Direct recognition and quantification by voltammetry of thiol/thioamide mixes in seawater

Thiols and thioamides form part of the pool of reduced sulfur substances (RSS) that modify the health of aquatic ecosystems acting as radical scavengers and heavy metal ligands. Their concentrations could be easily determined in seawater by cathodic stripping voltammetry (CSV) were it not be for the coalescence of their responses in a single peak. Here, we modified the traditional CSV method of RSS analysis to allow individual recognition and quantification in thiol/thioamide mixes. Glutathione, cysteine, thiourea and thioacetamide in UV digested seawater were repeatedly analyzed shifting the deposition potential (Edep) in the range +0.07 to −0.4 V at high resolution. The representation of peak height (ip) and peak potential (Ep) vs Edep resulted in different and distinctive profiles for each substance that allowed the selection of adequate Edep ranges for their separate quantification. Copper saturation modified thiol profiles and cancelled the response of thioamides. The vs Edep profiles explained the nature of the different thiols and thioamides present in the sample and permitted their individual quantification with excellent accuracy. The utility of the method was put to test with seawater modified with natural unknown RSS from pore waters and Posidonia oceanica exudates. Although both samples gave similar CSV signals, the vs Edep profiles unveiled completely different electrochemical behaviors incompatible with a similar nature. Based on those profiles we hypothesized that pore waters released a glutathione/thiourea mix and that one or several unidentified RSS formed part of P. oceanica exudates. The analytical scheme proposed here opens a new door to the use of direct voltammetry in the qualitative and quantitative determination of RSS in natural waters.