Anal. Chem. 2005, 77, 5440-5446
Articles
Trace Metal Measurements in Low Ionic Strength Synthetic Solutions by Diffusive Gradients in Thin Films Kent W. Warnken,* Hao Zhang, and William Davison
Institute of Environmental and Natural Science, Lancaster University, Lancaster LA1-4YQ, United Kingdom
In view of conflicting reports regarding the performance of DGT in low ionic strength solutions (I < 1 mM), further investigations have been carried out. Minimal washing of the diffusive gel and deployment in 1.0 and 10 mM NaNO3 solutions containing Cu and Cd gave the theoretical response of 1 for [C]DGT/[C]SOLN, where [C]DGT is the concentration of metal measured by DGT and [C]SOLN is the concentration of metal measured directly in the solution by an appropriate analytical method. Erroneously high values for [C]DGT/[C]SOLN were obtained when these same gels were deployed at I ) 0.1 mM, presumably due to a net negative charge on the gel, attributable to the presence of initiation products of polymerization. However, washing the diffusive gels completely, where the storage solution pH equaled that of deionized water, gave values of ∼0.5 for [C]DGT/[C]SOLN from deployments at I ) 0.1 mM, consistent with the lower measured value of the diffusion coefficients at this ionic strength. These results can be explained by the presence of a net positive charge on the gel when it is exhaustively washed, which reduces the effective diffusion coefficient of metal ions by changing their concentration at the gel-solution interface (Donnan partitioning). Diffusive gel equilibration experiments showed the presence of low capacity sites capable of binding metals irrespective of ionic strength. This binding within the diffusive gel does not affect most DGT measurements, as short (4 h) deployments at concentrations of 10 ppb gave theoretical results. Incomplete washing of the resin-gel caused a 5-15% measurement error and a decrease in precision, even at ionic strengths of 10 mM. A high level of accuracy and precision (typically 5 obtained with the unwashed gels (Figure 2). Thus, the excess reaction products, rather than pH, seem to be primarily responsible for the large negative charge within the gel that caused the gross overestimation of DGT concentrations. The fact that charge can be removed by washing suggests that it is not caused by hydrolysis of the polymer or impurities within the gel matrix but simply results from an excess of reaction products that can readily diffuse out of the gel when washed appropriately. Analytical Chemistry, Vol. 77, No. 17, September 1, 2005
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Figure 4. Diffusive gel binding results for various Cd concentrations (1, 10, 60, and 500 ppb) and ionic strengths (10, 1.0, 0.1, 0.01, and 0.00 mM). The horizontal line represents a ratio of 1 for [Cd]GEL/ [Cd]SOLN. Error bars represent the standard deviation of three diffusive gel disks at each of the indicated ionic strengths.
The work presented here involved only the APA gel type, which is most widely used for the DGT technique. We could only reproduce the data of Alfaro-de la Torre et al.,10 i.e., an approximate doubling in the diffusion coefficient of Cd at low ionic strengths of 0.2 mM, if we did not adequately wash the diffusive gel. Completely washed gels resulted in diffusion coefficients that were ∼50% of Dgel determined at 10 mM. However, this previous work was important in highlighting the potential problems of using APA hydrogels at ionic strengths below 1 mM and has led us to reevaluate our own washing and conditioning procedures. It is now clear that deviations in the wash procedures employed after casting of the diffusive gel can substantially affect DGT measurements. These results are consistent with those of Peters et al.,11 who reported values for the [C]DGT/[C]SOLN ratio ranging from 0.5 to 3 but did not report the pH of the final wash solution for the different ratios obtained. It is possible that the range in their reported values could have been due to variations in the diffusive gel washing procedures used during their work. Diffusive Gel Binding Experiments. To investigate the importance of gel binding rather than gel charge, experiments were carried out using APA gel disks that were allowed to equilibrate in 0.1-L solutions of varying ionic strength (0.01-10 mM NaNO3) and in deionized water (Figure 4). The solution concentration of Cd was varied between 1 and 500 ppb. These experiments were effectively the first performance tests for trace metals of the sister technique of diffusive equilibration in thin films (DET), which was originally developed for measuring major ions and Fe and Mn at elevated concentrations.19,20 The elevated concentrations of Cd in the gel at low concentrations of 1 ppb suggest that Cd may bind to the gel. The Cd binding at a concentration of 1 ppb (1 week equilibration time) was apparent, irrespective of ionic strength (Figure 4). There appear, therefore, to be two factors affecting APA gel-metal ion interactions: (1) a gel charge, which affects DGT measurements by Donnan partitioning modifying the flux of metal ions at low ionic strengths (
