The Use of Gas-Phase UV Spectra in the 168−330-nm Wavelength Region for Analytical Purposes. 1. Qualitative Measurements

A novel instrument, which combines gas chromatographic separation with ultraviolet detection, has been employed in the qualitative study of molecular UV absorption spectra in the gas phase. The wavelength range available with the current instrument made studies in the region of 168-330 nm possible. The absorption spectra between 168 and 330 nm for ∼1000 organic compounds and a number of inorganic gases were registered. For all the spectra recorded, 69% showed absorption maxima shorter than 190 nm, and as much as 87% shorter than 200 nm. This indicates the importance of the short UV wavelength region for analytical purposes. The results showed that each compound studied gave rise to unique absorption spectra. The influence of temperature on the shape of the spectra was studied in the range of 15-205 °C. A slight broadening effect on spectral absorption bands (0.3 nm) and the vibrational structure (maximally 1.4 nm) with increased temperature was observed. However, this effect was shown to have no influence on the analytical results. In the vast majority of cases, where structural isomers were investigated, there were clear differences between the recorded spectra. Compounds with the same functional group showed pronounced similarities. The influence of conjugation and various substituents could be demonstrated. A commercial computer program was used for a computer search of unknowns against the recorded reference spectra. This showed that the gas-phase UV spectra are very well defined and that a search always resulted in a very close agreement, even if concentrations and chromatographic conditions were different between the recording of reference and unknown. When a search was performed on a compound not present in the library, the result pointed out the correct class of compounds.