U,S, Geological Survey, Menlo Park, CA, USA.
Geochem Trans. 2012 Jun 25;13(1):6. doi: 10.1186/1467-4866-13-6.
Although uniquely capable of measuring multiple redox constituents nearly simultaneously with no or minimal sample pretreatment, voltammetry is currently underutilized in characterizing redox conditions in aquatic and terrestrial systems. Investigation of undisturbed media such as pore water requires a solid-state electrode, and such electrodes can be difficult to fabricate reproducibly. An approach to determine the concentrations of electroactive constituents using indirectly calibrated electrodes has been developed, but the protocol for and accuracy of this approach-the pilot ion method-has not been documented in detail.
A detailed procedure for testing electrode quality is provided, and the application and limitations of the pilot ion method have been documented. To quantify Fe(II) and Mn(II), subtraction of non-linear baseline functions from voltammetric signals produced better calibration curves than did linear baselines, enabled lower detection limits and reliable deconvolution of overlapping signals, and was successfully applied to sediment pore water signals. We observed that electrode sensitivities often vary by tens of percent, and that the sensitivity declines over time. The ratio of calibration slopes of Mn(II) to Fe(II) varied by no more than 11% from one Hg/Au electrode to another and Fe(II) concentrations predicted by the Mn(II) pilot ion were, on average, 13% different from their actual values. However, concentration predictions by the pilot ion method were worse for less than 15 μM Fe(II) (46% different on average). The ratio of calibration slopes of Mn(II) to S(-II) varied by almost 20% from one Hg/Au electrode to another, and S(-II) predicted concentrations were as much as 58% different from their actual values. These predictions of Fe(II) and S(-II) concentrations indicate that the accuracy of the pilot ion method depends on how independent calibration slope ratios are from the electrode used. At medium-to-high concentration for the ocean, naturally derived dissolved organic carbon did not significantly affect the baseline-corrected electrode response of Mn(II) and Fe(II), but did significantly affect the response of S(-II).
Despite their intrinsic variability, Hg/Au electrodes fabricated by hand can be used to quantify O2, S(-II), Fe(II), and Mn(II) without calibrating every electrode for every constituent of interest. The pilot ion method can achieve accuracies to within 20% or less, provided that the underlying principle-the independence of slope ratios-is demonstrated for all voltammetric techniques used, and effects of the physicochemical properties of the system on voltammetric signals are addressed through baseline subtraction.
尽管伏安法具有独特的能力，可以在几乎无需样品预处理的情况下同时测量多种氧化还原成分，但在水生和陆地系统中描述氧化还原条件时，它的应用并不广泛。对孔隙水等未受干扰的介质进行研究需要使用固态电极，而这种电极的制作可能难以重复。已经开发出了一种使用间接校准电极来确定电活性成分浓度的方法，但该方法的协议和准确性（即先导离子法）尚未详细记录。
提供了详细的电极质量测试程序，并记录了先导离子法的应用和局限性。为了定量 Fe(II) 和 Mn(II)，从伏安信号中减去非线性基线函数比线性基线产生更好的校准曲线，可降低检测限并可靠地解卷积重叠信号，并成功应用于沉积物孔隙水信号。我们观察到，电极灵敏度通常相差百分之几十，而且灵敏度随时间下降。从一个汞/金电极到另一个电极，Mn(II) 和 Fe(II) 的校准斜率之比变化不超过 11%，并且通过 Mn(II)先导离子预测的 Fe(II)浓度平均与其实际值相差 13%。然而，对于低于 15 μM 的 Fe(II)，先导离子法的浓度预测结果较差（平均相差 46%）。从一个汞/金电极到另一个电极，Mn(II) 和 S(-II) 的校准斜率之比变化近 20%，并且通过先导离子预测的 S(-II)浓度与其实际值相差高达 58%。这些 Fe(II)和 S(-II)浓度的预测表明，先导离子法的准确性取决于校准斜率之比与所用电极的独立性。在海洋中的中至高浓度下，天然存在的溶解有机碳不会显著影响 Mn(II)和 Fe(II)的基线校正后电极响应，但会显著影响 S(-II)的响应。
尽管存在固有变异性，但通过手工制作的汞/金电极可以在无需为每个感兴趣的成分校准每个电极的情况下，用于定量测量 O2、S(-II)、Fe(II) 和 Mn(II)。只要证明了所有使用的伏安技术的斜率比独立性这一基本原则，并通过基线扣除来解决系统物理化学性质对伏安信号的影响，先导离子法可以达到 20%或更低的精度。
