Development of advanced analytical methods for the analysis of food complex matrices by comprehensive two-dimensional liquid chromatography coupled to massspectrometry (LCxLC-MS)

Resumen

The importance of the development of novel analytical techniques lies in the growing needs and new applications that are arising in the analytical field, which require high resolving power in order to provide with enough information to characterize complex samples. In Food analysis, samples are commonly heterogeneous and are typically composed of multi-component mixtures of different chemical nature that are present in a wide range of concentrations. Moreover, some food samples contain significant amounts of compounds belonging to similar chemical families, involving very diverse mixtures of closely related compounds that are very difficult to be analyzed. Therefore, analytical techniques that are able to provide the maximum information about those samples are essential to address the new food analysis challenges. In this regard, multidimensional liquid chromatography groups very powerful separation techniques that provide significantly increased values of peak capacity compared to one-dimensional conventional analytical platforms. The use of multidimensional techniques coupled to mass spectrometry in food applications is gaining importance due to the benefits of the enhanced separation and identification potential that can be achieved for samples that present hundreds, or even thousands, of compounds. In this Doctoral Dissertation, the impressive analytical power of on-line comprehensive two-dimensional liquid chromatography (LC × LC) for the separation of very complex natural mixtures of compounds from different food sources is demonstrated. In particular, three main groups of secondary metabolites contained in seven different food-related samples deriving from plants and algae are exhaustively studied in this PhD Thesis, namely, proanthocyanidins, phlorotannins and triterpene saponins. These three groups have in common an important structural complexity due to the polymeric nature of proanthocyanidins and phlorotannins as well as to the very high variability of isomers and closely related structures in the case of triterpene saponins. Different on-line LC × LC methods based on the orthogonal coupling HILIC × RP were developed for each sample, and good separations, achieving high peak capacity values and orthogonality degrees were attained. Specifically, the separation of complex mixtures of proanthocyanidins together with other polyphenolic compounds in grape seeds, apples, chokeberries and grapevine canes is presented in Chapter 3. On the other hand, the separation and characterization of typical polymeric phenolic compounds exclusively found in brown algae, namely phlorotannins, are studied in detail in Chapter 4, and their content on two different algae species (i.e., Cystoseira abies-marina and Sargassum muticum) is determined. Lastly, a new method is developed for the profiling of the secondary metabolite pattern present in licorice (Ghlyzyrrhiza glabra), including the presence of triterpene saponins as well as different polyphenols. Subsequently, different strategies based on the use of focusing and non-focusing modulation procedures are studied with the aim to produce quantitative improvements on resolving power and sensitivity, using licorice as a model complex food sample. These last two studies are contained in Chapter 5. In addition, the development of LC × LC-based methods for the exhaustive chemical characterization of these interesting compounds helps to determine the native composition of the studied food-related samples, considering that sample preparation steps were reduced for their analysis. Thus, this analytical approach is demonstrated as very useful to correlate chemical composition with the potential biological properties that are attributed to the studied compounds. Moreover, this methodology is also capable to produce typical 2D profiles that could be applied to the determination of metabolic markers for geographical authentication assessment of valuable food products. In conclusion, the results contained in this PhD Dissertation contribute to increase the knowledge on the composition of the seven analyzed food-related samples for future researches as well as to support with new data and applications the use of LC × LC as a promising analytical technique in the Food analysis field. These contributions have given rise to the publication of 8 published/accepted original research articles included in international SCI journals within the Analytical Chemistry and Food Science and Technology categories.