Measurements of lead vapor and particulate in flames and post-flame gases

We describe several laser fluorescence methods used to measure lead in flames and in post-flame gases. Each relies on excitation of the 7s P-3(1) <-- 6p(2) 3P(0) transition from the ground state of lead and detection of fluorescence on the 7s P-3(1) --> 6p(2) P-3(1) and 7S P-3(1) --> P-6(2) P-3(2) transitions. Aqueous solutions of lead salts are atomized and injected into a lean flatflame. Dye laser excitation is used to detect the formation of Pb atoms at the flame front, and to obtain a concentration profile in the post flame gases. Excimer laser fragmentation-fluorescence spectroscopy (ELFFS) is used to detect molecular gas-phase and aerosol compounds in the post-flame gases. A single-shot detection limit of 220 ppb for Pb species for the single-laser ELFFS technique has been determined in post-flame gases; the sensitivity of the technique is not seriously affected by the Pb species injected into the flame. A sharp rise in ELFFS Pb signal in the 600 to 750 K region from molecular and aerosol forms of Pb is attributed to homogenous nucleation of PbO. Two-laser fragmentation-fluorescence measurements with a variable delay time between photofragmentation and Pb atom detection indicate that chemical reactions may be an important removal step for Pb at elevated temperatures. (C) 2002 by The Combustion Institute.