An atmospheric Pressure Field Effect (APFE) ionisation source for drift tube ion mobility spectrometry has been developed for operation in positive and negative polarities. The formation of negative and positive ions in synthetic air was studied and compared with the Atmospheric Pressure Corona Discharge (APCD) ionisation source. The APFE ionisation source is of point-to-plane geometry with a 10 μm Pt point electrode, a stainless steel plate electrode and ultra-high resistance (20 GΩ) current limiters. In the case of negative polarity, the ionisation source was able to generate Reactant Ions (RIs) O2-(H2O)n and O2-CO2(H2O)n, and in the case of positive polarity, stable production of H+(H2O)n RI was achieved in two different gas flow regimes of the IMS. RIs formed in the APFE in both polarities have made it a reliable chemical ionisation source at atmospheric pressure. The identification of the ions generated in the APFE was performed using an Ion Mobility Spectrometer coupled with an orthogonal acceleration Time of Flight Mass Spectrometer (IMS-oa-TOF-MS). The chemical ionisation of molecules was demonstrated for the APFE ionisation source in positive (2,6-di-tert-butyl-pyridine) and negative (tetrachloromethane) polarities.