Recent innovations and trends in analytical chemistry lead to the development and application of new instrumental systems, innovative sample treatment techniques, and application to problem solving in various fields of analysis. Likewise, the growing concern about the reduction of the environmental impact during the analytical process, without impairing benefits in terms of selectivity and sensitivity, have modulated the main topics in this field of research. In this context, the continuous application of miniaturized liquid chromatography (LC) systems, including capillary liquid chromatography (CapLC) and nano liquid chromatography (NanoLC), has represented important advances, due to their tangible advantages, related not only to their performance, such as high sensitivity, but also from an environmental point of view, reducing the use of solvents, electricity, involved material, and generated waste. Along these same lines, recent applications of portable liquid chromatography systems have a significant potential for solving analytical challenges in various applications in situ or at‐situ monitoring, due to their ability to perform measurements at the point of sampling. These potential applications are focused on the need to obtain results quickly, overcome problems in the preservation of the sample or the remote location of the monitoring site. Despite the great advances made at the instrumental level, sample treatment continues to be the stage of the analytical process with the greatest investment of time, sources of error and possible losses of the analytes of interest, therefore the replacing of conventional offline techniques by novel techniques, developed under the green chemistry approach, has become one of the main topics on which researchers have put much effort. An important group of these novel techniques is those based on the absorption of the analytes of interest in a solid, known as solid‐phase microextraction (SPME), which reduce the consumption of solvents and therefore minimize the generation of waste, as well as achieve higher productivity due to its automation capacity. In‐tube solid‐phase microextraction (IT‐SPME) corresponds to a type of dynamic SPME, with figures of merit related to the reduction of solvent consumption, easy automation, and miniaturization, allowing it to be easily coupled to miniaturized chromatographic systems. Currently, the development and application of new sorbent phases that allow increasing the selectivity and sensitivity of IT‐SPME is one of the main lines of investigation of this technique. In the framework of the development of this thesis, the research has been focused on the application of miniaturized LC systems for the analysis of highly polar and non‐polar compounds in various matrices such as biological samples, environmental waters, dietary supplements, and natural resins. Likewise, aspects such as the development of new sorbent phases and the use of new commercial phases for IT‐SPME have been evaluated. Furthermore, the evaluation of the portable NanoLC system for the analysis of emergent compounds in environmental waters has been successfully studied.