A charge-coupled device-based laser photofragment fluorescence spectrometer for detection of mercury compounds

The use of a charge-coupled device (CCD) camera for the detection of mercuric bromide (HgBr₂) vapor at low concentrations by laser photofragment fluorescence (PFF) spectroscopy was investigated. The CCD detection system (camera+monochromator) offers reasonable sensitivity plus spectral information, thus enhancing PFF as a technique for the environmental monitoring of airborne mercury compounds. The experiment used laser radiation at 222 nm to photolyze HgBr₂ and produce excited atomic mercury (Hg*). The PFF was monitored at 253.7 nm. Our unenhanced CCD detection limit was about 30 ppb HgBr₂ in the absence of air. The CCD response remained linear up to 20 ppm. Observed nonlinearity of the PFF signal at higher concentrations is discussed. With the same collection optics and under the same experimental conditions, the sensitivity of a photomultiplier tube (PMT) with interference filters (253.7 nm) was also investigated for comparison. The detection limit for our PMT system was 10 ppb without signal averaging, but the linear dynamic range ended at 0.7 ppm. It is expected that the CCD detection system would be more versatile for measuring metal compound species by PFF spectroscopy in any future airborne metals monitor.