Department of Inorganic and Analytical Chemistry , University of Geneva , Quai Ernest-Ansermet 30 , CH-1211 Geneva , Switzerland.
Center of Excellence for Innovation in Analytical Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand, and Department of Chemistry, Faculty of Science , Chiang Mai University , Chiang Mai 50200 , Thailand.
ACS Sens. 2018 Nov 26;3(11):2455-2462. doi: 10.1021/acssensors.8b01096. Epub 2018 Nov 12.
There is an urgent need for reliable seawater phosphate measuring tools to better assess eutrophication. Today, most accepted sensing approaches are based on the established colorimetric molybdenum blue assay. It requires one to modify the sample to strongly acidic conditions and to add various reagents, principally molybdate and reducing agent (e.g., ascorbic acid), to form a blue colored phosphate complex that is subsequently detected spectrophotometrically. The associated need for large sample and mobile phase reservoirs and mixing coils are, unfortunately, not ideally adapted for the development of operationally simple in situ sensing instruments. It is here demonstrated for the first time that the key reagents needed to achieve phosphate detection by the molybdate method may be delivered by passive counter transport across ion-exchange membranes. A cation-exchange Donnan exclusion membrane placed in contact with a sample flow (450 μm thick) is shown to provide the strongly acidic conditions (pH ∼ 1) necessary for phosphate determination. Proton transport is driven, via cation-exchange, by the high sodium content of the seawater sample. Molybdate was similarly released through an anion-exchange membrane by chloride counter transport. Consequently, an in-line flow system containing the two membrane modules in series was used for delivering both hydrogen and molybdate ions into the sample to form the desired phosphomolybdate complex for subsequent spectrophotometric detection. A linear calibration in the range of 0.1-10 μM phosphate (3-300 ppb inorganic P) was achieved, which is sufficiently attractive for environmental work. A range of seawater samples was tested and the results from this membrane delivery device showed no significant differences compared to the classical molybdate assay chosen as the reference method.
目前迫切需要可靠的海水磷酸盐测量工具，以更好地评估富营养化。如今，大多数公认的传感方法都是基于已建立的钼蓝比色法。这种方法需要将样品调至强酸性条件，并添加各种试剂，主要是钼酸盐和还原剂（如抗坏血酸），以形成蓝色的磷酸盐络合物，然后用分光光度法进行检测。不幸的是，与这种方法相关的需要大量样品和流动相储液器以及混合线圈，并不理想地适用于操作简单的现场传感仪器的开发。本文首次证明，实现钼酸盐法检测磷酸盐所需的关键试剂可以通过离子交换膜的被动反向输送来提供。将阳离子交换 Donnan 排斥膜与样品流（450 μm 厚）接触，可以提供测定磷酸盐所需的强酸性条件（pH ∼ 1）。质子通过阳离子交换，由海水中的高钠离子含量驱动。类似地，氯离子通过阴离子交换膜反向输送，从而将钼酸盐释放出来。因此，串联使用包含两个膜模块的在线流动系统，将两种离子同时输送到样品中，形成所需的磷钼酸盐络合物，以便随后进行分光光度检测。在 0.1-10 μM 磷酸盐（3-300 ppb 无机 P）范围内实现了线性校准，这对于环境工作非常有吸引力。对一系列海水样本进行了测试，结果表明，与作为参考方法选择的经典钼酸盐测定法相比，这种膜输送装置没有显著差异。
