A study of the analytical applicability of electron impact induced fragmentation fluorescence for nonfluorescent compounds

Fluorescence spectroscopy is a very sensitive qualitative and quantitative technique which, unfortunately, is limited to a rather small group of compounds. To overcome this problem, nonfluorescent molecules can be converted into fragments which have a higher tendency to fluoresce than their parent molecules. In this way many useful qualities of fluorescence spectroscopy can be extended to nonfluorescent compounds.

In this work, electron bombardment of gas phase molecules was used to create fragments, some of which were formed in an excited state. Fluorescence produced by those fragments, in the wavelength region between 250 and 500 nm, was collected and analyzed. Spectra of more than 30 compounds were examined and lists of the most intense bands from these compounds are presented. In addition, limits of detection, linear dynamic ranges, and relative standard deviations were collected for some of these compounds.

It was found that the precision of the method was -very good, with typical relative standard deviations of about 1%. The linearity of the method was reasonable throughout the pressure regions examined. The sensitivity of the method was only fair--highest for small compounds. Limits of detection on the order of 1 ng were obtained using photomultiplier tube detection of fluorescence. Suggestions for improving the instrumentation in such a way as to achieve improved sensitivity are discussed. Emission was obtained from all molecules which were examined, implying that this could be used as a universal detection method, unlike conventional fluorescence. It was also found that specific functional groups produce unique emissive fragments, such as OH emission from alcohols, CN emission from nitro and cyano groups and SH⁺ from thiol groups. These findings indicate that electron impact induced fragmentation fluorescence (EIFF) could be used as a group specific detector.

The selectivity of this method was not good, so a gas chromatograph was interfaced with the electron impact chamber in order to separate the components of complex mixtures. Helium was chosen as the carrier gas since it produced the least optical interference at the wavelengths being observed. Single wavelength detection at the most intense peak for most hydrocarbons, the hydrogen atom emission line at 486 nm, was performed. An optical multichannel analyzer was also used to collect "on the fly" spectra of fragment emission from the effluent.