The application of atmospheric pressure chemical ionisation liquid chromatography-mass spetrometry in the chemotaxonomic study of flavonoids: Characterisation of flavonoids from Ocimum gratissimum var. gratissimum

摘要

Liquid chromatography coupled to mass spectrometry (LC-MS), using atmospheric pressure chemical ionisation (APCI), was found to complement data obtained from diode array detection enabling common flavonoids in Ocimum gratissimum var. gratissimum to be characterised at the analytical level without complete purification, or even to be characterised in crudely purified ethanolic extracts. Xanthomicrol, cirsimaritin, rutin, kaempferol 3-O-rutinoside and vicenin-2 were identified as the major flavonoids, whereas luteolin 5-O-glucoside, luteolin 7-O-glucoside, apigenin 7-O-glucoside, vitexin, isovitexin, quercetin 3-O-glucoside and isothymusin were detected as minor constituents. FirSt-order positive ion APCI-MS provided not only the pseudomolecular ion of flavonoid O-glycosides, but also an ion corresponding to the aglycone and an intermediate ion if more than one sugar was attached to the flavonoid. Information on the type and sequence of sugars was therefore obtained without the need to undertake MS~2 product ion analyses. Product ion analysis was useful for confirming the glycoside fragmentation observed in first-order MS, especially with co-eluting compounds, and for obtaining fragmentation patterns of either free aglycones or the aglycone moiety of flavonoid O-glycosides; the latter could be used to support the aglycone identification achieved from the UV spectra. Negative ion APCI-MS was found useful for flavone C-glycosides and gave very characteristic product ions due to fragmentation of the C-sugars. For flavonoid O-glycosides analysed in the negative ion mode, there was evidence that the position and nature of the attached sugars could influence the aglycone product ion formed by MS~2 analysis and hence its product ion spectrum. APCI is considered to be a more useful ion source than electrospray in the chemotaxonomic study of flavonoids.