Free and immobilized matrix molecules: impairing ionization by quenching secondary ion formation in laser desorption MS

A thorough evaluation of matrix-free laser desorption ionization on structurally diverse alkaloids and their direct detection in plant extracts

Alkaloids represent a major group of natural products (NPs), derived from highly diverse organisms. These structurally varied specialized metabolites are widely used for medicinal purposes and also known as toxic contaminants in agriculture and dietary supplements. While the detection of alkaloids is generally facilitated by GC- or LC-MS, these techniques do require considerable efforts in sample preparation and method optimization. Bypassing these limitations and also reducing experimental time, matrix-free laser desorption ionization (LDI) and related methods may provide an interesting alternative. As many alkaloids show close structural similarities to matrices used in matrix-assisted laser desorption ionization (MALDI), they should ionize upon simple laser irradiation without matrix support. With this in mind, the current work presents a systematic evaluation of LDI properties of a wide range of structurally diverse alkaloids. Facilitating a direct comparison between LDI and ESI-MS fragmentation, all tested compounds were further studied by electrospray ionization (ESI). Moreover, crude plant extracts of Atropa belladonna, Cinchona succirubra, and Colchicum autumnale were analyzed by LDI in order to evaluate direct alkaloid detection and dereplication from complex mixtures. Finally, dose-dependent evaluation of MALDI and LDI detection using an extract of Rosmarinus officinalis spiked with atropine, colchicine, or quinine was conducted. Overall, present results suggest that LDI provides a versatile analytical tool for analyzing structurally diverse alkaloids as single compounds and from complex mixtures. It may further serve various potential applications ranging from quality control to the screening for toxic compounds as well as the build up of MS databases. Graphical abstract.