This works responds to the commitment acquired with the Ministerio de Economía y Competitividad (MINECO) of Spain for granting Alberto Celma Tirado with a predoctoral fellowship co-funded by European Social Fund (Grant number BES-2016-076914) within the project 'Investigación sobre el uso de Nuevas Sustancias Psicoactivas a través del análisis de los productos consumidos, de la orina de potenciales consumidores y de aguas residuales' (Project number CTQ2015-65603-P). Alongside his predoctoral fellowship, Alberto Celma Tirado also received additional funding to perform an international research stay in the Swedish University for Agricultural Sciences (SLU) in Uppsala ( Sweden) under the supervision of Dr. Johan Lundqvist from 29 th of March to 4 th of July 2019. The works included in this thesis have been funded by Ministerio de Economía y Competitividad (CTQ2015-65603-P), Ministerio de Ciencia e Innovación (RTI2018-097417-B-100), European Union's Justice Programme -Drugs Policy Initiatives (861602), Swedish Research Council (FORMAS) (2018-02256), European Commission (HOME/2014/JDRU/AG/DRUG/7086), Generalitat

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