Direct Determination of Mercury in Skin Lighteners Using Chemometric Assisted Energy Dispersive X-ray Fluorescence Spectroscopy

Abstract

The use of cosmetic products for skin colour lightening or bleaching is a common practice across the globe. However human health effects associated with mercury in these products such as skin cancer and kidney failure have been reported. Conventional techniques used in the analysis of mercury such as atomic absorption spectroscopy and mass spectroscopy are destructive, time consuming and expensive. Although the energy dispersive X-ray fluorescence (EDXRF) spectroscopy method is rapid, non-destructive and requires minimal or no sample preparations, it has a high detection limit for mercury and therefore quantification of trace levels of mercury below 1ppm is challenging. This is mainly due to spectral overlaps, weak mercury fluorescence signals and extreme matrix effects. In this work, a novel chemometrics-assisted EDXRF spectroscopy method was utilized to realize rapid, direct detection and quantification of both low (< 1 ppm) and high mercury levels in skin whitening creams and lotions. 50 simulate (mixture of distilled water and pure glycerol) samples spiked with mercury concentrations ranging from a blank sample to 500 ppm were used for method development. The samples were then analyzed in triplicates for 900 seconds using the NeX EDXRF spectrometer set-up. Two chemometric techniques namely, principal component analysis (PCA) and artificial neural networks (ANNs) were used to perform exploratory analysis of the measured EDXRF spectra and quantification of the mercury levels. From PCA, it was found that the spectral data forms distinct clusters for both the low and high mercury concentration levels in the 9.6-10.4 keV Hg Lα and 11.2-12.4 keV Hg Lβ regions. Two ANN Hg concentration models were developed, one for ppb concentrations (0-1000 ppb; 30 samples) and the other for ppm concentrations (0-500 ppm; 17 samples). The R2, RMSEP, LOD and LOQ values for the two models were 0.72, 20.6%, 527 ppb, 819 ppb and 0.98, 4.6%, 3 ppm and 11 ppm respectively. The ppm model was used to ascertain sample results acquired by utilizing the conventional EDXRF method, most of which were closely matching while the ppb model established that two of the real samples registered a Hg content equal to 731±151 ppb. It may therefore be concluded that the chemometrics – EDXRF approach has potential for rapid, non-destructive and trace quantitative analysis of mercury compared to traditional measurement approaches. The technique is recommended for quality control and assurance of consumer products by the relevant regulatory authorities such as the Kenya Bureau of standards.