An apparatus for conducting mass analysis of ions comprises: (a) an ion source (1) with an orifice (2) which produces ions from a sample substance; (b) a time-of-flight mass spectrometer comprising a two-stage ion accelerator with a first stage formed by a repeller electrode (11) and mesh electrode (12) and a second by mesh electrode (12) and a mesh electrode (13) with guard electrodes (15) between, a single stage ion reflector assembly (51) with a front electrode (20) a back electrode (21) with guard electrodes (22) between, first and second drift spaces (52) and a detector (40) with a sensitive ion conversion surface (41); and (c) a means to achieve increased resolution and sensitivity by setting the potentials on the electrodes on the time of flight mass spectrometer such that longitudinal focusing of first and second order is achieved for ions of equal mass to charge value arriving at the detector surface. Also claimed are: (i) atmospheric pressure ion sources selected from an electro-spray ion source, an atmospheric pressure chemical ionisation source and an inductively coupled plasma ion source; and (ii) an apparatus in which ions are generated inside the first stage of the accelerator region and processes using the apparatus.