Setting the Resolution of a Sector Instrument

Example The influence of relative slit width on peak shape and resolution is demonstrated on the second isotopic peak of toluene molecular ion, m/z 94 (Fig. 4.25). With the entrance slit at 50 pm and the exit slit at 500 pm the peak is flat-topped (left), because a narrow beam from the entrance sweeps over the wide open detector slit keeping the intensity constant as the scan proceeds until the beam passes over the other edge of the slit. Closing the exit slit to 100 pm increases resolution to 2000 without affecting the peak height (middle), but reduces the peak area by a factor of 4 in accordance with an increase in resolution by the same factor. Further reduction of the exit slit width to 30 pm improves 

Note The ultimate resolution of a magnetic sector mass spectrometer is reached when the slits are closed to a width of a few micrometers. Often, the slit height is also reduced, e.g., from 5 to 1 mm In daily work, the resolution will be set to fit the actual task, e.g., R = 1000-2000 for low resolution work, R = 3000-5000 if accurate mass determination at high scan rates is needed (GC-MS, Chap. 12) or isotopic patterns of high mass analytes have to be resolved, or R = 7000-15,000 in slow-scanning accurate mass measurements. 

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