Atmospheric Pressure Chemical Ionization (APCI) and electrospray ionization sources for the mass spectrometric analysis of solutions, and associated methods. The apparatus and methods are characterised in that ions generated by APCI or electrospray are directed such that their directions of travel immediately on formation can be resolved into two perpendicular components, one of which is aligned with a linear first trajectory which passes through an entrance orifice, an extraction chamber and into an evacuation port through which the extraction chamber is evacuated. The direction of travel is such that the component of velocity so aligned is smaller than the component perpendicular to it. Ions leave the chamber along a second trajectory which is inclined at an angle between 30.degree. and 150.degree. to the linear first trajectory and may pass into a mass analyzer. The apparatus and method provide improved sensitivity and a lower noise level in comparison with prior apparatus and methods using APCI and electrospray ionization sources.