How to overcome inter-electrode variability and instability to quantify dissolved oxygen, Fe(II), mn(II), and S(−II) in undisturbed soils and sediments using voltammetry
How to overcome inter-electrode variability and instability to quantify dissolved oxygen, Fe(II), mn(II), and S(−II) in undisturbed soils and sediments using voltammetry
复制标题
如何克服电极间的变异性和不稳定性，使用伏安法定量原状土壤和沉积物中的溶解氧、Fe(II)、mn(II) 和 S(−II)
作者：
A. J. Slowey;M. Marvin
背景虽然伏安法具有独特的能力，几乎同时测量多种氧化还原成分，无需或只进行最少的样品预处理，但伏安法目前在表征水生和陆地系统中的氧化还原条件方面尚未得到充分利用。研究孔隙水等未受干扰的介质需要固态电极，而这种电极很难重复制造。已经开发出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（引导离子方法）的协议和准确性尚未详细记录。结果提供了测试电极质量的详细程序，并且记录了引导离子方法的应用和局限性。为了量化 Fe(II) 和 Mn(II)，从伏安信号中减去非线性基线函数可产生比线性基线更好的校准曲线，实现更低的检测限和可靠的重叠信号解卷积，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会变化百分之几十，并且灵敏度会随着时间的推移而下降。从一个 Hg/Au 电极到另一个电极，Mn(II) 与 Fe(II) 的校准斜率之比变化不超过 11%，并且 Mn(II) 引导离子预测的 Fe(II) 浓度与实际值平均相差 13%。然而，对于低于 15μM Fe(II) 的浓度，引导离子法的浓度预测较差（平均差异 46%）。从一个 Hg/Au 电极到另一个电极，Mn(II) 与 S(−II) 的校准斜率之比相差近 20%，并且 S(−II) 预测浓度与其实际值相差多达 58%。 Fe(II) 和 S(−II) 浓度的这些预测表明，引导离子方法的准确性取决于校准斜率与所用电极的独立程度。在海洋的中高浓度下，天然溶解的有机碳不会显着影响 Mn(II) 和 Fe(II) 的基线校正电极响应，但会显着影响 S(−II) 的响应。 结论 尽管存在固有的可变性，手工制作的 Hg/Au 电极可用于量化 O2、S(−II)、Fe(II) 和 Mn(II)，而无需针对每种感兴趣的成分校准每个电极。如果所有伏安技术都证明了基本原理（斜率的独立性），并且通过基线减法解决了系统物理化学性质对伏安信号的影响，则引导离子方法的准确度可以达到 20% 或更低。
BackgroundAlthough 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.ResultsA 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).ConclusionsDespite 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.