Speciation of chromium and vanadium in environmental samples using HPLC-DRC-ICP-MS

Abstract

A HPLC separation procedure has been developed for the speciation of Cr(III), Cr(VI) and V(IV), V(V) in soil and plant samples. The species were separated on a reversed phase column (C₈) in gradient elution mode. The mobile phase consisting of EDTA and TBAP in methanol at pH 6.85, operated in a linear ramp, yielded well resolved chromatograms of all the species within 8 min with retention times of less than 2% RSD. The parameters such as concentration of EDTA in mobile phase, hold time and ramp time in gradient elution have been optimized to obtain lower retention times and better resolution. The analyses were carried out using dynamic reaction cell-inductively coupled plasma-mass spectrometer (DRC-ICP-MS). The DRC conditions have also been optimized to obtained interference free measurements of ⁵²Cr⁺, ⁵³Cr⁺ and ⁵¹V⁺, which were otherwise interfered by: ³⁵Cl¹⁶O¹H⁺, ³⁵Cl¹⁷O⁺, ⁴⁰Ar¹²C⁺ on ⁵²Cr⁺; ³⁷Cl¹⁶O⁺, ⁴⁰Ar¹³C⁺, ⁴⁰Ar¹²C¹H⁺ on ⁵³Cr⁺; and ³⁵Cl¹⁶O⁺, ³⁸Ar¹³C⁺ on ⁵¹V⁺. The detection limits of the procedure were 0.06 ng V ml⁻¹ and 0.05 ng Cr ml⁻¹. The accuracy of the method has been validated by comparing the sum of the concentrations obtained for individual species with total concentration of the elements in two NIST SRMs, San Joaquin Soil SRM 2709 and Tomato Leaves SRM 1573a. The method has also been applied on two real samples, namely farmland soil and leaves of Persea americana collected locally, in which case the comparison has been made with total concentrations determined after complete dissolution of the samples. The species were extracted from soils and leaves using a mixture of mobile phase B (5 mmol l⁻¹ EDTA, 0.5 mmol l⁻¹ TBAP in 4% v/v CH₃OH at pH 6.85) and HF using open microwave extraction procedure.