Theory of bulk and flow electrolysis and approach to parameter optimisation for chromatographic electrochemical detection

Abstract

IntroductionE l e c t ro chemical (coulometric and ampero m e t ric) detection in chromatography is known to have several advantages such as high sensitivity, wide linear range, the possibility to distinguish among different oxidation states of the same analyte and the applicability to a wide range of electroactive solutes [1], from organic substances (e.g. amines, phenols, mercaptans, aromatic nitro and halogen compounds, aldehydes and ketones) [2-5] to inorganic ones (anions and cations) [6][7]. Nowa d ays electro chemical detection is ex t e n s ive ly adopted in biochemistry for organic analytes (sugars, pept i d e s , c at e ch o l a m i n e s , v i t a m i n s , d rugs...) [8][9][10][11][12]; it is also e m p l oyed for env i ronmental analysis of organic contaminants (phenols, pesticides...) [13][14], whereas it is less commonly applied to metal ion determination [15][16].Though this extensive use of electrochemical detection for applicative purposes, in no case in the current literature has the behaviour of an electrochemical detector been studied as a function of mobile phase parameters especially in particular ch ro m at ographic mechanisms such as ion pair ch romatography. The knowledge of such behaviour is important to optimise electrochemical detection after chromatographic separation. Thus in this work several parameters related to analyte detection after HPLC separation, were investigated and the results obtained are reported hereafter. In particular hy d roquinone was chosen as a re fe rence solute since its ch ro m at ographic retention and re d ox mechanism are we l l known.The paper consists of two sections. In the first one (detector characterisation) the behaviour and performance of the detector are described: the results obtained can be of practical use in order to establish the operative conditions also for the detection of other analytes. The second section (bulk and flow electrolysis) deals with the effect of flow rate on the ch ro m at ographic signal. The process occurring in the detector cell in the presence of an electroactive species is actually an electrolysis. Therefore, starting from the theory of bulk and fl ow electro ly s i s , a model for predicting the detector response as a function of eluent fl ow rate wa s applied and its agreement with ex p e rimental results wa s compared. Materials and methodsThe chromatographic system used was a Gilson Model 302 pump fitted with a Gilson Model 208 manometric module, a Rheodyne Model 7125 injection valve with a 100 µL loop, an ESA Coulochem 2 ampero m e t ri c / c o u l o m e t ric detector equipped with an ESA electrolysis cell Model 5011.The cell was equipped with two porous graphite working electrodes, each coupled with a platinum counter electrode and a palladium reference electrode, enclosed in two stainless steel chambers. The cell volume was 5 µL. The chromatograms were recorded on a personal computer with the aid of a dedicated software (Dionex AI-450) which allows peak area integration.An Orion EA-920 pH meter equipped with a ...