The origin of the annotated chemicals is explored in Figure , revealing that most are endogenous. Since exogenous species are typically present at trace levels compared to endogenous metabolites, it is challenging to capture both concurrently; furthermore, detection in CSF requires exogenous species to cross the blood brain barrier (BBB), which regulates the passage of substances to the CNS and CSF . unique features between Level 1 and 3 (Table ). The same compounds were identified using the AD-database except for metoprolol acid (Level 2a) and L-beta-homolysine (Level 3a). Despite only 18,677 chemicals overlapping between PubChemLite and the AD-database (Figure ), most of the features annotated in the CSF samples were within this overlap. Although both databases (PubChemLite and the AD-database) focus on the exposome and include primarily exogenous compounds (Figure ), most of the annotations by the HILIC method were categorized as endogenous (Figure ), revealing that the annotation results were not biased by the nature of the database. The RPLC method captured a lower number of endogenous chemicals (Figure ).