Development and characterisation of a colorimetric binding phase for the diffuse gradients in thin films technique

Passive sampling is a useful method for contaminant analysis and monitoring, and of the techniques available the Diffusive Gradients in Thin Films (DGT) technique is particularly applicable for use in transition metal studies. However, analysis of the metal-accumulating DGT 'binding phase' is typically expensive and time consuming, which limits the technique's application. In order to improve on the cost and speed of analysis, and to allow in-field analysis, a colorimetric binding phase was developed. By employing metallochromic reagents adsorbed onto ion-exchange resins a colour change was obtained as a result of the binding of metal ions, with the degree of change being proportional to the quantity of complexed metal. This reagent-adsorbed resin was immobilised on adhesive paper labels to provide a robust, cheap and effective colorimetric binding phase. Quantification of metal loading on these binding phases was undertaken through use of UV-Vis reflectance or Computer Imaging Densitometry (CID), the latter employing a flat-bed scanner for improved ease of analysis. Analytically robust metal quantification was found to be achievable using these methods, with the CID method equivalent in performance to the reflectance method. The use of a variety of metallochromic reagents was explored, with different reagents noted to provide colorimetric responses for different metal ions. A typical detection limit, provided by Methylthymol Blue adsorbed to Dowex 1x8 resin, was 10 ng Cu(II) per cm\(^{-2}\) of disc area. Linear response was seen up to a stoichiometrically defined metal loading, typically providing a linear response from the detection limit to 2 µg of Cu(II) per cm\(^{-2}\). Selectivity was reagent dependant, in particular Zincon was seen to provide a selective response to Cu(II) at neutral pHs and is a promising candidate for deployment of these colorimetric DGT binding phases