Mass Spectrometric Method and Apparatus

US Patent #5,015,848

This patent describes a novel method for increasing the information content of a mass spectrum by varying the energy used to produce the ions. For the usual case of electron impact ionization, the energy can be varied just by changing the voltage of the filament relative to the ionization region.

The ion formation yield and mass pattern both depend on the ionization energy. Near threshold, yields are low and the ions have the same mass as the parent compounds. At higher energies, the yield increases and fragment ions can be observed. Changes in yield and mass pattern are characteristics of each compound present. Analyzing the resulting "two dimensional" spectrum--ion yield vs. mass and vs. energy-- results in a more reliable determination of the chemicals present in a mixture. As an demonstration of this technique, mixtures of CO and N₂ were analyzed using only the information at mass 28. Without ionization energy modulation, these compounds are indistiguishable. The addition of ionization energy modulation provided an accurate determination of the composition.

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