Molecular fluorescence spectroscopy of phosphorus monoxide in flames studied by a SIT-OMA system

Molecular fluorescence of PO produced in flames and excited by an Eimac xenon arc continuum source has been investigated. An SIT image detector combined with an optical multichannel analyzer (OMA) was used as the detector. Molecular fluorescence of PO could be observed near 246 nm in the air—acetylene, air—hydrogen, oxygen/argon—hydrogen, and the corresponding argon-separated flames, while PO fluorescence enhanced by molecular emission was observed in the nitrous oxide—acetylene flame and in the argon-separated nitrous oxide—acetylene flame. The fluorescence spectrum of PO near 246 nm is identical to the emission and absorption spectra of PO and is ascribed to the γ system, which is caused by the electronic transition A 2 Σ- X 2 Π. PO emission observed near 324 nm, a result of the transition D 2 Σ X 2 Π, was very weak in the present experiment and was not accompanied by measurable quantities of fluorescence. The fluorescence intensity of PO depends primarily upon the flame temperature, and so the quenching effect of nitrogen on the fluorescence intensity could not be observed. The best detection limit for phosphorus was obtained in the argon separated air—hydrogen flame, which generated the least amount of flame background noise, and was about 80 μg P/ml. Finally, because phosphine (PH 3 ) is an impurity in commercial acetylene, PO fluorescence bands in the 230–270 nm region may obscure the atomic fluorescence signals in this region excited via continuum sources and thus deleteriously affect atomic fluorescence signals in certain analyses.