An atmospheric pressure rf plasma source which operates in a variety of different buffer gases has been developed as an ionization method for organic samples introduced by liquid injection into atmospheric pressure ionization mass spectrometry (API/MS). The rf source can operate in He at < 1 W of load power at 165 kHz. It can also be sustained in Ar, N2, air, and CO2 at a load power of < 15 W. In most cases studied, the protonated molecule, MH+, is observed with little or no fragmentation even under the relatively high current conditions of the discharge. However, using increasingly higher acceleration voltages between the skimmers in the differentially pumped region between atmospheric pressure and high vacuum, one can induce fragmentation via collision-induced dissociation. This can be assisted in these experiments via the use of a heavy buffer gas. The detection limits achieved for rf/API plasma detection are typically in the low femtomole region for small organic molecules including neurotransmitters, PTH-amino acids, steroids, drugs, pesticides, and explosives. The detection can be performed with quantitation over at least 4 orders of magnitude.