Determination of hexavalent chromium by species specific isotope dilution mass spectrometry and ion chromatography–1,5-diphenylcarbazidespectrophotometry

Abstract

The accuracy of the determination of hexavalent chromium (Cr⁶⁺) in solid matrices remains a challenging field of effort and improvement. An alkaline digestion (0.5 M NaOH–0.28 M Na₂CO₃) followed by ion chromatography and spectrophotometric determination by post-column derivatisation with 1,5-diphenylcarbazide (DPC) provides accurate and reproducible results. To gain a better insight into the species interconversion during the digestion, the concept of species specific isotope dilution mass spectrometry by adding enriched chromium isotopes that have been chemically converted into trivalent (Cr³⁺) and hexavalent chromium was used. For the analysis of the digestion solutions ion chromatography coupled to inductively coupled plasma-mass spectrometry (IC-ICP-MS) was peformed. The limit of detection in the digestion solution by the latest technique amounted 0.8 ng ml⁻¹ Cr⁶⁺. The accuracy of an alkaline digestion on solid waste materials was compared with a water extraction (leaching test industrial waste), indicating that the alkaline digestion is accurate from the point of view of minimal species interconversion on one hand and the maximum amount of Cr⁶⁺ extracted on the other, while significant oxidation and reduction reactions were observed during leaching tests with water. In the framework of European Directive 94/62/EC on Packaging and Packaging Waste, the developed analytical method has been used to compare the concentrations of Cr⁶⁺ on 40 packaging materials against the regulatory limit value of 100 mg kg⁻¹ by weight (sum of concentration levels of lead, cadmium, mercury and hexavalent chromium).