Development and characterization of a single particle laser ablation mass spectrometer (SPLAM) for organic aerosol studies

[摘要] A single particle instrument was developed for real-time analysis of organicaerosol. This instrument, named Single Particle Laser Ablation MassSpectrometry (SPLAM), samples particles using an aerodynamic lens system forwhich the theoretical performances were calculated. At the outlet of thissystem, particle detection and sizing are realized by using two continuousdiode lasers operating at λ = 403 nm. Polystyrene Latex (PSL),sodium chloride (NaCl) and dioctylphtalate (DOP) particles were used tocharacterize and calibrate optical detection of SPLAM. The optical detectionlimit (DL) and detection efficiency (DE) were determined using size-selectedDOP particles. The DE ranges from 0.1 to 90% for 100 and 350 nm DOPparticles respectively and the SPLAM instrument is able to detect andsize-resolve particles as small as 110–120 nm. During optical detection,particle scattered light from the two diode lasers, is detected by twophotomultipliers and the detected signals are used to trigger UV excimerlaser (λ = 248 nm) used for one-step laser desorption ionization(LDI) of individual aerosol particles. The formed ions are analyzed by a 1 mlinear time-of-flight mass spectrometer in order to access to the chemicalcomposition of individual particles. The TOF-MS detection limit for gaseousaromatic compounds was determined to be 0.85 × 10⁻¹⁵ kg(∼4 × 10³ molecules). DOP particles were also used to test the overall operation ofthe instrument. The analysis of a secondary organic aerosol, formed in asmog chamber by the ozonolysis of indene, is presented as a firstapplication of the instrument. Single particle mass spectra were obtainedwith an effective hit rate of 8%. Some of these mass spectra were foundto be very different from one particle to another possibly reflectingchemical differences within the investigated indene SOA particles. Our studyshows that an exhaustive statistical analysis, over hundreds of particles,and adapted reference mass spectra are further needed to understand thechemical meaning of single particle mass spectra of chemically complexsubmicrometer-sized organic aerosols.

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