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Exploring femtosecond laser ablation in single-particle aerosol mass spectrometry

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 (107–1010 W cm−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 193 nm, pulse width 8 ns, pulse
energy 4 mJ), and coupled it to an fs laser (Spectra Physics Solstice-100F
ultrafast laser) with similar pulse energy but longer wavelengths (266 nm
with 100 fs and 0.2 mJ, 800 nm with 100 fs and 3.2 mJ). 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 ∼ 1013 W cm−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.
