New strategies for the simultaneous voltammetric quantification of Pb and Zn in hair cosmetics samples employing chemically modified composite electrodes

Given the possibility of exposure to various metals by inappropriate use of some commercially available hygiene products, it becomes necessary to develop simpler and more cost-affordable procedures to quantify those analytes in different matrices, allowing a local quality control even for smaller producers and industries. In this context, electroanalytical techniques, especially using stripping voltammetry, have been an alternative due to the relatively low cost of implementation and operation, high sensitivity, accuracy and applicability in environments with high ionic strength. Transitory modifications based on bismuth films stands for a viable alternative in gaining sensitivity and resolution, replacing mercury films, with simpler preparation and operation. Electrode modification can also be accomplished with organic films, which can be made, for example, by reduction of diazonium salts. In this paper, we present the development and application of two different modified systems, using a manufactured composite electrode composed of graphite (65%, m/m) dispersed in epoxy resin, for determination of Pb2+, Cd2+ and Zn2+ in color wash lotion and anti-dandruff shampoo. Two modifications were implemented: in situ bismuth film and ex situ organic film by diazonium salt reduction. Determinations were carried out in acetate buffer pH 6.0 with deposition potential of −1.4 V (applied during 120 s, with stirring), followed by voltammetric square wave measurements and subsequent cleaning with application of 0.3 V potential. Detection limits of 0.07, 0.05 and 0.06 µmol L−1 were achieved for lead, cadmium and zinc, respectively. The method was applied to hair cosmetic products such as color wash lotion (with lead acetate as its active), and anti-dandruff shampoo (with zinc in the forms of pyrithione and carbonate as actives). All studied samples have shown results in accordance with local regulations and product labels. The results were compared to those found by applying flame atomic absorption spectrometry, being considered statistically equivalent.