Simultaneous analysis of some transition metals at ultra-trace level by ion-exchange chromatography with on-line preconcentration

The quantitative analysis of six transition metals at concentrations below 1 mu g l(-1) was achieved using ion-exchange chromatography with on-line preconcentration. The metal ions are selectively retained on an iminodiacetate chelating stationary phase, then desorbed and transferred to a mixed-bed ion-exchange column for separation. Both the desorption from the concentrator column and the analysis on the separator column are made possible by the use of a complexing agent in the mobile phase, namely pyridine-2,6-dicarboxylic acid, which forms stable negatively charged complexes with the metals. The detection is spectrophotometric after addition of a post-column reagent (pyridyl-azo-resorcinol). Optimum uptake conditions (sample pH, maximum volume of preconcentration) as well as performance of the method (dynamic range, within-laboratory precision, recovery, limit of detection) were determined. It was found that recoveries would drop below 90% for preconcentration volumes larger than 25 ml and for sample pHs lower than 2.8 or higher than 3.5. Under optimum conditions, limits of detection of some tens of ng l(-1) were achieved with precision better than 9% at the mu g l(-1) level. The accuracy was tested with a reference Canadian river water and with bottled spring and mineral waters; the results showed good agreement with the certified values. For highly mineralized waters, the influence of major cations (Na, Ca) was studied. Limitations are given and attempts to reduce the interference with calcium are discussed.