Indirect determination of thimerosal after graphene quantum dots assisted photo-degradation and trace level determination of mercury by multipath optical absorption spectrophotometry

Thimerosal (C9H9HgNaO2S) has been used as a preservative for cosmetics and pharmaceuticals since 1930, preventing contamination and guaranteeing the long-term stability of the products. Thimerosal contains 49.55% mercury in its composition and inside the human body it is metabolized producing ethylmercury (EtHg+) and thiosalicylic acid1. In recent years there has been controversy over the use of thimerosal in vaccines and pharmaceuticals as it is considered a direct means of mercury contamination in humans and environment. The objective of this work was to determine thimerosal in industrial effluent samples after the photodegradation of thimerosal by visible light mediated by graphene quantum dots (GQDs). Volumes of 10 mL of the thimerosal solution (at the hundreds of ng L-1 level) were placed in 15-mL glass vials where a specific volume of GQDs dispersion was inserted, before pH adjustment and sealing. These flasks were placed in a photochemical reactor containing commercial standard fluorescent lamps as a source of visible light. Optimization studies showed a higher degradation efficiency using 17 μL of GQDs synthesis dispersion (average size of 5.2 ± 1.2 nm and 6 mg L-1 of total carbon), pH of the medium adjusted to 4.5 and after 20 min of light exposure. The determination of thimerosal was made indirectly by the quantification of the total mercury released as the final product of the degradation. Quantification was made by atomic absorption spectrophotometry (with a multipath cell) using a classic sample reduction procedure using SnCl2. The monitoring of EtHg+, as one of the degradation product of thimerosal, was made by gas chromatography using a dedicated system based on mercury cold vapor fluorimetric detection after chemical derivatization with sodium tetrapropyl borate. Quantification was made using analytical curves in the concentration range between 19 ng L-1 (limit of quantification) and 300 ng L-1 of thimerosal. Interfering assays showed that EtHg+ interfered with the method. This method was applied in the determination of thimerosal in samples of a pharmaceutical industry effluent. The results obtained were statistically comparable to those obtained by the standard method based on high performance liquid chromatography (HPLC)