ent-Kaurenoic acid (KA) is a key intermediate connected to a phytohormone gibberellin. To date, the general procedure for quantifying KA is by using traditional gas chromatography-mass spectrometry (GC-MS). In contrast, gibberellins, which are more hydrophilic than KA, can be easily quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In this study, we have established a new method to quantify KA by LC-MS/MS by taking advantage of a key feature of KA, namely the lack of fragmentation that occurs in MS/MS when electrospray ionization (ESI) is in the negative mode. Q1 and Q3 were adopted as identical channels for the multiple reaction monitoring of KA. The method was validated by comparing with the results obtained by selected ion monitoring in GC-MS. This new method could be applicable for the quantification of other hydrophobic compounds.
Keywords: ESI, electrospray ionization; GA, gibberellin; GC, gas chromatography; Gibberellin; KA, ent-kaurenoic acid; KAO, ent-kaurenoic acid oxidase; KO, ent-kaurene oxidase; LC, liquid chromatography; LC–MS/MS; LOD, limit of detection; LOQ, limit of quantitation; MRM; MRM, multiple reaction monitoring; MS/MS, tandem mass spectrometry; Moss; Phytoalexin; SIM, selected ion monitoring; ent-Kaurenoic acid.