An Atmospheric Pressure Chemical Ionisation ion source is disclosed wherein the current applied to a corona needle 2 is repeatedly varied between two or more settings during a single experimental run or acquisition. Two or more separate sets of mass spectral or mass analysed data are obtained. The ion source in one embodiment comprises a heated tube 1 arranged to discharge gas and analyte into a housing 7 enclosing corona needle 2. The housing 7 comprises a gas outlet port 3 via which analyte ions exit. According to another embodiment the corona needle does not have to be enclosed within a housing.