SPLAT (Single Particle Laser Ablation Time-of-flight mass spectrometer)

S. Borrmann (Uni-Mz/MPCH)
A. Wollny (FZJ)
M. Bläsner (FZJ)
T. Böttger (FZJ/MPCH)
J. Schneider (MPCH)

The chemical composition of aerosol particles determines their influence on the solar radiation and the chemical processes in the atmosphere (Borrmann et al., 1996). Its knowledge will improve the understanding of climate, cloud microphysics and chemical gas phase processes and could help to improve and verify numerical models.
A newly designed instrument currently under development determines the size and the chemical composition of individual particles by means of laser induced ablation mass spectrometry. This SingleParticle LaserAblation Time-of-flight mass spectrometer (SPLAT) is especially designed for use on research aircraft in the upper troposphere and lower stratosphere.
The SPLAT instrument introduces the ambient aerosol through an inlet system into a vacuum chamber where the individual aerosol particle passes through a permanent laser beam. The scattered light provide the size information und trigger a pulsed excimer laser. The laser pulse vaporizes the aerosol particle and ionizes the fragments. The ion cloud produced by this UV laser ablation is subsequently led to mass spectrometric analysis in two time of flight drift tubes (one each for positive and negative ions). Figure 1 shows the principle of the method.
In recent years several instruments have been built following the above described concept. At NOAA (Aeronomy Laboratory, Boulder) a compact instrument based on an aircraft is introduced by Murphy et al. (1998). The first bipolare ground-based instrument is developed by Hinz et al. (1996). SPLAT combines the advantages of these two instruments, a bipolar airborne aerosol mass spectrometer.
The first version of SPLAT is semi automated and will be operated on a research aircraft reaching up to 13 km altitude. The next stage will be a fully automated version for a single pilot aircraft.

Inlet system:
The ambient aerosol is introduced from the free flow around the aircraft through a differentially pumped inlet system into the vacuum chamber necessary for the mass spectrometry. The first stage of the inlet system is either a capillary or an aerodynamic lens (Schreiner et al., 1999).
Particle sizing:
The size of an individual aerosol particle is determined in two independent ways. The first one is to measure scattered light from the detection laser (Mie-theory), the second is to investigate the particle velocity through the vacuum chamber (aerodynamic particle sizing). The necessary dual-trace optic serves, together with an electronic control, as the triggering unit for the excimer laser.
Ionization process:
The laser pulse from the excimer hits the aerosol particle exact between the acceleration grids of the bipolar time-of-flight mass spectrometer. The deposited energy vaporizes the particle and ionizes the evaporated molecules.
Data acquisition:
The ions are detected depending on their mass-to-charge ratio in the two time-of-flight tubes. A digital oscilloscope collects the two mass spectra and sends them to the data acqusition system, a PC-104.

Recently, first gas phase measurements were made to test the properties of the time-of-flight mass spectrometer and the excimer laser. Figure 2 shows a picture of the aircraft rack with the SPLAT system. vacuum chamber connected with with one of the two time-of-flight mass spectrometers and the turbo moleculare pumps as used for these tests.
For example propanol was introduced into the vacuum chamber and ionized by the excimer laser. In Figure 3 a multiphoton ionization spectrum of propanol taken with SPLAT is shown.
 
 
 

This instrument development is financially supported by the German Helmholtz Gesellschaft der Forschungseinrichtungen through the Innovationsfonds at the Research Center Jülich. The gas phase mass spectrometer tests were made with friendly help of R. Flesch and E. Rühl from the University of Osnabrück.
 

References

Borrmann, S., S. Solomon, J.E. Dye and B. Luo, The potential of cirrus clouds for heterogeneous chlorine activation, Geophys. Res. Lett., 23, 2133-2136, 1996.

Hinz, K.-P., R. Kaufmann and B. Spengler, Simultaneous Detection of Positive and Negative Ions From Single Airborne Particles by Real-time Laser Mass Spectrometry, Aerosol Science and Technology, 24, 233-242, 1996.

Murphy, D.M., D.S. Thomson, and M.J. Mahoney, In situ measurements of organics, meteoritic material, Mercury and other elements in aerosols at 5-19 km, Science, 282, 1664-1669, 1998.

Schreiner, J., U.Schild, C.Voigt and K. Mauersberger, Focusing of Aerosols into a Particle Beam at Pressures from 10 to 150 Torr, Aerosol Science and Technology,31, 373-382, 1999.