In-Source Fragmentation for the Identification of Compounds by CE-ESI-TOF in Human Plasma. L-Proline as Case Study

One of the aims of untargeted metabolomics is searching for selective biomarkers of different pathophysiological conditions. Modified amino acids originated from the posttranslational modification of proteins play a key role as potential biomarkers; however, they are very often still classified as unknown after metabolite annotation. We have developed an analytical workflow for the targeted screening of these compounds using CE-ESI-MS. The workflow is based on the in-source fragmentation of molecules that produces diagnostic ions that we have collected in an open-source library. In this chapter, we describe in detail the strategy for the targeted screening of modified amino acids (MAAs), using as an example L-proline and its modified derivatives. We illustrate the strategy with two case studies in human plasma.