OPTIMIZATION OF SKIMMER VOLTAGES OF AN ELECTROSPRAY ION-SOURCE COUPLED WITH A MAGNETIC-SECTOR INSTRUMENT

An electrospray ion source designed for a magnetic sector instrument usually has two skimmers, which are located behind a capillary exit with separate pumping stages to reduce the pressure from that of ambient air to a valve suitable for the sector instrument. This work describes an evaluation of these two skimmer voltages with respect to the charge-state distributions and the focusing efficiency of ions. The skimmer voltages were found to be closely related to the m/z values of the detected ions. The optimal difference voltage between skimmers 1 and 2 increased in proportion to the m/z values of the ions, irrespective of the molecular masses of the associated substances. This phenomenon is ascribable to the ion-focusing effect in this region rather than to the effects of energetic collisions in the region of the skimmers, and suggests that the skimmer voltages should be adjusted in accordance with the m/z values of the detected ions for efficient acquisition of all distributed ions. The work demonstrates the functional role of the skimmers in electrospray ionization mass spectrometry with a magnetic sector instrument by examining several types of peptides and proteins, as well as an analysis of a complex mixture of peptides.