A DUAL IONIZATION SOURCE WITH LASER-DESORPTION SAMPLE INTRODUCTION IN A TIME-OF-FLIGHT MASS-SPECTROMETER

A new mass spectrometric detection system has been developed for the analysis of non-volatile chemical species such as biochemicals. This system involves the use of a novel dual ionization source, consisting of laser-induced electron ionization (EI) and multiphoton ionization (MPI), to provide versatile ionization for a wide range of chemicals in a time-of-flight mass spectrometer. Laser desorption is incorporated into the system for sample vaporization. The experimental design of this system is described. It is demonstrated that two ionization modes can be interchanged very readily by simply adjusting the position and focus of the laser beam. EI and MPI mass spectra of a number of amino acids and small peptides are shown to illustrate the detection capability of the system. For chemicals amenable to both ionization methods, EI and MPI can provide sensitive detection with different fragmentation patterns. Thus, the operation in a dual ionization mode provides enhanced structural information. In addition, laser-induced EI is shown to be a more general ionization method while MPI provides selective ionization with controlled fragmentation. The combination of these unique properties is demonstrated in the study of laser desorption process of an organic salt.