ABSTRACT
MS is an analytical technique that measures the mass-to-charge (m/z) ratio of charged atoms, molecules, and molecular clusters/fragments. MS is one of the fastest and most reliable methods for the high-accuracy determination of the mass of analyte particles. A typical mass spectrometer is composed of several functionally distinct components-an ion source, a mass analyzer, and a detector. Within the ion source, the analyte atoms or molecules are ionized by means of various ionization techniques; the analyzer then separates the ions on the basis of their m/z values and the detector outputs electrical signals in response to the reception of separated ions. The mass spectra can then be constructed from these signals (Fig. 1). The development of analyte ionization techniques, including recent progress in the soft-ionization technique, particularly MALDI and electrospray ionization (ESI), has opened up the possibility for the analysis of quite a wide range of molecules using a simple procedure, especially for large biopolymers such as proteins, nucleotides, and polysaccharides. MALDI, in particular, permits the analysis of solid phase samples and has therefore has been adopted in IMS from early studies mainly for biological tissue samples (Stoeckli et al. 2001). The details of this application are further discussed in a subsequent section. The MALDI process is triggered by a laser beam. An organic matrix compound is used to protect the large
biomolecules from being destroyed by direct contact with the laser beam and to facilitate vaporization and ionization. The currently derived, general versatility of MALDI was established by a momentous effort directed at the development of novel matrix compounds suitable for the ionization of various molecules of interest. This effort has, therefore, facilitated the utilization of MALDI-IMS for the visualization of a variety of molecules (Sugiura and Setou 2010a).
