ELECTROANALYSIS OF ASCORBIC-ACID - A COMPARATIVE-STUDY OF LASER-ABLATION VOLTAMMETRY AND SONOVOLTAMMETRY

The electroanalytical detection of L-ascorbic acid via its two electro n oxidation in aqueous solution is the subject of a comparison of two voltammetric methodologies both of which rely in part on electrode abr asion. First in sonovoltammetry cavitational collapse at the electrode -solution interface can lead to electrode erosion and activation. Seco nd in Inse, ablation voltammetry (LAV) a 10 Hz pulsed laser (532 nm, 0 .7 mJ per pulse) is used to abrade the electrode. In both cases the co ncomitant agitation of the solution leads to regular refreshment of th e diffusion layer so that at sufficiently extreme potentials sustained currents (transport limited currents) are observed which scale with t he concentration of ascorbic acid present and permit quantitative elec troanalysis. Comparison of the methods as applied to ascorbic acid sho ws that for LAV the laser light intensity can be adjusted for maximum ablation of surface adsorbed blocking species but with minimum damage to the platinum surface itself. In contrast the sonovoltammetry techni que does not facilitate selective erosion of surface adsorbed species. Instead the mass transport, amount of cavitation and damage to the el ectrode are all interlinked with the intensity of ultrasound employed. Thus while the amount of cavitation at the electrode surface can be c ontrolled the relative adsorbate/electrode abrasion cannot. The limiti ng currents under insonation were found to be substantially (ca. 15 ti mes) larger than for LAV suggesting that the major benefit of sonovolt ammetry is in terms of enhanced mass transport whereas LAV shows more selective cleaning activation. The development of a novel sono-LAV is therefore reported which retains the merits of both the separate exper iments in isolation. Thus the cleaning potential of LAV is coupled wit h the mass transport enhancement of ultrasound. Application to the qua ntitative electroanalysis of ascorbic acid is reported.