Results are presented on the optimization and characterization of a plasma-assisted desorption ionization (PADI) source for ambient mass spectrometry. It is found that by optimizing the geometry we can increase ion intensities for valine and by tuning the plasma power we can also select a more fragmented or less fragmented spectrum. The temperature of the surface rises linearly with plasma power: at 19 W it is 71 °C and at 28 W it is 126 °C. To understand if the changes in signal intensity are related to thermal desorption, experiments using a temperature-controlled sample stage and low plasma power settings were conducted. These show markedly different signal intensities to experiments of equivalent surface temperature but higher plasma power, proving that the mechanisms of ionization and desorption are more complicated than just thermal processes. Four different polymers, poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate) (PET), poly(lactic acid) (PLA), and poly(tetrafluoroethylene) (PTFE), are analyzed using PADI. Mass spectra are obtained from all the polymers in the negative ion mode and from PMMA and PLA in the positive ion mode. For each polymer, characteristic ions are identified showing the ability to identify materials. The ions are formed from bond cleavage with O and CH(2) as common adducts. Ions were detected up to m/z 1200 for PTFE.