The interaction of low-energy electrons with electrophilic molecules in the production of negative ions is an important chemical process. Electron capture negative ion mass spectrometry (ECNI-MS) is a powerful system used for the analysis of very small amounts of such compounds, but in its present forms it can provide only limited information on the processes that produce negative ions. In this application we propose to construct a versatile gas chromatography/electron monochromator-time-of- flight mass spectrometer (GC/EM-TOF-MS). This instrument will be used to elucidate mechanisms of negative ion formation and decomposition via metastable ions. The electron monochromator (EM) to be used generates monoenergetic electron beams with currents of approximately 400 microamps tunable in the range 0- 15eV with maximum energy spreads of O.leV. The electron source when interfaced to the time-of-flight (TOF) analyzer will yield reproducible 3-dimensional spectra whose axes are resonance electron energy mass to charge, and ion signal intensity. The gas chromatographici nlet system, will provide a means to resolve compound mixtures and a stable supply of buffer gas to stabilize resonance electron capture ions. A reflectron in the analyzer will provide the capability to study metastable ions. Experimental resonance electron energies, when correlated with the virtual orbital energies of the molecules calculated by density functional theory methods, will provide information on the orbitals involved during initial ion formation. Electron attachment to several classes of electrophilic compounds including organophosphates, sulfonamides, dinitro [2,2]para- cyclophane, aliphatic ketones, haloethenes, vinyl chloride and chloroacetaldehyde will be investigated. In addition to providing a unique system for studying the mechanistic details of negative ion formation, the sensitivity, specificity, extra dimension of analytical information from resonance electron energies and "fast" GC/MS capability of the proposed GC/EM-TOF MS instrument will make it a valuable new tool for analyzing environmental health-related chemicals, biochemical and biomedical-related compounds that can be derivatized with electrophores. The analysis of proteins, peptides and other biopolymers with electron-capturing posttranslational modifications is another potentially exciting application for the GC/EM-TOF-MS system.