Characterization of trace metals with the SP-AMS: detection and quantification

Abstract. A method to detect and quantify mass concentrations of metals by the Aerodyne Soot Particle – Aerosol Mass Spectrometer (SP-AMS) was developed and evaluated in this study. The generation of monodisperse Regal black (RB) test particles with trace amounts of 13 different metals (Na, Al, Ca, V, Cr, Fe, Mn, Ni, Cu, Zn, Rb, Sr and Ba) allowed the determination of the relative ionization efficiency of each metal (RIEmeas). The ratio RIEtheory / RIEmeas presented values larger than the unity for Na, Rb, Ca, Sr and Ba due to the thermal surface ionization (TSI) on the surface of the RB particles. Values closer to the unity were obtained for the transition metals Zn, Cu, V and Cr. Mn, Fe and Ni presented the lowest RIEtheory / RIEmeas ratio and highest deviation from the unity, which was most likely related to different losses. The RIEmeas values obtained in this study were applied to the data of emission measurements in a heavy fuel oil fired heating station. Emission measurements revealed various fragmentation patterns for sulfate, probably because sulfate was mainly in the form of metallic salts (vanadium sulfate, calcium sulfate, iron sulfate and barium sulfate), which were also identified in the high-resolution mass spectrum. The response of the metals to the laser power was also investigated and the results indicated that a minimum current of 0.6 A was needed in the laser in order to vaporize the metals and the rBC. Isotopic pattern of metals was resolved from high-resolution mass spectra and the mass size distribution information of each individual ion was obtained using the high-resolution particle time-of-flight (HR-PToF).