We have developed a mass microscope (mass spectrometry imager with spatial resolution higher than the naked eye) equipped with an atmospheric pressure ion-source chamber for laser desorption/ionization (AP-LDI) and a quadrupole ion trap time-of-flight (QIT-TOF) analyzer. The optical microscope combined with the mass spectrometer permitted us to precisely determine the relevant tissue region prior to performing imaging mass spectrometry (IMS). An ultraviolet laser tightly focused with a triplet lens was used to achieve high spatial resolution. An atmospheric pressure ion-source chamber enables us to analyze fresh samples with minimal loss of intrinsic water or volatile compounds. Mass-microscopic AP-LDI imaging of freshly cut ginger rhizome sections revealed that 6-gingerol ([M + K]+at m/z 333.15, positive mode; [M - H] - at m/z 293.17, negative mode) and the monoterpene ([M + K] + at m/z 191.09), which are the compounds related to pungency and flavor, respectively, were localized in oil drop-containing organelles. AP-LDI-tandem MS/MS analyses were applied to compare authentic signals from freshly cut ginger directly with the standard reagent. Thus, our atmosphereimaging mass spectrometer enabled us to monitor a quality of plants at the organelle level.
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