The MS can operate in either the positive or negative ion polarity mode. You can obtain positive-ion scans, negative-ion scans, or when using polarity switching, a combination of positive and negative ion scans. The MS controls whether positive ions or negative ions are transmitted to the mass analyzer by changing the polarity of the voltage potentials applied to the API source, ion optics, and ion detection system. For a schematic of the ion transmission path, see Ion Optics.
The information obtained from a positive-ion mass spectrum is different from, and complementary to, the information from a negative-ion spectrum. Therefore, the ability to obtain both positive-ion and negative-ion mass spectra aids you in the qualitative analysis of your sample. You can choose the ion polarity mode and ionization mode to obtain maximum sensitivity for the particular analyte of interest.