Synchronous Extraction and Quantitative Speciation of Arsenic and Chromium in Sediments by High-Performance Liquid Chromatography – Inductively Coupled Plasma – Mass Spectrometry (HPLC-ICP-MS)

Abstract

The optimum chromatographic separation conditions, polyatomic ions interference correction, and extraction parameters were investigated for simultaneous extraction and quantification of As(III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), As(V) and Cr(VI) with excellent resolution. The ammonium salts (NH₄H₂PO₄ and NH₄NO₃) were examined as mobile phases at different pH values by altering the composition during the chromatographic run in gradient elution mode using high-performance liquid chromatography – inductively coupled plasma – mass spectrometry (HPLC-ICP-MS). The 10 mM and 70 mM NH₄NO₃ composition at pH 8.7 enabled sufficient baseline separation of the target As and Cr species at a total run time of 18 minutes. The identified polyatomic ion interferences were corrected by an inter-element method using mathematical equations. The optimum extraction conditions containing a mixture of 0.375 M (NH₄)₂HPO₄ and 50 mM EDTA at neutral pH as an extraction buffer, extraction temperature of 150 °C and volume of 15 mL were attained by a factorial design technique. The lowest concentrations of the selected elemental species that can be reliably detected by the procedure were in the range from 0.064 to 0.682 ng g⁻¹. The accuracy of the developed procedure was demonstrated by achieving percentage recoveries of 96.8% for As(V) and 97.7% for Cr(VI) using San Joaquin Soil standard reference material 2709a. Proposed method was validated by achieving percentage recoveries from 84.9 to 104% for As species and Cr(VI) using spiking and recovery studies at 3x and 25x the limit of quantification values of the target species. The selected As and Cr species in real sediment were monitored using the optimized analytical method. The differences in concentrations of Cr(VI), As(III), MMA and As(V) between the seasonal state of the river were statistically significant (p ˂0.05) at a 95% confidence level as determined by analysis of variance (ANOVA).

Keywords:

• Arsenic
• chromium
• high-performance liquid chromatography – inductively coupled plasma – mass spectrometry (HPLC-ICP-MS)
• sediment analysis
• simultaneous speciation

Acknowledgements

We would like to acknowledge the University of Johannesburg Research Centre for Synthesis and Catalysis and Spectrum for the facility. Mokgehle R. Letsoalo is grateful to the University of Johannesburg Global Excellence and Stature and DSI-NRF Innovation Doctoral scholarships for the financial support.

Additional information

Funding

This work is based on research supported in part by the National Research Foundation (NRF) of South Africa under the Thuthuka Programme Grant Number 117673 and the Water Research Commission (WRC) of South Africa Project Number K5/2515//1.