Charge-Number Discrimination Mass Analysis Using Superconducting Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometer -Analysis of Fragment Impurities in Immunoglobulin G

Conventional time-of-flight mass spectrometers equipped with microchannel plate (MCP) detectors give only m/z values, which remains ambiguous over charge number (z). In contrast, the matrix-assisted laser desorption/ionization (MALDI) TOF mass spectrometer (TOF-MS) with a superconducting tunnel junction (STJ) detector—Super-TOF-MS—enables kinetic energy measurement with mass-independent detection efficiency. Therefore, since kinetic energy is proportional to z, it is possible to determine mass (m) unambiguously.The Super-TOF-MS is effective for analysing high-mass fragments in immunoglobulin solution. Specifically, unknown peaks appearing on mass spectra for immunoglobulin G (IgG) samples can be identified as IgG fragments using kinetic energy measurements and the mass-independent detection efficiency of the STJ detector. A complete IgG molecule has two heavy and two light chains that are connected by disulfide bonds. The heavy and light chains include four and two structural domains, respectively (12 domains in total). The charge-state discrimination mass analysis enables the assignment of m values to the unknown peaks. The existence of pairs for which the m-value sum is equal to that of the intact IgG reveals that there are several fragment types consisting of different numbers of structural domain units.