Optimizing the detection, ablation and ion extraction efficiency of a single particle laser ablation mass spectrometer for application in environments with low aerosol particle concentrations
Authors: Clemen Hans-Christian , Schneider Johannes , Klimach Thomas , Helleis Frank , Köllner Franziska , Hünig Andreas , Rubach Florian , Mertes Stephan , Wex Heike , Stratmann Frank , Welti André , Kohl Rebecca , Frank Fabian , Borrmann Stephan 
Published: June 2020

Abstract. The aim of this study is to show how a newly developed aerodynamic lens system (ALS), a delayed ion extraction (DIE) and better electric shielding improve the efficiency of the Aircraft-based Laser ABlation Aerosol MAss spectrometer (ALABAMA). These improvements are applicable to single particle laser ablation mass spectrometers in general. To characterize the modifications extensive size-resolved measurements with spherical polystyrene latex particles (PSL; 150–000 nm) and cubic sodium chloride particles (NaCl; 400–1700 nm) were performed. Measurements at a fixed ALS position show an improved detectable particle size range of the new ALS compared to the previously used Liu-type ALS, especially for super-micron particles. At a lens pressure of 2.4 hPa, the new ALS achieves a PSL particle size range from 230 nm to 3240 nm with 50 % detection efficiency and between 350 nm and 2000 nm with 95 % detection efficiency. The particle beam divergence was determined by measuring the detection efficiency at variable ALS positions along the laser cross sections and found to be minimal for PSL at about 800 nm. Compared to measurements of SPMS instruments using Liu-type ALSs, the minimum particle beam divergence is shifted towards larger particle sizes. However, there are no disadvantages compared to the Liu-type lenses for particle sizes down to 200 nm. Improvements achieved by using the DIE and an additional electric shielding could be evaluated by size-resolved measurements of the hit rate, which is the ratio of laser pulses yielding a detectable amount of ions to the total number of emitted laser pulses. In particular, the hit rate for multiply charged particles smaller 500 nm is significantly improved by preventing an undesired deflection of these particles in the ion extraction field. Moreover, it was found that by using the DIE the ion yield of the ablation, ionization and ion extraction process could be increased, resulting in up to seven times higher signal intensities of the cation spectra. The enhanced ion yield results in a larger effective width of the ablation laser beam, which in turn leads to a hit rate of almost 100 % for PSL particles in the size range from 350 nm to 2000 nm. Regarding cubic NaCl particles the modifications of the ALABMA result in a up to two times increased detection efficiency and a up to five times increased hit rate. The need for such instrument modifications arises in particular for measurements of particles that are present in low number concentrations such as ice nucleation particles (INP) in general, but also aerosol particles at high altitudes or in pristine environments. Especially for these low particle number concentrations improved efficiencies help to overcome statistical limitations of single particle mass spectrometer measurements. As an example laboratory INP measurements carried out in this study show that the application of the DIE alone increases the number of INP mass spectra per time unit by a factor of two to three for the sampled substances. Overall, the combination of instrument modifications presented here resulted in an increased measurement efficiency of the ALABAMA for different particle types, particles shapes and for highly charged particles.