Quantitative Detection of Gas-Phase NaOH Using 355-nm Multiple-Photon Absorption and Photofragment Fluorescence

Abstract

The third-harmonic output of a Nd:YAG laser is utilized in the quantitative detection of gas-phase NaOH. Species selectivity and sensitivity are achieved via multiple-photon absorption and photo-dissociation of NaOH—in combination with monitoring of the subsequent Na photofragment fluorescence. Detection limits of ∼0.5 ppm of gas-phase NaOH have been achieved under atmospheric conditions. The fluorescence intensity has been found to be proportional to the square of the laser power, which is consistent with a second-order absorption process. Although the technique is less sensitive than the more widely used excimer laser-induced photofragment fluorescence approach, there are many technical advantages which make it more suitable for use in modern coal combustion systems. These combustion systems require accurate alkali measurement and control to combat fouling and corrosion of process surfaces and turbine blades.