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

A new mass spectrometric detection system has been developed for the a nalysis of non-volatile chemical species such as biochemicals. This sy stem involves the use of a novel dual ionization source, consisting of laser-induced electron ionization (EI) and multiphoton ionization (MP I), to provide versatile ionization for a wide range of chemicals in a time-of-flight mass spectrometer. Laser desorption is incorporated in to 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 foc us of the laser beam. EI and MPI mass spectra of a number of amino aci ds 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 p atterns. Thus, the operation in a dual ionization mode provides enhanc ed structural information. In addition, laser-induced EI is shown to b e a more general ionization method while MPI provides selective ioniza tion with controlled fragmentation. The combination of these unique pr operties is demonstrated in the study of laser desorption process of a n organic salt.