The risk associated with the use of PPD has motivated scientists to develop multiple analytical techniques to detect and quantify this chemical in cosmetic products to ensure their safety. Furthermore, because hair dyeing is based on oxidative chemical reactions, one can expect a variety of chemicals and intermediates with structural similarities, making their detection cumbersome. Consequently, several chromatographic methods have been reported for the analysis of the intermediates of oxidative hair dyes [18,22,23,24]. Some of these methods may be suitable only for checking the purity of the raw materials, while others may also be used for the analysis of intermediates of hair dyes in cosmetic formulations. ICP/MS, HPLC/MS, and GC-MS are reported analytical methods used to separate and determine the hair dye’s PPD intermediates [4]. However, these methods are costly, time-consuming, and some of them involve complicated procedures for sample preparation, such as extraction and chemical derivatization [22]. Because of that, a convenient and relatively easy-to-use technique is demanded in order to be used for routine analysis, and hence the advantage for the HPLC technique becomes visible. Practically, reported HPLC methods for the analysis of PPD in hair dye products suffer from major problems related to the quality of the chromatographic peak. Severe tailing, asymmetric chromatographic peak, and the inherent instability of PPD in hair dyes are common issues associated with HPLC analysis for any PPD-contained products. Fortunately, reversed-phase ion-pairing liquid chromatography has proven to counteract those issues with chromatographic separation [24]. In the current study, we used the ion-pairing technique in which we adapted the previous method with minor modifications. In order to improve peak shape, enhance detection sensitivity, and obtain a high response, a specified amount of ion-pair reagent was needed. We used 1-heptane sulfonic acid sodium salt as an ion-pair reagent to minimize the peak tailing. During method optimization, the resolution between the oxidizing hair dye components of commercial products was challenging. The original method used a flow rate of 1.0 mL/min, and authors achieved satisfactory resolution between some oxidizing components, but not for others [18]. In order to overcome this problem, authors recommended decreasing the flow rate from 1.0 to 0.8 mL/min, and this is the strategy we used in the current study.
A method for PPD quantification was successfully implemented and validated, circumventing the severe tailing and asymmetric chromatographic peak of PPD by using reversed-phase ion-pairing liquid chromatography. All tested samples complied with the current regulatory limits of PPD. The analysis of unreacted PPD revealed as much as 77% prevalence following extended dyeing time longer than what is recommended by the manufacturer. Eventually, higher PPD levels would likely be observed after strictly applying the manufacturers’ recommended dyeing times. Keeping in mind the health risks associated with PPD, as well as the carcinogenic potential of PPD-related chemicals, unconsumed PPD needs to be further explored and evaluated by regulatory bodies. Ideally, consumers are expected to be exposed to minimum levels of hazardous chemicals in hair dyes while obtaining the desired hair color. Collaborative efforts between industry, regulatory bodies, and health-related decision makers are deemed necessary to establish safe concentrations of sensitizing chemicals in hair dyes, particularly PPD.