Optimum phase angle for laser desorption ion trap mass spectrometry is dependent on the number of ions produced

Fur laser desorption sampling within a quadrupole ion trap, the phase and amplitude of the rf potential used to trap the ions, as well as the helium bath gas pressure, are important factors governing sensitivity. This article is concerned with investigating the dependence of trapping efficiency on the phase angle at the time that the laser fires. New data have been acquired demonstrating how the distribution of phase values that yield successful trapping, as well as the optimum phase for trapping, vary with the number of ions produced during the laser desorption event. It will also be shown that the position on the probe where the ions are created is a further factor in determining the optimum phase for trapping. Additional evidence taken from the laser desorption mass spectrometry literature will be used to propose a model for the dependence of the signal intensity versus phase relationship on the number of ions produced. It will be argued that trends in the data observed here are due to the effects of Debye shielding that accompany the desorption of substantial quantities of positive and negative ions. The dependency of the optimum phase angle on the position on the probe where the ions originate is not well understood at this time. Last, it will be shown how the pressure of helium within the trap does not influence the optimum phase value for trapping, but the effects of the bath gas pressure on trapping efficiency and fragmentation are interesting and will be discussed briefly. (Int J Mass Spectrom 190/191 (1999) 59-80) (C) 1999 Elsevier Science B.V.