Hydrophobic Eutectic Solvent with Antioxidant Properties: Application for the Dispersive Liquid–Liquid Microextraction of Fat-Soluble Micronutrients from Fruit Juices

Abstract

Despite the great interest devoted to eutectic solvents with significant negative deviations from ideality (namely, deep eutectic solvents), many hydrophobic liquid mixtures are ideal or quasi-ideal systems. In this regard, we propose the introduction of a hydrophobic eutectic solvent based on l-menthol and butylated hydroxytoluene, blended in a molar ratio of 3:1. The physicochemical characterization by means of differential scanning calorimetry and infrared spectroscopy has identified that it is an ideal eutectic solvent. Compared to other hydrophobic mixtures, this one represents an advanced solvent system due to its intrinsic antioxidant activity, which makes it an ideal green choice for the extraction and preservation of easily oxidizable lipophilic compounds. In order to evaluate its efficiency, it was applied for the first time as an extractant for the dispersive liquid–liquid microextraction of carotenoids and fat-soluble vitamins from fruit juices, carried out at 298 K. All extracts were analyzed by high-performance liquid chromatography–tandem mass spectrometry. The developed method was then validated, providing precise (4–8%) and accurate (4–6%) results on a commercial fruit juice containing declared values of β-carotene and α-tocopherol acetate. Recoveries were ≥70%, while the detection limits were 0.05 μg L–1 for β-carotene and 0.28 μg L–1 for α-tocopherol acetate. Here, we also propose an original approach to properly determine the antioxidant activity of eutectic mixtures since the available commercial kits cannot be applied to such systems without altering their nature. Last but not least, a circular process of reuse of this ideal eutectic solvent has been developed using a low-cost activated carbon material obtained from coconut shells.

Hydrophobic Eutectic Solvent-Based Dispersive Liquid-Liquid Microextraction Applied to the Analysis of Pesticides in Wine

“…Therefore, the ethanol volume to be added depended on the alcoholic content of the selected sample: for example, for a wine with 13% (v/v) alcohol, it was 550 µL, i.e., 1850 µL (the total volume)-1300 µL (the endogenous amount). An overall volume of ethanol lower than 1850 µL could not prevent solidification during the extraction and the storage of the extracts at temperatures ≤ 19 • C. This is due to the fact that the liquid state of pure MEN:BHT(3:1) is thermodynamically stable at room temperature and above (≥25 • C) [27], but the partition of ethanol in the eutectic mixture lowers the melting point (up to ≤4 • C). Extractions performed on untreated wine gave an unclear phase separation; therefore, instead of diluting the sample and its useful ethanol content, filtration was preferred as a pretreatment.…”

“…DLLME experiments were performed on the basis of our previous experience with this eutectic solvent, applied to the extraction of fat-soluble micronutrients from fruit juices [27]. In that case, it was found that the best volumes for extracting and dispersing solvents were 150 μL and 1850 μL, respectively.…”

“…Although many DLLME methods are still based on the use of toxic organic solvents, one of the current trends in analytical chemistry is their replacement with safer alternatives [22]. In particular, eutectic solvents (ESs), including both the deep (DESs) and the ideal (IESs) ones, as well as low transition temperature mixtures (LTTMs) and ionic liquids (ILs), are widely used to make DLLME an even greener procedure [23][24][25][26][27]. When dealing with highly hydrophobic ESs, the use of the dispersant solvent is not always necessary, but it can conveniently lower the melting point of some ideal mixtures, such as the one composed of L-menthol and butylated hydroxytoluene at a 3 to 1 molar ratio (MEN:BHT (3:1)).…”

“…L-menthol has been chosen for its natural derivation, absence of toxicity, and tendency to form hydrophobic eutectic mixtures with selected compounds, such as thymol; BHT, which is a hindered phenolic compound used as antioxidant by food, cosmetic, and pharmaceutical industry, has been preferred to thymol for its lower cost and greener character (penalty points calculated by the analytical Eco-Scale are 1 for BHT, and 4 for thymol), as well as for its additional antioxidant value. This IES with marked antioxidant properties has successfully been employed as an extraction solvent to perform a green DLLME of fat-soluble vitamins and carotenoids from fruit juices [27]. The use of ethanol as a dispersive solvent prevented the solidification of the extracts at temperatures lower than 19 • C.…”

Application of a Low Transition Temperature Mixture for the Dispersive Liquid–Liquid Microextraction of Illicit Drugs from Urine Samples

“…ESs, which include both deep (DES) and ideal (IES) eutectic solvents, are mixtures with a melting point lower than that of the individual solid components, while LTTM is a mixture exhibiting a glass transition [12,[20][21][22]. IESs [23], DESs, and LTTMs are of special interest for analytical purposes, especially when liquid is at room temperature.…”