Pulsed electrochemical detection of organic sulfur compounds at gold electrodes

摘要

Many classes of organic sulfur compounds are catalytically oxidized at gold electrodes, including any compound having a free lone electron pair on the sulfur atom. The oxidation is a transient process, requiring concomitant formation of surface oxide, which results in a large background signal and the rapid poisoning of anodic activity. A three-step pulsed electrochemical detection (PED) waveform can be applied to maintain electrode activity and detect sulfur compounds following their separation by liquid chromatography. The waveform consists of a detection step, an oxidative cleaning step and a cathodic regeneration step. A short delay is used in the detection step prior to current sampling to allow for decay in current from charging and surface oxide formation. The measured signal has contributions from preadsorbed and mass transported sulfur species, and from the formation of Au surface oxide;PED at gold electrodes shows selectivity for sulfur compounds as a result of their strong adsorption to the gold surface. In addition, further selectivity may be gained by careful selection of pH and organic modifiers. PED is commonly practiced at pH \u3e 9 for carbohydrates and amines, but by lowering the pH to below 9, response from carbohydrates and amines is virtually eliminated without a significant loss of response for sulfur compounds. Selectivity for the sulfur may be further enhanced by the addition of an organic modifier, such as acetonitrile, which adsorbs to the electrode surface, blocking the activity of other PED-active compounds which adsorb less strongly;A novel PED waveform, integrated voltammetric detection (IVD), integrates electrode current throughout an entire cyclic scan, including analyte oxidation and Au-surface oxide formation on the forward scan and Au-surface oxide reduction on the return scan. IVD cancels the background from surface oxide, thereby decreasing the background signal and reducing baseline perturbations caused by changes in eluent composition. In addition, the use of an IVD waveform results in an improvement in the limits of detection. Limits of detection of organic sulfur compounds using the IVD waveform are on the order of one picomole.