Evaluating the influence of laser wavelength and detection stage geometry on optical detection efficiency in a single-particle mass spectrometer

摘要

Single-particlemass spectrometry (SPMS) is a useful tool for the onlinestudy of aerosols with the ability to measure size-resolved chemicalcomposition with a temporal resolution relevant to atmospheric processes. InSPMS, optical particle detection is used for the effective temporal alignmentof an ablation laser pulse with the presence of a particle in the ion source,and it gives the option of aerodynamic sizing by measuring the offset ofparticle arrival times between two detection stages. The efficiency of theoptical detection stage has a strong influence on the overall instrumentperformance.A custom detection laser system consisting of a high-powered fibre-coupledNd:YAG solid-state laser with a collimated beam was implemented in thedetection stage of a laser ablation aerosol particle time-of-flight (LAAP-TOF)single-particle mass spectrometer without majormodifications to instrument geometry. The use of a collimated laser beampermitted the construction of a numerical model that predicts the effects ofdetection laser wavelength, output power, beam focussing characteristics,light collection angle, particle size, and refractive index on the effectivedetection radius () of the detection laser beam. We compare the modelpredictions with an ambient data set acquired during the Ice in CloudsExperiment – Dust (ICE-D) project.The new laser system resulted in an order-of-magnitude improvement ininstrument sensitivity to spherical particles in the size range 500–800 nmcompared to a focussed 405 nm laser diode system. The model demonstrates thatthe limit of detection in terms of particle size is determined by thescattering cross section () as predicted by Mie theory. In addition,if light is collected over a narrow collection angle, oscillations in themagnitude of with respect to particle diameter result in avariation in R, resulting in large particle-size-dependent variation indetection efficiency across the particle transmission range. This detectionbias is imposed on the aerodynamic size distributions measured by theinstrument and accounts for some of the detection bias towards sea saltparticles in the ambient data set.