Doehlert matrix for the optimization of ultrasound dispersive liquid–liquid microextraction of melatonin in Argentine and Brazilian wine samples

Abstract In the present work, a method based on the Ultrasound-dispersive liquid-liquid microextraction (US-DLLME) for the preconcentration of melatonin in Brazilian and Argentinian wine samples prior to the determination by reverse phase high performance liquid chromatography with diode array detector HPLC-DAD was developed. Sample prep optimization was carried out in two stages. First, a full factorial design was executed to evaluate the variables significance. The second step consisted of significant variables optimization (pH, extraction volume, sonication time, centrifugation time) using a Doehlert matrix to determine the critical values. Optimal US-DLLME extraction conditions were as follows: pH of 4.5, sonication time of 6.8 min, centrifugation time of 2.5 min, extractor volume of 250 µL and 5.0 mL of sample. The organic phase was retrieved with a microsyringe to a glass tube, dried and diluted with the mobile phase prior injection. The method presented a limit of detection (LOD) of 0.23 μg L-1 and a limit of quantification (LOQ) of 0.70 μg L-1. The precision expressed as relative standard deviation (R.S.D.) was 0.8 % and 1.9 % at 10.0 and 1.0 μg L-1, respectively. Accuracy was confirmed by comparison of the results found by the proposed methods with an alternative method, and spike tests with recuperations ranging from 92 to 103%. the analyzed samples showed concentrations varying from 2.2 to 5.0 μg L−1.