Anal. Chem. 2009, 81, 9153–9157
Visualization of Volatile Substances in Different Organelles with an Atmospheric-Pressure Mass Microscope Takahiro Harada,† Akiko Yuba-Kubo,‡ Yuki Sugiura,§,| Nobuhiro Zaima,§ Takahiro Hayasaka,§ Naoko Goto-Inoue,§ Masatoshi Wakui,‡ Makoto Suematsu,‡ Kengo Takeshita,† Kiyoshi Ogawa,† Yoshikazu Yoshida,† and Mitsutoshi Setou*,§ Technology Research Laboratory, Shimadzu Corporation, 3-9-4 Soraku-gun, Seika-cho, Kyoto 619-0237, Japan, Department of Biochemistry and Integrative Medical Biology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan, Department of Molecular Anatomy, Molecular Imaging Frontier Research Center, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashi-ku, Hamamatsu, Shizuoka 431-3192, Japan, and Department of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama, Kanagawa 226-8501, Japan 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. Imaging mass spectrometry (IMS) is used to visualize the distribution of multiple biomolecules in tissue sections. IMS has exceptional advantages with regard to the imaging of metabolites in tissues and cells. First, IMS does not require any labels or specific probes. Second, IMS is a nontargeted imaging method. Thus, unexpected metabolites can be imaged. Finally, many types of metabolites can be simultaneously imaged. These benefits are pertinent to metabolite imaging. Most of the systems utilize the * Corresponding author. Tel/Fax: +81-53-435-2292; E-mail: setou@ hama-med.ac.jp. † Shimadzu Corporation. ‡ Keio University. § Hamamatsu University School of Medicine. | Tokyo Institute of Technology. 10.1021/ac901872n CCC: $40.75 2009 American Chemical Society Published on Web 09/29/2009
scanning mode, but some use the stigmatic mode with truly “microscope” ion optically.9,10 This technique was initially applied to large proteins due to its ability to detect high-molecular-weight compounds. Further, there has recently been an increase in research regarding MALDI-imaging of small organic molecules.11-14 All of the above-mentioned features of IMS are especially useful for localizing the metabolites in plants, which may be very diverse. IMS coupled with matrix-assisted laser desorption/ionization (MALDI) has been used to visualize the distribution of various biomolecules including small metabolite molecules1-5 and much larger5-8 proteins in cells and tissues. Despite its clear promise, MALDI-imaging of small metabolites is still a challenging prospect. A critical problem is that the lowm/z region (
