Ionisation probabilities per unit of time, for a gas atom in the close vicinity of a supertip-gas field ion source, are analytically calculated and interpreted accordingly with the operational conditions of these devices. The results obtained for realistic values of the local electric field intensities are significantly smaller than the ones said to be expected and necessary for an "instantaneous" ionisation of a gas atom. Considerations about the effect of the local electric field and ionisation probabilities on the beam narrowness and intensity are also done when discussing their dependence on position and direction at the vicinity of the emitter.