Dispersive Solid Phase Extraction of Melatonin with Graphene/Clay Mixtures and Fluorescence Analysis in Surfactant Aqueous Solutions.

In this work, the dispersive solid phase extraction (dSPE) of melatonin using graphene (G) mixtures with sepiolite (SEP) and bentonite (BEN) clays as sorbents combined with fluorescence detection has been investigated. The retention was found to be quantitative for both G/SEP and G/BEN 4/96 and 10/90 / mixtures. G/clay 4/96 / mixtures were selected to study the desorption process since the retention was weaker, thus leading to easier desorption. MeOH and aqueous solutions of the nonionic surfactant Brij L23 were tested as desorbents. For both clays and an initial sample volume of 25 mL, a percentage of melatonin recovery close to 100% was obtained using 10 or 25 mL of MeOH as desorbent. Further, using a G/SEP mixture, 25 mL as the initial sample volume and 5 mL of MeOH or 60 mM Brij L23 solution as the desorbent, recoveries of 98.3% and 90% were attained, respectively. The whole method was applied to herbal tea samples containing melatonin, and the percentage of agreement with the labeled value was 86.5%. It was also applied to herbal samples without melatonin by spiking them with two concentrations of this compound, leading to recoveries of 100 and 102%.