed oxidation states, +3 (Cr(III)) and +6 (Cr(VI)). Since hexavalent chromium is carcinogenic, concentration limits were accordingly established for several commodities (drinking water, toys, tanned leather, automobiles, cement) and in the field of occupational health (e.g. welding fumes). These limits are often dictated by the sensitivities of the analytical protocols used in routine laboratories (e.g. photometry) together with their selectivity for the two species. The most often approach used in research laboratories (HPLC-ICP-MS) is considered to be costly not only from the point of instrumentation but also from the point of analysis time and efforts. In this hyphenated technique, the separation module (HPLC) keeps the ICP-MS instrument busy for many minutes for the measurement of species for a single element. The fastest method for LC-ICP-MS reported up to now is using 3 min. (Lesniewska and Godlewska-Zylkiewicz 2019).

In 2019, Spanu et al. demonstrated the potentialities of frontal chromatography (FC) as a separation module providing fast separation of species for on-line detection by ICP-MS (see the EVISA News below).  The hyphenated technique FC-ICP-MS can enable not only a substantial reduction of the analysis time, but also very significant simplification of the instrumental setup. The method can be applied, whenever the requirement for speciation can be reduced to the differentiation of two species that can be achieved by using very short columns with limited separation power.

The separation of the cationic or neutral Cr(III) species from anionic Cr(VI) species fits perfectly with the field of application of FC-ICP-MS, if other chromium species (e.g. organic species) can be excluded. A team of Italian researchers therefore decided to exploit this technique to develop a sensible, fast and reliable method for the determination of Cr(VI).

The developed method is based on the use of a homemade column filled with a strong cation exchange resin, completely blocking Cr(III) leaving Cr(VI) as the only unretained species. As a result of such high selectivity, the determination of Cr(VI) can be performed in the outstanding short time of one minute. The authors investigated the applicability of the new approach with two sample types, bottled mineral water and leachates of toys made from polymeric materials. Sample preparation was very simple: mineral water samples were prepared by adding Ge as internal standard and addition of HNO3. Toy samples were leached, as described in EN-71-3 standard, by immersion for two hours in a 0.07 M HCl solution at 37 °C.

A highly selective quantification of Cr(VI) ultra-traces was obtained with a detection limit of LOD = 0.026 μg/kg - defined as 3 s of 10 replicated measurements of a 0.050 μg/kg solution) over a wide linearity range (tested up to 1024 μg/kg), even in the presence of Cr(III) concentration as high as 50 mg/kg. Since the column is totally blocking Cr(III) the column has a limit in the number of samples that can be analysed. However, due to the relatively high capacity off the column material, about 1700 samples containing 100 µg/L of Cr(III) can be analysed using the same column. Since the column material is with 0.10 € very cheap, the replacement of the material is preferred as regeneration strategy.

Related studies
O. Mihai, M.S. Kawamoto, K.L. LeBlanc, P. Grinberg, A.R. Nogueira, Z. Mester, Determination of chromium picolinate and trace hexavalent chromium in multivitamins and supplements by HPLC-ICP-QQQ-MS. J. Food Compos. Anal. 87 (2020) 103421.  DOI: 10.1016/j.jfca.2020.103421.

C.D. Quarles, M. Szoltysik, P. Sullivan, M. Reijnen, A fully automated total metals and chromium speciation single platform introduction system for ICP-MS. J. Anal. At. Spectrom. 34 (2019) 284–291. DOI: 10.1039/c8ja00342d

B. Lesniewska, B. Godlewska-Zyłkiewicz, Speciation of chromium in alkaline soil extracts by an ion-pair reversed phase HPLC-ICP MS method. Molecules 24 (2019) 1172.
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October 9, 2019: Fast speciation analysis of inorganic arsenic by using frontal chromatography for separation

last time modified: September 19, 2024