Freeze enrichment protocol based on voltammetric probing of liquid-phase growth in frozen aqueous electrolyte...

Voltammetry of the reversible redox couple, Fe(CN)6(4-)/Fe(CN)6(3-), using a microdisc electrode, is employed to probe the dynamic process of the growth of the liquid phase (LP) in frozen aqueous NaCl solutions. Critical factors are the temperature history of a frozen sample and a freeze-concentration ratio. When the temperature is directly set to the working temperature after freezing at -25.0 °C, CV measurements are often impossible at high concentration ratios. Even if measurements are possible, the shapes of the CVs and the currents change with time over 3 h. Combined measurements of CV and chronoamperometry clearly indicate that the growth of the LP near the working electrode is responsible for these phenomena. The development of an LP along the electrode surface leads to a change in the shape of the CV due to an increase in the contribution from the radial diffusion. When the LP mainly grows orthogonal to the electrode surface, the peak-shaped CV representing linear diffusion is maintained over several hours. In contrast, when the temperature of a frozen sample increases up to -2.0 °C for annealing, reproducible cyclic voltammograms (CVs) are always measured at temperatures higher than -18 °C. This procedure allows us to handle frozen solutions as a deterministic system rather than a stochastic one. The present results strongly suggest that annealing of frozen samples is critical for successful uses of the freeze enrichment of trace solutes. Up to 1000-fold enrichments for voltammetric measurements are demonstrated by the proposed procedure involving an annealing step.