Preservation and determination of trace metal partitioning in river water by a two-column ion exchange method

A one-step preconcentration method using two columns in series was used to partition trace metals in natural waters into several operationally defined fractions. The water passed through a Chelex-100 cation exchange column to collect dissolved labile trace metals and then through an AG MP-1 macroporous resin column to collect trace metals complexed with natural organic matter. A third fraction (inert) was obtained from the difference between total dissolved trace metal concentrations, determined after UV irradiation of a separate aliquot, and the sum of the fractions retained on the two columns. This analytical scheme was successfully applied to the determination of Al, Cd, Co, Cu, Fe, Mn, Ni, Pb, V, and Zn in river waters collected from across the state of Texas. Trace metal partitioning between fractions was found to be stable for at least 8 days after the addition of ammonium acetate buffer (pH 5.5). In most samples collected from relatively pristine sections of Texas rivers, the sum of fractions retained on the two columns accounted for better than 80% of the total dissolved trace metal concentrations. In rivers where anthropogenic inputs were more evident (San Antonio and Trinity Rivers), the inert fraction became pronounced, in some cases accounting for up to 95% of the total dissolved concentrations. The preconcentrate is free of matrix interference and can be easily analyzed by atomic absorption spectrometer or inductively coupled plasma mass spectrometer. Using a 1-L sample allows preconcentration factors of 150 or higher for both labile and organic fractions, and therefore, trace metal concentrations and speciation in natural waters at ambient levels can be accurately and precisely determined.