Parts-per-trillion detection of harmala alkaloids in Undaria pinnatifida algae by on-line solid phase extraction capillary electrophoresis mass spectrometry

Abstract

β-carboline alkaloids of the harmala group (HAlks)—a family of compounds with pharmacologic effects—can be found at trace levels (<25 μg kg−1 algae) in the edible invasive algae Undaria pinnatifida, known commonly as wakame. In this study, we present a simple and sensitive method to detect and quantify at low parts-per-trillion levels the six HAlks more frequently found in those plants. The method is based on on-line solid phase extraction capillary electrophoresis mass spectrometry using a C18 sorbent. First, the methodology was optimized and validated with standard solutions through the use of ultraviolet (UV) and mass spectrometry (MS) detection. Second, the optimized method for MS detection was applied to an analysis of the HAlks in U. pinnatifida extracts. The method achieved limits of detection between 2 and 77 pg mL−1 for standards, producing an analyte preconcentration of about 1000-times in comparison to CE-MS. Some matrix effects were observed for the complex wakame extracts, especially for the most polar HAlks (harmol and harmalol), which bear aromatic hydroxyl groups. Harmine, harmaline, and norharmane were not detected in the algal extracts, whereas harmane was found at 70 pg mL−1 (70 ng kg−1 dry algae). The results underscored that C18-SPE-CE-MS may be considered as a powerful method to detect trace levels of alkaloids and other bioactive small molecules in complex plant extracts.