Abstract: An Atmospheric Pressure Ion (API) source is configured for Electrospray (ES) and Atmospheric Pressure Chemical Ionization (APCI) operating modes. The API source includes a multipurpose heater assembly mounted in the API source chamber. The multipurpose heater supplies heat to the API chamber endplate, bath gas and entrance end of the capillary orifice into vacuum as well as supplying electrical connections to the API source elements. An additional heater is configured at the exit end of the capillary orifice into vacuum. With this configuration, the bath gas, endplate and capillary entrance temperature can be set independent of gas flow rate. The capillary exit and entrance temperatures can be set independently as well. The multipurpose endplate heater combined with the capillary exit heater allows the optimization of API source performance over a wide range of operating conditions and analytical applications.