Exploring femtosecond laser ablation in single particle aerosol mass spectrometry

Abstract

Keywords: nanosecond laser ablation, femtosecond laser ablation, single particle mass spectrometry, atmospheric aerosols, aerosol mass spectrometry 10 AbstractSize, 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 (10 7 W cm -2 -10 10 W cm -2 ) in nanosecond (ns) pulses at ultraviolet (UV) 15wavelengths, 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 gets 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, and systematically investigates the influence of power density and pulse duration 20 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 4 mJ, 25 respectively). Generally, mass spectra exhibit an increase in ion intensities (factor 1 to 5) with increasing laser power density (~10 8 W cm -2 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. Quantification of ablated material remains difficult due to incomplete ionization of the particle. Furthermore, the fs-laser application still suffers from limitations in 30 triggering it in a useful timeframe. Further tests are needed to test potential advantages of fs-over ns-laser ablation in SPMS.

Laser ablation aerosol particle time-of-flight mass spectrometer (LAAPTOF): performance, reference spectra and classification of atmospheric samples

“…The LAAPTOF has been described in several recent publications (Ahern et al, 2016;Gemayel et al, 2016;Marsden et al, 2016Marsden et al, , 2018Reitz et al, 2016;Wonaschuetz et al, 2017;Zawadowicz et al, 2017). Therefore, here we only briefly review the general operation steps that yield size and composition information of individual aerosol particles.…”

“…Based on the particle time of flight between the two laser beams, its d va can be determined and recorded. After detection by the second sizing laser, a nanosecond excimer laser pulse (wavelength: 193 nm; pulse duration: 4 to 8 ns; maximum pulse energy: ∼ 8 mJ; beam diameter: ∼ 300 µm when it hits the particle (Ramisetty et al, 2017); power density: ∼ 10 9 W cm −2 ; ATLEX-S, ATL Lasertechnik GmbH) can be triggered to desorb and ionize particle compounds. A laser pulse energy of 4 mJ was used for all the measurements in this study.…”

“…Online single-particle mass spectrometers (SPMSs) commonly use pulsed lasers for particle desorption and ionization (LDI), with the advantage of ionizing nearly all atmospheric particle components, including both non-refractory and refractory materials (Kulkarni et al, 2011). To the best of our knowledge, so far there is no quantitative analysis of particle composition by SPMS, since the ablation/ionization laser cannot interact with the entire particle and the generated ion fragments/clusters are susceptible to matrix effects (Ramisetty et al, 2017). In addition, ionization mechanisms are not fully understood (Murphy, 2007).…”

“…Gemayel et al (2016) presented spectra from ambient particles collected in the city centre of Marseille, France; spectra (positive only) from pure soot and SOA-coated soot particles were shown by Ahern et al (2016); spectra from potassium rich feldspar, soot, Argentinian soil dust, and Snomax (commercial ice nuclei) were shown by Zawadowicz et al (2017); PSL and potassium rich feldspar spectra were measured by Marsden et al (2018); and radiolytically formed particles spectra were measured by Wonaschuetz et al (2017). Reitz et al (2016) presented peak assignments for pure ammonium nitrate and sulfate particles, as well as for ambient particles measured at a suburban site of Düsseldorf, Germany, but did not show any spectra. Marker ions generated from SPMS are likely instrument specific, as pointed out by Schmidt et al (2017).…”

“…There exists several techniques to group the large number of individual particle types and spectra resulting from SMPS measurements -such as k-means, c-means, and hierarchical clustering algorithms; neural-network-based methods such as ART2-A; and the most recent algorithm of ordering points to identify the clustering structure (OP-TICS) -to help analyse the data (Hinz et al, 1999;Murphy et al, 2003;Reitz et al, 2016;Zelenyuk et al, 2006b;Zhao et al, 2008). There also exist target (reference spectra/predefined clusters)-oriented methods (Hinz et al, 1999;Gleanta GmbH;LAAPTOF AnalysisPro, AeroMegt GmbH).…”