Comparison of the application of higher mass resolution and cool plasma conditions to avoid spectral interferences in Cr(III)/Cr(VI) speciation by means of high-performance liquid chromatography – inductively coupled plasma mass spectrometry

Abstract

In Cr(III)/Cr(VI) speciation by means of high-performance liquid chromatography – inductively coupled plasma mass spectrometry (HPLC–ICPMS), accurate quantification is often jeopardised as a result of spectral overlap to be attributed to the occurrence of C- ( 40 Ar 12,13 C + ) and/or Cl-based ( 37 Cl 16 O + and 35 Cl 16 OH + ) molecular ions. In the present work, a microbore anion exchange column, hyphenated to a sector field ICP — mass spectrometer by means of a microconcentric nebuliser, was used for a challenging ‘real-life’ application: Cr speciation in industrial process solutions, characterised by a heavy matrix. The capabilities of both high mass resolution ( R=3000) and cool plasma conditions (at R=300) to overcome the aforementioned spectral interferences were studied and compared to one another and the effect of both approaches on the analytical figures of merit was evaluated. Under cool plasma conditions, the occurrence of ArC + ions, which interfere with the determination of Cr(VI) with the anion exchange column used (co-elution of Cr(VI) and HCO 3 −), can be sufficiently reduced to permit reliable Cr(VI) determination. The presence of ClO(H) + ions, which interfere with the determination of Cr(III) on the other hand (co-elution of Cr(III) and Cl −), cannot be avoided by cool plasma conditions. Increasing the mass resolution is an elegant way to avoid the detrimental influence of both interferences, but leads to a 15-fold reduction in transmission efficiency (sensitivity). Nevertheless, for both species involved, sub-μg limits of detection — 50 ng/l for Cr(III) and 120 ng/l for Cr(VI) — could be obtained. As a result, application of a higher mass resolution ( R=3000) was preferred for the analysis of industrial process solutions by means of HPLC–ICPMS. The accuracy of the results obtained was demonstrated by the good agreement between (i) the sum of the concentrations of Cr(III) and Cr(VI) and (ii) the total Cr content, determined by means of pneumatic nebulisation ICPMS.