Suitable conditions for liquid-phase microextraction using solidification of a floating drop for extraction of fat-soluble vitamins established using an orthogonal array experimental design

A simple, rapid and efficient microextraction method for the extraction and determination of some fat-soluble vitamins (A, D2, D3) in aqueous samples was developed. For the first time orthogonal array designs (OADs) were employed to screen the liquid-phase microextraction (LPME) method in which few microliters of 1-undecanol were delivered to the surface of the aqueous sample and it was agitated for a selected time. Then sample vial was cooled by inserting it into an ice bath for 5 min. The solidified solvent was transferred into a suitable vial and immediately melted. Then, the extract was directly injected into a high-performance liquid chromatography (HPLC) for analysis. Several factors affecting the microextraction efficiency such as sample solution temperature, stirring speed, volume of the organic solvent, ionic strength and extraction time were investigated and screened using an OA16 (45) matrix. Under the best conditions (temperature, 55 °C; stirring speed, 1000 rpm; the volume of extracting solvent, 15.0 μL; no salt addition and extraction time, 60 min), detection limits of the method were in the range of 1.0–3.5 μg L−1. The relative standard deviations (RSDs) to determine the vitamins at μg L−1 levels by applying the proposed method varied in the range of 5.1–10.7%. Dynamic linear ranges of 5–500 μg L−1 with good correlation coefficients (0.9984 < r2 < 0.9991) were observed. Finally, the study was applied to determine the vitamins in several real aqueous samples including mixed juice fruit, urine and tap water samples and relatively good results were obtained.