Molecular Emission Spectrohetry ih a Low Pressure Electrical Discharge

A low pressure electrical discharge for excitation of polycyclic aromatic hydrocarbons, PAHs, without fragmentation is described. The discharge characteristics as well as the plasma background spectra are given. The application of the discharge for the excitation of molecular spectra of PAHs is described. Difficulties with correction of the analyte emission spectrum for the background emission spectrum of the gas (He) are mentioned. Analytical use of the discharge for detection of PAH3 introduced by gas chromatography is described and is shown to be of marginal analytical use because of the rather poor detection limits of about 1 μg.

Jurgensen and Winefordner¹ have reviewed both reduced pressure and atmospheric pressure nitrogen discharges for detection of atomic and molecular species. The same authors² have also described an active nitrogen-induced chemiluminescence system for detection of polycyclic aromatic hydrocarbons, PAHs, and achieved detection limits in the range of 0.01 - 2000 ng. The intriguing aspect of the active nitrogen—induced chemiluminescence approach² for detection of PAHs was that the discharge was gentle resulting in little fragmentation of the PAHs and in the appearance of emission spectra characteristic of the parent molecules. Because of the poor understanding of the mechanism of excitation of the large organic molecules and the minimal fragmentation and because of the lack of control of the direct discharge as opposed to the afterglow in the studies of Jurgensen and Winefordner², the present authors began a careful study of the physical and analytical characteristics of this discharge in the fall of 1984. The physical characteristics were reported by Yu and Winefordner³ and indicated that the previous workers², were using the direct discharge plasma as a result of llpoorlv grounding. In addition, the same workers³ showed that the active nitrogen afterglow was not capable of exciting PAHs. In this note, we report on the analytical capabilities of an improved active gas (nitrogen or helium) electrical discharge system for gaseous PAHs.