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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 7 | Copyright
Atmos. Meas. Tech., 11, 4345-4360, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Jul 2018

Research article | 23 Jul 2018

Exploring femtosecond laser ablation in single-particle aerosol mass spectrometry

Ramakrishna Ramisetty1, Ahmed Abdelmonem1, Xiaoli Shen1, Harald Saathoff1, Thomas Leisner1, and Claudia Mohr1,2 Ramakrishna Ramisetty et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden

Abstract. Size, composition, and mixing state of individual aerosol particles can be analysed in real time using single-particle mass spectrometry (SPMS). In SPMS, laser ablation is the most widely used method for desorption and ionization of particle components, often realizing both in one single step. Excimer lasers are well suited for this task due to their relatively high power density (107–1010Wcm−2) in nanosecond (ns) pulses at ultraviolet (UV) wavelengths and short triggering times. However, varying particle optical properties and matrix effects make a quantitative interpretation of this analytical approach challenging. In atmospheric SPMS applications, this influences both the mass fraction of an individual particle that is ablated, as well as the resulting mass spectral fragmentation pattern of the ablated material. The present study explores the use of shorter (femtosecond, fs) laser pulses for atmospheric SPMS. Its objective is to assess whether the higher laser power density of the fs laser leads to a more complete ionization of the entire particle and higher ion signal and thus improvement in the quantitative abilities of SPMS. We systematically investigate the influence of power density and pulse duration on airborne particle (polystyrene latex, SiO2, NH4NO3, NaCl, and custom-made core-shell particles) ablation and reproducibility of mass spectral signatures. We used a laser ablation aerosol time-of-flight single-particle mass spectrometer (LAAPTOF, AeroMegt GmbH), originally equipped with an excimer laser (wavelength 193nm, pulse width 8ns, pulse energy 4mJ), and coupled it to an fs laser (Spectra Physics Solstice-100F ultrafast laser) with similar pulse energy but longer wavelengths (266nm with 100fs and 0.2mJ, 800nm with 100fs and 3.2mJ). We successfully coupled the free-firing fs laser with the single-particle mass spectrometer employing the fs laser light scattered by the particle to trigger mass spectra acquisition. Generally, mass spectra exhibit an increase in ion intensities (factor 1 to 5) with increasing laser power density (∼109 to ∼1013Wcm−2) from ns to fs laser. At the same time, fs-laser ablation produces spectra with larger ion fragments and ion clusters as well as clusters with oxygen, which does not render spectra interpretation more simple compared to ns-laser ablation. The idea that the higher power density of the fs laser leads to a more complete particle ablation and ionization could not be substantiated in this study. Quantification of ablated material remains difficult due to incomplete ionization of the particle. Furthermore, the fs-laser application still suffers from limitations in triggering it in a useful time frame. Further studies are needed to test potential advantages of fs- over ns-laser ablation in SPMS.

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Short summary
In this study we coupled a laser ablation aerosol time-of-flight (LAAPTOF) single-particle mass spectrometer, originally equipped with an excimer laser, to a femtosecond laser. The objective was to assess the influence of the higher laser power density of the femtosecond laser on ablation–ionization of atmospheric particles, ion signal, and ultimately quantitative abilities of the single-particle mass spectrometer.
In this study we coupled a laser ablation aerosol time-of-flight (LAAPTOF) single-particle mass...