An experimental surface ionization tandem mass spectrometer has been constructed and is being used for the analyses of polar organic compounds in complex mixtures of biological origin. Phospholipids, drug metabolites and small peptides have been partially purified and analyzed by liquid surface ionization techniques. Metabolites of MPTP have characterized (see Z01MH00279-02) using crude tissue homogenates and tandem mass analysis to separate compounds of interest from background. Presently, the system is being re-configured to utilize it for spatial analysis of polar organic compounds as an organic ion microprobe. The application of the microwave interface to problems in drug metabolism has been initiated.