Aerosol mass spectrometry

Aerosols play a significant role in the earth climate, evoke substantial disease burden and environmental effects. A current WHO study attributes 7 million premature deaths per year to air pollution with particulate matter [1]. The aerosols physical and chemical complexity as well as permanent changes by the surrounding atmosphere and solar radiation (aging) complicates impact assessments. For sampled aerosols, detailed chemical analyses are possible but they can neither acquire its high temporal variability nor the physico-chemical structure of a higher number of individual particles. Our research focuses on novel real-time techniques for physical and chemical characterization of single particles in ambient aerosols.

Key approach is the Aerosol-Time-of-Flight (ATOF) method [2,3,4]. 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.

In order to extend the chemical classification to several classes of (health-relevant) molecules and to smaller particle sizes our group investigates complex laser desorption and ionization schemes. In a recent experiment [5] 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 [5]
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). [5]
References

References

[1] Ambient air pollution: A global assessment of exposure and burden of desease; World Health Organization, Report, 2016.

[2] Hinz, K. P., Kaufmann, R., and Spengler, B. Anal. Chem. 1994, 66:2071-2076.

[3] Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403–1407.

[4] Pratt, K. A.; Prather, K. A. Mass Spectrom. Rev. 2012, 31, 17–48.

[5] Passig, J., Schade, J. and Zimmermann, R. Anal. Chem. 2017 89 (12), 6341-6345.