Solid-phase extraction and HPLC-UV detection of Cr(iii) and Cr(vi) using ionic liquid-functionalized silica as a hydrophobic sorbent

Abstract

A new method for speciation and separation of Cr(III) and Cr(VI) in aqueous medium has been developed using solid-phase extraction and HPLC analysis using UV detection. The simultaneous extraction of a pre-chelated Cr species with ammonium pyrrolidine dithiocarbamate (APDC) was achieved on a single mini-column packed with N-methylimidazolium hexafluorophosphate ionic liquid-functionalized silica as a hydrophobic sorbent. Separation of the eluted Cr species was accomplished on a RP-ODS column using a mixture of methanol–acetonitrile–water (40 : 30 : 30, v/v) as the mobile phase at a flow rate of 1.0 mL min⁻¹. Various factors that affect the chelation and extraction of the Cr species were screened using fractional factorial design. Afterwards, the influence of the significant factors was also studied using a central composite design under response surface methodology in order to find out more reliable models. Additionally, a desirability function was applied to define the combined response of the Cr(III) and Cr(VI) responses, and then non-linear optimization of the multiple responses was performed. Under the optimal conditions (APDC amount of 0.15% w/v, chelation treatment temperature of 50 °C, chelation treatment time of 10 min, elution flow rate of 1 mL min⁻¹, pH 5.5, and sample flow rate of 1.5 mL min⁻¹), the linear dynamic range was 50–300 ng mL⁻¹ with detection limits of 12.12 and 15.07 ng mL⁻¹ and repeatability (RSD%, n = 5) of 1.85 and 2.54%, for Cr(III) and Cr(VI), respectively. The performance of the method was evaluated for the speciation of Cr in water and urine samples in nanograms per milliliter with satisfactory recoveries.