Determination of neodymium–fulvic acid binding constants by capillary electrophoresisinductively coupled plasma mass spectrometry (CE-ICP-MS)

Abstract

Capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS) has been used to study neodymium–Suwannee river fulvic acid (FA) interaction at pH 6–9 and 0.001–0.1 mol L⁻¹ ionic strength. The use of a strong ligand (EDTA) competition method allowed us to succesfully manipulate a bimodal species distribution of NdFA and NdEDTA. These two species have sufficiently different charge and shape characteristics to be easily separable by CE, even in the presence of nebulizer induced siphoning. The NdFA and NdEDTA complexes were found to be stable on the timescale of a CE separation and quantitative artifacts due to complex dissociation were therefore negligible. A method development scheme for quantitative CE-ICP-MS addresses metal–ligand dissociation specifically and is oulined in this study. Siphoning induced laminar flow was optimized and used to our advantage by speeding up separations. Log values of conditional binding constants for Nd–FA complexation at 0.1 mol L⁻¹ ionic strength varied gradually from 9.1 to 12.5 over the pH range 6–9. Detection limits for Nd speciation were 500 pM.