Matrix‐assisted laser desorption/ionization with an external ion source Fourier‐transform mass spectrometer

Mass spectra of several model oligopeptides (substance P, [Arg⁸]‐vasopressin, tyrothricin, and the B‐chain of bovine insulin) have been obtained with a matrix‐assisted laser desorption/ionization (MALDI) source and a custom‐designed Fourier‐transform mass spectrometer (FTMS). The MALDI source is outside the magnetic field in a separately pumped external chamber, and ions are injected through the fringing fields of the magnet and into the FTMS analyzer cell by a long quadrupole mass filter that is operated in the RF‐only mode. A large window on the ion source housing makes it easy to point and focus the laser beam onto the sample probe tip. A good quality mass spectum is achieved for 0.5 pmol of substance P, and the mass resolution for the B‐chain of bovine insulin is 19000.