Liquid Matrices in MALDI-MS

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) (Karas, M. & Hillenkamp, F., 1988; Tanaka, K. et al., 1988), in addition to elecrospray ionization mass spectrometry (ESI-MS) (Fenn J. B. et al., 1989) have developed into practical analytical tools in proteomics and glycomics as both have higher throughput and sensitivity than previous mass spectrometric techniques in this area. In MALDI-MS, one benefit is the detection of mainly singly charged ions, whereas ions are detected in a multiply charged state in ESI-MS. This benefit enables easy interpretation of the mass spectra acquired and thus MALDI-MS finds particular use in mixture analysis. On the other hand, a weak point of MALDI is difficulty in selecting appropriate matrices and preparation methods for each sample. Therefore development of an analyte-specific, polarity independent matrix is still required. In MALDI the matrix is essential for ionizing the analytes that are then detected as ion peaks in the mass spectrometer. Thus the general structure of matrices has a benzene ring for absorbing N2 laser energy and hydroxyl groups to have high affinity with analytes such as peptides, proteins and carbohydrates. In addition, the matrices have some functional groups to work as a proton donor and/or a proton acceptor to help with ionizing analytes. So far, many matrices have been reported for biopolymers and synthetic polymers. The most common matrices for biopolymers are -cyano-4-hydroxycinnamic acid (CHCA) for peptides, 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid or sinapinic acid, SA) for proteins and 2,5-dihydroxybenzoic acid (DHB) for carbohydrates (Fig. 1). Typically, the matrices are dissolved in solvent and then mixed with analytes solution on MALDI sample plate. After volatilization of the solvent, solid crystals containing analytes and matrix are formed on the plate. The crystals are irradiated by the laser in mass spectrometer to ionize the analytes. On the other hand, a “liquid matrix” forms a liquid droplet containing analytes and matrix on the plate. We show here the properties of liquid matrices and differences from conventional solid matrices.