Patent ID: 8426805

Claim:
A method for operating a mass spectrometer having an ion volume and an electron source for supplying electrons to the ion volume, the method comprising: (a) performing a tuning or calibration of the mass spectrometer using a tuning standard or a calibration standard provided in the ion volume; (b) selecting an operational characteristic so as to be a function of an ion intensity produced by ionization of an analysis standard comprising the same or a different tuning standard or the same or a different calibration standard provided in the ion volume during a subsequent plurality of analytical runs, wherein the analysis standard is selected so as to include an internal standard that is used to evaluate a target ion having a target mass-to-charge ratio; (c) setting, in response to the tuning or calibration, one or more operating parameters that influence a flow of electrons from the electron source into the ion volume so as to establish a low nominal value for an effective emission current from the electron source into the ion volume such that ions produced from a material in the ion volume in response to electrons correspond to a user-specified fixed level of a sensitivity of the mass spectrometer; and (d) repeatedly performing the steps of: (d1) performing the plurality of analytical runs using the mass spectrometer with the electron source and using the most-recent setting or settings of the one or more operating parameters while monitoring said operational characteristic during the analytical runs, wherein the monitoring includes comparing the ion intensity of the target ion with a predefined ion intensity associated with the internal standard; and (d2) adjusting the setting or settings of the one or more operating parameters in response to the monitoring so as to increase the effective emission current so as to compensate for a degradation of the sensitivity relative to the fixed sensitivity level over time, said adjusting carried out as a function of a difference between the ion intensity of the target ion and the predefined ion intensity.