Near-infrared laser desorption/ionization aerosol mass spectrometry for measuring organic aerosol at atmospherically relevant aerosol mass loadings

Abstract

Abstract.A new method, near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS), is described for the real time analysis of organic aerosols at atmospherically relevant total mass loadings. Particles are sampled with an aerodynamic lens onto an aluminum probe. A moderate energy NIR laser pulse at 1064 nm is directed onto the probe to vaporize and ionize particle components. Delayed pulse extraction is then used to sample the ions into a reflectron time of flight mass spectrometer for chemical analysis. The soft ionization afforded by the NIR photons results in minimal fragmentation (loss of a hydrogen atom) producing intact pseudo-molecular anions at [M-H] − . The limit of detection measured for pure oleic acid particles (geometric mean diameter and standard deviation of 180 nm and 1.3, respectively) was 140 fg (or 1.7 ng m −3 per minute sampling time). As an example of the utility of NIR-LDI-AMS to measurements of atmospheric importance, the method was applied to laboratory chamber measurements of the secondary organic aerosol formation from ozonolysis of α-pinene. High quality mass spectra were recorded with a 2-min time resolution for total aerosol mass loadings ranging from 1.5 to 8.7 µg m −3 . These results demonstrate the potential of NIR-LDI-AMS to allow for more accurate measurements of the organic fraction of atmospheric particulate at realistic mass loadings. Measurements at ambient-levels of SOA mass loading are important to improve parameterizations of chamber-based SOA formation for modeling regional and global SOA fluxes and to aid in remediating the discrepancy between modeled and observed atmospheric total SOA production rates and concentrations.

New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

“…Geddes et al [139] described a near-infrared (IR) laser desorption ionisation method with sufficient sensitivity to produce mass spectra of OA at realistic atmospheric mass concentrations ($1 μg/m 3 ) and with good time resolution (2 min). The aerosols first are collected on an aluminum probe then desorbed/ionised with a 1064-nm laser pulse.…”

Online atmospheric pressure chemical ionization ion trap mass spectrometry (APCI-IT-MS<sup>n</sup>) for measuring organic acids in concentrated bulk aerosol – a laboratory and field study

“…Other approaches towards soft-ionization AMS are described by Dreyfus and Johnston (2008), Dreyfus et al (2009), andGeddes et al (2010). An overview article on soft-ionization mass spectrometry of atmospheric organic aerosols was recently published by Zahardis et al (2011).…”

Ozonolysis Chemistry and Phase Behavior of 1-Octen-3-ol-Derived Secondary Organic Aerosol

“…Chemical analysis of SOA was carried out using a custom built near-infrared laser desorption/ionization aerosol mass spectrometer (NIR-LDI-AMS). , In this method, the OTL-derived SOA was sampled from the chamber following ozonolysis of the previously introduced OTL standard. Sampling occurred through a Liu-type aerodynamic lens and SOA was collimated into a particle beam that was aligned to deposit aerosol mass onto the surface of an aluminum probe (99.9% purity, ESPI metals, Ashland, OR) suspended under high vacuum.…”