Detection limits for sulfates and nitrates in aerosol particles by Raman spectroscopy

Abstract

Detection limits by Raman spectroscopy are reported for sulfates and nitrates as trace constituents in aerosol particles. The aerosol particles under investigation are either single suspended solid particles with a diameter of 15 μm or present in a monodisperse particle stream generated by a vibrating orifice with a droplet diameter of 45 μm. The visible lines of a continuous wave (cw) Argon ion laser are used as the excitation source. Using the state-of-the art charge coupled device (CCD) detector, high sensitivity is achieved even for spontaneous Raman scattering. For suspended solid particles containing sodium nitrate as the trace constituent, the detection limit ( S N = 1 ) is about 4 × 10 −15 moles or 3 × 10 9 molecules. For solution droplets, the detection limit is 2.5 millimolar for both sulfates and nitrates ions in aqueous solutions. A comparison of the sensitivities between spontaneous and stimulated Raman scattering (SRS) shows that the sensitivity of the spontaneous Raman scattering is at least 20–40 times higher than that of SRS.