TOPIC VII "Aerosol/Laser-MS"

Single-particle mass spectrometry of aerosols

For the assessment of potential health effects and the understanding of atmospheric processes, the mixing state of aerosols is of importance. In particular, potential harmful substances can be distributed as internal mixture (low concentration on all particles) or as external mixture (single particles with high concentration and e.g. potential mutagenic activity). Single particle techniques can inherently address this issue, but are technically challenging. Key approach is the Aerosol-Time-of-Flight (ATOF) method (Hinz et al., 1994; Prather et al., 1994; Pratt & Prather, 2012). Herein, individual particles are aerodynamically accelerated into vacuum and detected via Mie-scattering in a pair of laser beams (see Fig. 1a). The flight time between both laser beams provides information on the particle size and its (real-time calculated) arrival time in the center of a bipolar mass spectrometer. Here the particle is hit by an UV laser pulse leading to desorption and ionization (LDI) of elements and molecular fragments. Their typical signatures in mass spectra allow for a classification and apportionment of the individual particles to specific sources. Our goal is the extension of this method to smaller particles below 200 nm size, which are e.g. important condensation nuclei in the atmosphere and can be transported deep into the human lung, and to bioaerosols, e.g. pollen, for future environmental monitoring.

Fig. 1 (a) ATOF-principle: Particles are aerodynamically accelerated and sized via laser velocimetry. Approaching the dual MS ion source, the multi-step pulse sequence is started: (b) The particle is heated by an IR-pulse. (c) The plume of desorbed PAHs is selectively ionized and analyzed in one MS tube. (d) Fast field inversion. (e) An UV pulse hits the particle for LDI+ of inorganic compounds being detected in the second MS tube. Modified from Passig et al., (2017). Copyright (2017) American Chemical Society.

Detection of Polycyclic Aromatic Hydrocarbons on individual particles

In order to extend the chemical classification of the ATOF-MS (see tab above) to several classes of (health-relevant) molecules our group investigates complex laser desorption and ionization schemes. In a recent experiment (Passig et al., 2017) we hit single particles with a sequence of three consecutive laser pulses of different wavelength to desorb and selectively ionize the health-relevant polycyclic aromatic hydrocarbons (PAHs) while the refractive elements from the particle core are exclusively ionized by the last, intense UV pulse (Fig. 1b-e). In order to assign the resulting ions to the respective ionization process, the extraction electrodes polarity is reversed within a few hundred nanoseconds between the laser pulses leading to an opposite acceleration of the ions into one of the respective ion flight tubes of the mass spectrometer. Our approach provides both a fully-fledged mass spectrum of (carcinogenic) PAHs in a single particle (Fig. 2, red) and the elemental composition of its core (blue). Consequently, the individual PAH-distribution of single-particles in aerosols and its assignment to specific pollution sources become accessible for the first time.

Fig. 1 (a) ATOF-principle: Particles are aerodynamically accelerated and sized via laser velocimetry. Approaching the dual MS ion source, the multi-step pulse sequence is started: (b) The particle is heated by an IR-pulse. (c) The plume of desorbed PAHs is selectively ionized and analyzed in one MS tube. (d) Fast field inversion. (e) An UV pulse hits the particle for LDI+ of inorganic compounds being detected in the second MS tube. Modified from Passig et al., (2017). Copyright (2017) American Chemical Society.

Fig. 2: Combined mass spectra of two exemplary ambient air particles: (a) Typical sea-salt particle (b) A PAH-containing particle. Combined LDI+ and REMPI information features the assessment of specific health risks, here by a high amount of (carcinogenic) PAHs. In this case, apportionment to wood or biomass burning is possible by a dominant K+ peak combined with retene (m/z=234). Modified from Passig et al., (2017). Copyright (2017) American Chemical Society.

Strong field ionization of complex gaseous mixtures

Among the ionization techniques in mass spectrometry, photoionization stands out as a very ‘soft’ method, i.e. it causes only minimal molecular fragmentation. This, and the advantage of avoiding adducts and clustering makes it very suitable for fast and direct mass spectrometry analyses of complex mixtures without pre-separation. However, suitable light sources for single-photon ionization are technically difficult and the methods sensitivity is limited. Alternatively, laser sources allow for Resonance-Enhanced Multiphoton Ionization (REMPI), being more sensitive but restricted to specific classes of molecules (e.g. aromatics). Ultrashort laser pulses (femtosecond-laser) may help to fill this gap by delivering an enormous number of photons in a very short time (Gigawatt pulse power) that allow for (non-resonant) multiphoton ionization of any molecular target (Hamachi, A. et al., 2015; Mehdi, S. et al., 2008). The disadvantage of increased fragmentation can at least partially be compensated by elevated pressure in the ion source (Peng, J. et al., 2012). In our new fs-laser lab, we will work on advanced strong field ionization schemes for analytics of trace gases in environmental- and health-relevant questions. 

 

Fig. 1: Construction of the atmospheric pressure ion source for femtosecond mass spectrometry source: Fasmatec Science and Technology SA, Greece. Image: Fasmatec, Athens

Fig. 2: Snapshot of our fs-laser system. Image: Dr. Johannes Passig

Femtosecond laser ablation and ionization of particles

Femtosecond lasers allow for precise laser ablation (Chichkov, B. N. et al., 1996) and matrix-assisted laser desorption/-ionization (MALDI) (Cui, Y. et al., 2015). In particular, its short duration minimizes structural changes and heat damage of the target. This feature, frequently exploited in laser micromachining, will be applied for desorption and ionization of aerosols in order to open new perspectives for future techniques in environmental monitoring.

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