Some organic compounds are investigated by atmospheric pressure ionization(API)mass specmetry. Organic molecules are ionized by proton or ion transfer reactions to produce many cluster ions. These ions can be reduced to quasimolecular ions by means of collisional dissociation at high pressure. Relation curves of ion intensity and sample concentration are determined. for the ion producing process. In most cases, those for organic compounds are linear in a relatively small sample size region. Ionization efficiency is so high that the detection limits is influenced only by overallion transmission efficiency.