Extractive Freezing-out of Caffeine from Aqueous Solutions in a Centrifugal Force Field
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Extractive Freezing-out of Caffeine from Aqueous Solutions in a Centrifugal Force Field V. N. Bekhterev* Faculty of Environmental Engineering, Sochi State University, Sochi, 354000 Russia *e-mail: [email protected] Received November 22, 2019; revised March 12, 2020; accepted March 24, 2020
Abstract—The effect of the pH of the medium, the volume ratio of the extractant to water on the efficiency of caffeine extraction with acetonitrile from aqueous solutions by extractive freezing-out in a centrifugal force field is studied. The results can be explained in terms of the previously proposed theoretical process model. The highest recovery of caffeine was achieved at pH 6–7. The limit of determination of caffeine in water is 0.015 μg/mL. Keywords: extractive freezing-out, centrifugation, caffeine, regularities, gas chromatography DOI: 10.1134/S1061934820090051
The use of liquid–liquid extraction in chemical analysis is usually associated with tasks of eliminating the interference of the sample matrix and of the preconcentration of analytes [1–3]. There is a need in expanding the methods to the extraction of soluble, including polar and ionogenic, organic substances from aqueous media, because of a need in increasing the efficiency of their extraction, reduction of chemical and thermal effects on the test sample (elimination of salting out, distillation, and evaporation), shortening of some steps (filtering, dehydration of extracts), and improving economical characteristics. The method proposed in 2005 for the extraction of soluble organic compounds from aqueous media [4], which combined extraction with freezing and was named “extractive freezing-out” (EF) [5, 6], successfully solves most of the above problems. To date, EF is already used in a number of chemical analysis procedures [7–12]. Laboratory protocols have been proposed for the isolation of saponins from roots of butcher’s-broom in the isolation of biologically active substances (BASs) [13]. The established regularities of the extraction of carboxylic acids and phenols using EF were explained within the framework of the proposed theoretical model [5, 6]. It is based on an analyte redistribution mechanism between the previously added non-freezing solvent (extractant) and the surface of the crystalline ice phase formed during temperature reduction. As a result, Eq. (1) was obtained that relates the concentration of the target component in the extract corg with its initial mass in the sample mo and volume of the extractant Vextr:
corg = ( K eqβ α )( mo Vextr ) = *K eq ( mo Vextr ) , (1) where Keq is the constant of an adsorption–desorption equilibrium, established for the concentration of an analyte in the extract and on the forming ice surface; α is a coefficient relating the concentration of free adsorption sites with the volume of extractant added to the sample Vextr, cm–5; β is a coefficient relating the concentration of the occupied adsorption sites with the initial mass of the extracted substance in the sample mo, μg–1 cm–2. As a result, it becam
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