Online extraction-LC-MS/MS is an alternative imaging tool for spatial-resolved metabolomics: Mint leaf as a pilot study.

An attempt was made here to a complemental analytical tool for classical MSI approach. OLE-LC-MS/MS imaging was proposed to plot the spatial-resolved metabolome through deploying mint leaf as a proof-of-concept. A dried leaf underwent chemical composition characterization using OLE-LC-Qtof-MS. Another dried leaf was cut into small pieces, and all pieces were successively packed into a suitable cartridge to undergo OLE-LC-SRM measurements. Fifty-two compounds were observed and identified. Special attention was paid onto isomeric identification using fragment ion intensity ranking style, e.g., 3-O-caffeoylquinic acid vs. 4-O-caffeoylquinic acid. Thereof, 23 abundant ones were involved for relatively quantitative analysis. Quantitative settings were optimized using online ER-MS program. Following spatial metabolome imaging, regioselective distributions were observed for most concerned metabolites. Particularly, isomer-specific occurrences were observed for luteolin-7-O-glucuronide and luteolin-3'-O-glucuronide. Together, OLE-LC-MS/MS is alternative for spatial metabolome imaging due to the advantages at isomeric separation, identification confidence, and quantitative accuracy.