Exploring femtosecond laser ablation in single-particle aerosol mass spectrometry

Abstract

Abstract. Size, composition, and mixing state of individual aerosol particles can be analysed in real time using singleparticle 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 (10 7 -10 10 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, SiO 2 , NH 4 NO 3 , 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 freefiring 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 (∼ 10 9 to ∼ 10 13 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 fslaser 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.

Sources and nature of ice-nucleating particles in the free troposphere at Jungfraujoch in winter 2017

“…More specifically, the different lasers lead to different power densities. ALABAMA generates a power density of 1x10 9 W cm -2 , with an assumed effective diameter of the laser at the ablation spot of the particles of about 400 µm (Clemen et al, 2020), whereas the LAAPTOF uses a power density of ~1x10 10 W cm -2 , with a laser beam diameter of 99 (31) μm at 210 the ablation spot (Ramisetty et al, 2018). The C-ToF-AMS has been described in detail in the literature (e.g., Drewnick et al 2005, Canagaratna et al, 2007.…”

Understanding atmospheric aerosol particles with improved particle identification and quantification by single-particle mass spectrometry

“…The LAAPTOF is a commercially available SPMS and has been described elsewhere (Ahern et al, 2016;Gemayel et al, 2016;Marsden et al, 2016;Ramisetty et al, 2018;Reitz et al, 2016;Shen et al, 2018;Wonaschuetz et al, 2017). In brief, aerosols are sampled with a flow rate of ∼ 80 cm 3 min −1 via an aerodynamic lens, focusing and accelerating particles in a size range between 70 and 2500 nm d va .…”

“…Due to the relatively complex laser desorption and ionization (LDI) mechanisms, including charge and proton transfer, as well as ion-molecule reactions that may occur in the plume with many collisions (Murphy, 2007;Reilly et al, 2000;Reinard and Johnston, 2008;Zenobi and Knochenmuss, 1998), some mass spectroscopic signature peaks do not necessarily reflect the primary composition of the particles. Gallavardin et al (2008) used a pair of peak area ratios, such as Ca 2 O + /Ca + vs. CaO + /Ca + and SiO − /SiO − 2 vs. SiO − 3 /SiO − 2 , to differentiate calcium-/silicon-containing mineral dust.…”

Composition and origin of PM<sub>2.5</sub> aerosol particles in the upper Rhine valley in summer

“…These instruments were used to provide real-time information on size and chemical composition for individual particles and bulk samples, respectively. The LAAPTOF is a commercially available singleparticle mass spectrometer and has been well described in recent publications (Gemayel et al, 2016;Marsden et al, 2016;Ramisetty et al, 2018;Shen et al, 2018). In brief, aerosol particles with a size range of 70 nm to 2.5 µm in vacuum aerodynamic diameter (d va ) are sampled with a flow rate of ∼ 80 cm 3 min −1 , then focused and accelerated via an aerodynamic lens (ADL).…”

“…Panels (b) to (d) are time series of vertical profiles above the measurement site for simulated SO 2 , NO x , and total particle number. C 5 H 7 NO + 3 arises from the recombination of inorganic nitrate and organics due to the laser desorption and ionization (LDI) matrix effect in single-particle MS (Murphy, 2007;Reilly et al, 2000;Reinard and Johnston, 2008;Zenobi and Knochenmuss, 1998). It is even stronger when it arises from organonitrates formed under NO x -rich conditions (refer to Fig.…”