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: November 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; 150–6000 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 supermicron
particles. At a lens pressure of 2.4 hPa, the new ALS achieves
a PSL particle size range from 230 to 3240 nm with
50 % detection efficiency and between 350 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 by single-particle mass spectrometry (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 than 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 7 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 to 2000 nm. Regarding cubic NaCl particles the
modifications of the ALABAMA result in an up to 2 times increased
detection efficiency and an up to 5 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-nucleating particles (INPs) 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 the 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 2 to 3 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 and particles shape as well as for highly
charged particles.