A method of background reduction for supersonic jet spectroscopy (SJS) and multiphoton ionization mass spectrometry (MPIMS) is described. This method is based on the ion trajectory difference of the sample ions, generated from the jet-cooled molecules, and the background ions during the flight from the acceleration region to the detector in a reflectron time-of-flight mass spectrometer (TOFMS). Studies of the ionization signal profiles of the sample pulse and background indicate that the jet-cooled sample molecules have much higher linear velocities in the jet expansion axis than the background molecules. On the basis of this fact, a computer simulation on the ion trajectory in the TOFMS is performed. The simulation results suggest that it is possible to selectively detect the sample ions if a sufficient flight distance is provided. It is demonstrated that, with a reflectron TOFMS which provides a longer flight length and a less diffused sample-ion packet, the intense background signals can be significantly reduced with the use of an ion deflector and the proper adjustment of the time delay between the laser and the sample pulse. Finally, an example of the application of this method for background reduction in SJS and MPIMS with the fast atom bombardment and laser desorption techniques for sample volatization is given.
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