Microextracción dispersiva con nanopartículas magnéticas y en fase líquida acoplada a técnicas cromatográficas

Abstract

In recent years, miniaturized techniques have focused on a very important part of research in the area of Analytical Chemistry, and, particularly, in the sample pretreatment step. The analytical methods developed in this Doctoral Thesis follow to this trend, and procedures based on liquid phase microextraction (LPME) and magnetic solid phase microextraction (MSPE) have been developed, leading to improved sensitivity of the analytical procedures, since the compounds of interest are isolated from the sample and preconcentrated in a microvolume of extractant that can be found in liquid or solid phase. Furthermore, a cleaning effect is simultaneously produced in the sample, which results in improved method selectivity. Among the different variants of LPME, dispersive liquid-liquid microextraction (DLLME) has shown marked advantages over others, and is the technique selected in this Thesis. In addition to the classical version using extraction solvents denser than water, some modifications have been applied to minimize the use of toxic organic solvents: extraction phases which are lighter than water and compatible with the mobile phase used in reversed phase-liquid chromatography (LC); the in situ generation of ionic liquids and the possibility of omitting the use of dispersants in certain samples. Although the analysis of extracts obtained in the microextraction stage can be approached through various instrumental systems, chromatographic methods offer the possibility of separating closely related compounds. The combination of microextraction procedures with LC and gas chromatography (GC) enabled the development of selective, sensitive and accurate methods in this Thesis, and they can, in addition, be included in the framework of Green Analytical Chemistry, since the consumption of both toxic organic solvents and amount of sample is very low, thus generating little waste. One drawback of the combination of microextraction with chromatographic techniques is the possible incompatibility between the extract enriched with the analytes and the separation system. The evaporation of the extractant solvent and the reconstitution of the dry residue in a phase compatible with the selected LC mode or the use of thermal desorption systems for GC analysis of ionic liquids containing the analytes are the solutions applied in this PhD Thesis. The detection of the separated compounds was performed with mass spectrometry. For GC, the ionization of the analytes was produced by an electronic impact source and the analyzer was a single quadrupole, while for LC, the electrospray source allowed the coupling of two incompatible systems, and the analyzer used was a triple quadrupole, which enabled unambiguous identification following fragmentation patterns. The selection of optimal microextraction conditions was carried out by applying multivariate statistical methods, since there are directly related variables. Taguchi and Central Compound designs proved to be very good tools for the selection of the experimental conditions that provided the best preconcentration efficiency. Six analytical methods are presented in the Thesis, thus demonstrating the applicability of the combination of microextraction techniques with chromatographic separations for the resolution of interesting issues. Thus, within the field of food analysis, the determination of terpenes in alcoholic beverages and grapes, differentiating between free and glycosylated forms through enzymatic hydrolysis is proposed (Chapters I and II). Concerning food safety, the determination of PDE-5 enzyme inhibitors has been addressed in energy drinks and dietary supplements, with the ensuing detection of frauds due to the presence of undeclared compounds in the composition of commercial products (Chapter V). The determination of these inhibitors in human hair may be of interest in the forensic field. Analytical procedures for environmental control are proposed in Chapters III, IV and VI for the determination of chlorobenzenes, marine toxins and herbicides in soils and waters of different origins. All the methods developed have been validated, according to international guidelines based on the most relevant analytical characteristics (linearity intervals, accuracy, precision and sensitivity) and applied to the analysis of real samples of very different nature.