PHASE I MASS SPECTROMETRY -- COLD ELECTRON EMITTER

A NON-REACTIVE PHOTOCATHODE, BACK-ILLUMINATED BY VACUUM ULTRAVIOLET (VUV) LIGHT, IS PROPOSED AS A KEY ELEMENT IN A MASS SPECTROMETER IONIZATION SOURCE IN RESPONSE TO NIMH TOPIC 2A. USING THIS ELEMENT BOTH ELECTRON IMPACT AND PHOTOIONIZATION CAN BE EMPLOYED AND MULTIPLEXED TO PROVIDE TWO "SIMULTANEOUS" MASS SPECTRA. IN ADDITION, IONS SORBED ON THE PHOTOCATHODE CAN BE IONIZED OR CAN ATTACH ELECTRONS TO FORM NEGATIVE IONS. THESE IONS CAN BE DESCRIBED WITH MODEST ELECTRIC FIELDS IF EITHER A THIN 100A DIELECTRIC LAYER OVERCOATS THE PHOTOCATHODE OR THE ENTIRE PHOTOCATHODE IS A DIELECTRIC PHOTOCONDUCTOR. BY THIS MEANS LARGE BIOLOGICALLY IMPORTANT MOLECULES NEED NEVER BE VOLITALIZED TO BE IONIZED AND INJECTED INTO A MASS SPECTROMETER. THE CHARACTERISTICS OF THE PHOTOCATHODE (FLUX, ENERGY DISTRIBUTION, DESORPTION ENERGY) WILL BE STUDIED IN DETAIL AS A FUNCTION OF PHOTOCATHODE MATERIAL AND ITS THICKNESS BY INCREMENTAL EVAPORATION ONTO A MGF(2) WINDOW OF A VACUUM ULTRAVIOLET SOURCE IN A HIGH-VACUUM EVAPORATOR, BY MEASURINGEMISSION CURRENTS AND ARRIVAL TIMES TO A SIMPLE, GRID SHIELDED, PLANAR ELECTRODE. A CYLINDRICALLY SHAPED PHOTOCATHODE WILL ALSO BE INVESTIGATED AND USED WITH A QUADRAPOLE MASS SPECTROMETER TO PROVIDE DIRECT COMPARISON OF SENSITIVITY AND EASE OF USE RELATIVE TO COMMERCIAL HOT FILAMENT IONIZATION SOURCES. IT IS EXPECTED THAT THESE FORMS OF MASS SPECTROMETER IONIZATION SOURCES UTILIZING VUV RADIATION WILL BE AS EFFICIENT AS HEATED FILAMENT ION SOURCES, BUT MUCH MORE VERSATILE. IN PARTICULAR, THE SURFACE PHOTO-ION SOURCE SHOULD BE VERY SUITABLE FOR TIME- OF-FLIGHT MASS SPECTROMETERS. MASS SPECTROMETERS UTILIZING THE PROPOSED ION SOURCE WOULD BE SUPERIOR EXPLOSIVE DETECTORS, CHEMICAL AGENT DETECTORS, ENVIRONMENTAL MONITORS,AND GENERAL ANALYTICAL INSTRUMENTS FOR BIOLOGICALLY IMPORTANT SPECIES. THE COMMERCIAL MARKET FOR SUCH DEVICES IS IN EXCESS OF $100 MILLION/YEAR.