Spatially Shaped Laser Pulses for the Simultaneous Detection of Polycyclic Aromatic Hydrocarbons as well as Positive and Negative Inorganic Ions in Single Particle Mass Spectrometry

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are toxic organic trace components in atmospheric aerosols that have impacts on climate and human health. They are bound to airborne particles and transported over long distances. Observations of their distribution, transport pathways, and degradation are crucial for risk assessment and mitigation. Such estimates would benefit from online detection of PAHs along with analysis of the carrying particles to identify the source. Typically, laser desorption/ ionization (LDI) in a bipolar mass spectrometer reveals the inorganic constituents and provides limited molecular information. In contrast, twostep ionization approaches produce detailed PAH mass spectra from individual particles but without the source-specific inorganic composition. Here we report a new technique that yields the single-particle PAH composition along with both positive and negative inorganic ions via LDI. Thus, the complete particle characterization and source apportionment from conventional bipolar LDI-analysis becomes possible, combined with a detailed PAH spectrum for the same particle. The key idea of the method is spatiotemporal matching of the ionization laser pulse to the transient component distribution in the particle plume after laser desorption. The technique is robust and field-deployable with only slightly higher costs and complexity compared to two-step approaches. We demonstrate its capability to reveal the PAH-distribution on different particle types in combustion aerosols and ambient air.

Resonance-enhanced detection of metals in aerosols using single-particle mass spectrometry

“…Beyond these direct applications, more studies are required to elucidate the promising implications for SPMS quantification approaches (Healy et al, 2013;Gemayel et al, 2017). Of note, the Fe-containing particles can further be characterized with regard to their organic content using multistep ionization techniques (Schade et al, 2019;Czech et al, 2017). This is of importance for health-related studies, as two of the most relevant adverse aerosol compounds, transition metals and PAHs, can be addressed with the same easily accessible KrF-excimer laser wavelength.…”

“…Diesel exhaust particles from an old van (Volkswagen Transporter 1.7 D, 1988 model) were collected from the inner surface of the exhaust tube. These particles exhibit a rather uniform chemical composition, as demonstrated in previous experiments (Passig et al, 2017;Schade et al, 2019). Model particles for mineral dust were Arizona test dust 0-3 µm diameter (Powder Technology Inc., USA), and complex anthropogenic aerosols with trace metals were mimicked using NIST urban dust 1649b (Gonzalez and Choquette, 2016).…”

“…For example, the particle's humidity and its main composition can have a strong effect on the detection of particle compounds (Neubauer et al, 1998), known as matrix effects. These effects are associated with several poorly determined interactions at the particle surface and in the desorbed plume affect ion formation (Reilly et al, 2000;Reinard and Johnston, 2008;Hinz and Spengler, 2007;Murphy, 2007;Wade et al, 2008;Hatch et al, 2014;Schoolcraft et al, 2000), reduce detection efficiencies, and complicate quantification approaches (Healy et al, 2013;Gemayel et al, 2017;Gross et al, 2000;Fergenson et al, 2001;Qin et al, 2006;Zhou et al, 2016;Shen et al, 2019). These difficulties can be mitigated if the desorption and ionization are separated in a two-step process and ions are formed in the gaseous plume as demonstrated for aromatic hydrocarbons (Morrical et al, 1998;Bente et al, 2008;Woods et al, 2001).…”

“…In such a two-step approach, thermal or laser desorption (LD) is often followed by resonance-enhanced multiphoton ionization (REMPI), a gas-phase ionization technique that is highly sensitive and selective for aromatic molecules (Gunzer et al, 2019). The LD-REMPI approach yields detailed mass spectra of the health-relevant polycyclic aromatic hydrocarbons (PAHs) -ubiquitous trace compounds of combustion particles (Bente et al, 2009;Li et al, 2019;Passig et al, 2017;Schade et al, 2019). Resonant laser ablation of metals, where the leading edge of the laser pulse ablates atoms from a solid sample that are then ionized by the same pulse, have been studied some time ago for laser microprobe mass analysis (LAMMA) from surfaces (Verdun et al, 1987;McLean et al, 1990).…”

Laser Mass Spectrometry

“…16 Schade et al achieved the simultaneous detection of PAHs as well as positive and negative inorganic ions in singleparticle mass spectrometry. 17 At first, a particle is hit by an infrared laser pulse for the desorption of organics. After 14 μs, the expanding plume is illuminated with a broad ultraviolet laser beam for MPI of PAHs.…”