Excimer laser fragmentation-fluorescence spectroscopy as a method for monitoring ammonium nitrate and ammonium sulfate particles

Abstract

Excimer laser fragmentation-fluorescence spectroscopy (ELFFS) is shown to be an effective detection strategy for ammonium nitrate and ammonium sulfate particles at atmospheric pressure and room temperature. Following photofragmentation of the ammonium salt particle, fluorescence of the NH fragment is observed at 336 nm. The fluorescence signal is shown to depend linearly on particle surface area for laser intensities varying from 1.2×10 8 to 6×10 8 W/cm 2. The 100 shot (1 s) detection limits for ammonium nitrate range from 20 ppb for 0.2 μm particles to 125 ppb for 0.8 μm particles, where these concentrations are expressed as moles of ammonium ion per mole of air. For ammonium sulfate, the 100 shot (1 s) detection limits vary from 60 ppb for 0.2 μm particles to 500 ppb for 1 μm particles. These detection limits are low enough to measure ammonium salt particles that form in the exhaust of combustion processes utilizing ammonia injection as a nitric oxide control strategy.