A two-in-one device for online monitoring of direct immersion single-drop microextraction: an optical probe as both microdrop holder and measuring cell

Abstract

For the first time an optical probe is proposed as the microdrop holder and simultaneously the measuring cell in a direct immersion single-drop microextraction (DI-SDME) procedure. This approach enables the analytical signal (absorbance of organic phase) to be monitored and recorded online during the extraction process. Based on the suggested approach, a novel on-probe DI-SDME method for the determination of thiocyanate ions using the extraction of the ion pair formed between SCN⁻ and Astra Phloxine reagent in acidic medium into 40 µL of amyl acetate is suggested. The final experimental conditions were found to be: the pH of the aqueous phase being 3, the stirring rate at 500 rpm, a 150 s extraction time, the absorption maximum at 555 nm and 700 nm as the reference wavelength. The calibration plot was linear in the range of 0.29–4.35 mg L⁻¹ of SCN⁻, with a correlation coefficient of 0.9981. The limit of detection (LOD) was found to be 0.12 mg L⁻¹. The method was applied for analysis of human saliva samples. Furthermore, the possibility of using the optical probe not only for DI-SDME but also for liquid–liquid–liquid microextraction (LLLME) and reverse mode microextraction techniques is shown in separate experiments.