A mass spectrometer having a thermospray ion source, suitable for the analysis of liquid samples is disclosed. The source comprises a strongly heated atomizing nozzle (4) through which the sample is pumped into a spray chamber (5, 11), and an aperture in a cone (26), through which the ions pass through an electrostatic lens (15, 16, 17) into a region (23) in which a mass analyzer is situated. Electrode means (27, 45) generate a repulsive electrostatic field which improves the efficiency of the transmission of ions through the hole in the cone (26). The electrode is situated downstream of the hole in cone (26). By further increasing the potential on the electrode (27, 45) the nature of the ions passing through the hole in the cone (26) can be changed from largely adduct and cluster ions formed by association of solvent molecules with sample ions in the thermospray to largely pseudomolecular sample ions similar to those formed by conventional chemical ionization.