A highly sensitive ultra-high performance liquid chromatography/tandem mass spectrometry method with in-source fragmentation for rapid quantification of raspberry ketone

Raspberry ketone (RK) is the characteristic aromatic compound in raspberry (Rubus idaeus L.) with wide applications as food additive and anti-obesity agent. However, quantification of RK has presented difficulties in MS detection and reliable LC-MS method for RK analysis in literature is in limit to date. In order to facilitate quality control of raspberry derived products and RK metabolomics study, this study aimed to develop a validated and sensitive UHPLC-MS/MS method. Strong in-source fragmentation was noted and the fragmental ion of 107 m/z produced was selected as the precursor ion for MRM detection, and as such the electrospray ionization performance was optimized by fractional factorial design to accommodate such ion-source dissociation behavior as well as its moderate volatility. A pathway involving the formation of quinone-like structure with strong conjugation was proposed to explain the intense in-source fragmentation. The MRM transition was optimized with product ion of 77 m/z selected as the quantifier ion. The method featured low limit of quantification of ～2 ng/mL and allowed for rapid detection of RK in fresh raspberries following direct sample preparation. RK contents were found to be higher from locally grown and harvested farm sources compared to commercial products shipped into the state, and higher in those at late-stage compared with early-stage maturity. No correlations in RK content between organic and non-organic labels were noted.