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

Nicholas Marsden, Michael J. Flynn, Jonathan W. Taylor, James D. Allan, Hugh Coe
2016 Atmospheric Measurement Techniques Discussions  
Single particle mass spectrometry (SPMS) is a useful tool for the on-line study of aerosols with the ability to measure size resolved chemical composition with a temporal resolution relevant to atmospheric processes. In SPMS, optical particle detection is used for the effective temporal alignment of an ablation laser pulse with the presence of a particle in the ion source, and gives the option for aerodynamic sizing by measuring the offset of particle arrival times between two detection stages.
more » ... o detection stages. The efficiency of the optical detection stage has a strong influence on the overall instrument performance. </br></br> A custom detection laser system consisting of a high powered fibre coupled Nd:YAG solid state laser with a collimated beam was implemented in the detection stage of a LAAP-TOF single particle mass spectrometer without major modifications to instrument geometry. The use of a collimated laser beam permitted the construction of a numerical model that predicts the effects of detection laser wavelength, output power, beam focussing characteristics, light collection angle, particle size and refractive index on the active detection volume in the detection stage. We compare the model predictions with an ambient data set acquired during the Ice in Clouds Experiment - Dust (ICE-D) project. </br></br> The new laser system resulted in an order of magnitude improvement in instrument sensitivity to spherical particles in the size range 500&ndash;800&thinsp;nm compared to a focussed 405&thinsp;nm laser diode system. The model demonstrates that the limit of detection in terms of particle size is determined by the scattering cross section (<i>C</i><sub>sca</sub>) as predicted by Mie theory. In addition, if light is collected over a narrow collection angle, oscillations in the magnitude of <i>C</i><sub>sca</sub> with respect to particle diameter result in a variation in the active detection volume, resulting in large particle size dependent variation in detection efficiency across the particle transmission range. This detection bias is imposed on the aerodynamic size distributions measured by the instrument and accounts for a detection bias towards sea salt particles in the ambient data set.
doi:10.5194/amt-2016-150 fatcat:dcffqz57xjbcxomjvgt3h275zy