Qu Hui, Arai Yuta, Harada Makoto, Okada Tetsuo
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan.
Anal Chem. 2015 Apr 21;87(8):4314-20. doi: 10.1021/acs.analchem.5b00747. Epub 2015 Apr 8.
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.
使用微盘电极对可逆氧化还原对Fe(CN)6(4-)/Fe(CN)6(3-)进行伏安法测量，以探究冷冻NaCl水溶液中液相（LP）生长的动态过程。关键因素是冷冻样品的温度历史和冷冻浓缩比。当在-25.0°C冷冻后直接将温度设置为工作温度时，在高浓缩比下通常无法进行循环伏安（CV）测量。即使可以进行测量，CV的形状和电流在3小时内也会随时间变化。CV和计时电流法的联合测量清楚地表明，工作电极附近LP的生长是这些现象的原因。沿着电极表面LP的发展会导致CV形状的变化，这是由于径向扩散的贡献增加所致。当LP主要垂直于电极表面生长时，代表线性扩散的峰形CV会在几个小时内保持不变。相比之下，当冷冻样品的温度升至-2.0°C进行退火时，在高于-18°C的温度下总是可以测量到可重复的循环伏安图（CV）。该程序使我们能够将冷冻溶液作为确定性系统而非随机系统来处理。目前的结果强烈表明，冷冻样品的退火对于成功使用痕量溶质的冷冻富集至关重要。通过包含退火步骤的提议程序，伏安测量的富集倍数高达1000倍。
