Mass spectrometry is a technique that ionizes chemical species and then sorts them by mass. While useful, spectrometry does not distinguish between chemical isomers, species with the same number and types of atoms as another chemical species. This is important; isomers possess distinct properties because their atoms are arranged into different chemical structures. Isomers may differ, for instance, in reactivity, vapor pressure, and the identity of products. This grant will support work by Delphine Farmer, Associate Professor of Chemistry at Colorado State University, in collaboration with Ellison Carter, Assistant Professor of Civil and Environmental Engineering, to develop and test novel software for time-of-flight chemical ionization mass spectrometry that will allow researchers to identify isomers based on differences in binding energy. Funded work includes software development, calibration, and validation using both individual isomers and mixtures of isomers, and field testing in an unoccupied residence. The project will result in new software for both data acquisition and analysis, as well as field datasets, for sharing with the broader scientific community. The findings will be shared through publications from the instrument development component of the proposal, and additional publications when the instrument is used in an indoor study. The project will train at least one Ph.D. student in indoor chemistry and mass spectrometry instrument development.