Mass spectrometry based metabolomics of volume-restricted in-vivo brain samples: Actual status and the way forward

Abstract Brain metabolomics is gaining interest because of the aging of the population, resulting in more central nervous system disorders such as Alzheimer's and Parkinson's disease. Most often these diseases are studied in vivo, such as for example by analysing cerebrospinal fluid or brain extracellular fluid. These sample types are often considered in pre-clinical studies using animal models. However, the scarce availability of both matrices results in some challenges related to sampling, sample preparation and normalization. Much effort has been made towards the development of alternative, less invasive sampling techniques for collecting small sample volumes (pL till mid μL range) over the past years. Despite recent advances, the analysis of low volumes is still a tremendous challenge. Therefore, proper preconcentration and sample pretreatment strategies are necessary together with sensitive analysis and detection techniques suitable for low-volume samples. In this review, an overview is given of the state-of-the-art mass spectrometry-based analytical workflows for probing (endogenous) metabolites in volume-restricted in-vivo brain samples. In this context, special attention is devoted to challenges related to sampling, sample preparation and preconcentration strategies. Finally, some general conclusions and perspectives are provided.