Modern approaches for the analytical speciation of chromium at very low concentration in waters and leachates

The speciation of chromium in waters and leachates obtained from soils and sediments has aroused interest in the last years. The element may be present in two oxidation degrees that have quite different toxicity. While chromium (III) is even essential for human beings due to its role in the metabolism of glucose and lipids, Cr(VI) is toxic due to its oxidant properties. The concentration of chromium in waters is usually of a few micrograms per liter, and the difficulty of carrying out the measurement at such low levels is further increased due to the distribution of the total element in the two mentioned forms. The technique commonly used nowadays for the purpose is inductively coupled plasma mass spectrometry (ICP-MS), a powerful analytical tool, but expensive both in acquisition and maintenance. Speciation, in addition, requires some type of previous separation or suitable strategy since the signal obtained by ICP-MS depends on the total amount of the metal present.

Recent advances in microextraction techniques have demonstrated that the determination and speciation is also possible by using electrothermal atomization atomic absorption spectrometry (ETAAS), an analytical technique which is of lesser cost than ICP-MS and is present in most laboratories worldwide. This communication summarizes some recent studies carried out in our laboratory based on the use of dispersive solid-phase microextraction to concentrate chromium. The small volume of liquid extract finally obtained can be injected into the electrothermal atomizer, and a very sensitive chromium determination is achieved. The extreme sensitivity in this way obtained is the consequence of combining the efficient preconcentration step with the sensitivity inherent to ETAAS measurement. Selectivity is also guaranteed by the characteristics of ETAAS. Speciation can be carried out by means of simple previous redox treatments without the need for a chromatographic separation. Three procedures are compared, one of them using graphene oxide as the active micro-solid phase, other based in the use of cellulose, an inexpensive reagent. Both procedures require a centrifugation step to separate the micro-solid phase from the supernatant. The third procedure uses freshly prepared ferrite particles and avoids the centrifugation step since the magnetic characteristics of the solid material permit an easy separation of phases with an external magnet. In all cases, chromium is measured after desorption from the micro-solid phase by a small volume of a suitable reagent. The limits of detection are close to 0.01 micrograms/L. The reliability of the three procedures is checked by using several reference samples with a certified chromium content. Data for the speciation of the metal are also given, a point that may be of practical interest for those involved in risk assessment or toxicity studies, since the dealers of the reference materials only provide the total chromium content.