Performance of a mixed binding layer for measuring anions and cations in a single assay using the diffusive gradients in thin films technique.

The performance of a mixed binding layer (MBL) for use in diffusive gradients in thin films (DGT) was investigated. The MBL consisted of ferrihydrite and Chelex-100 cation-exchange resin combined together in a binding gel in an attempt to allow measurement of anions and cations in a single assay. Results from the MBL were compared to experiments performed using individual Chelex gels and ferrihydrite gels that have been shown to work successfully for DGT methodology. To facilitate combined analysis of P and cations by ICP-MS, HCl (1 M) was used for gel elution to minimize interferences from 14N16OH or 15N16O on 31P. All elements tested (Cd, Cu, Mn, Mo, P, and Zn) were bound successfully to the MBL. An elution efficiency of 0.92 was obtained for all elements, apart from Mo (0.79). This is higher than the elution efficiencies obtained previously for pure Chelex or ferrihydrite gels using HNO3 (1 M) as the eluent. Uptake of cations by DGT using the MBL was consistent across the pH range 5-9, which compares well with results using pure Chelex. Below pH 5, accumulated masses were lower for Mn, Cu, and Zn. Uptake of P and Mo was unaffected by pH in the range 3-8, and the amount absorbed compared well with results obtained previously for pure ferrihydrite gels. Performance of the MBL at different ionic strengths (0.001, 0.01 M) was comparable to performance using the pure Chelex gel. DGT measurements obtained using the MBL on agricultural soils correlated well (r2 = 0.95) with separate measurements obtained using either pure Chelex or ferrihydrite binding layers. This suggests that the MBL could be used for simultaneous measurement of cationic and anionic element availability in soils.