HIGH-RESOLUTION SELECTED-ION MONITORING IN A QUADRUPOLE ION-TRAP MASS-SPECTROMETER

A new method of selected ion monitoring was described that was capable of unit mass isolation throughout the operational mass range of an ion trap mass spectrometer. The trap was first selectively filled with ions via the application of a broad-band waveform to the end caps during ionization. A second isolation step completed the unit mass isolation, leaving only the desired ions in the trap. Following the accumulation and isolation steps, the intensity of the isolated ions was determined by resonantly scanning them from the trap for detection. As many as 10 different masses could be monitored by sequentially applying these steps. The ionization time for each mass was individually adjusted to optimize the filling of the trap with the selected ions; this increased the dynamic range for the set of ions. The individual spectra for each mass could then be added together to form a mass spectrum containing all specified ions.