CI-Orbitrap: An Analytical Instrument To Study Atmospheric Reactive Organic Species

Abstract

While acknowledged as key components in the formation of new particles in the atmosphere, the accurate characterization of gaseous (highly) oxygenated organic compounds remains challenging, and requires analytical developments. Earlier studies have successfully used the nitrate ion (NO3 -) based chemical ionization (CI) coupled to atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) for monitoring these compounds. Despite many breakthroughs in recent years, the CI-APi-TOF has many limitations, preventing for instance the unambiguous ion identification of overlapping peaks. To tackle this analytical challenge, we developed a CI interface coupled to an ultrahigh-resolution Orbitrap mass spectrometer (CI-Orbitrap). We show that the CI-Orbitrap has similar sensitivity and selectivity as the CI-APi-TOF, but with over an order of magnitude higher mass resolving power (up to 140 000). Equally importantly, the CI-Orbitrap allows tandem mass spectrometry, providing the first possibility for structural elucidation of the highly oxygenated molecules (HOM). As a proof of concept, we characterized HOM formed during the ozonolysis of two biogenic compounds (α-pinene and limonene), under different environmental conditions in a flow reactor. The CI-Orbitrap exhibited high sensitivity to both HOM and radical species, while easily separating ions of different elemental composition in cases where the more common TOF applications would not have been able to distinguish all ions. Our tandem mass spectrometry analyses revealed distinct fingerprint spectra for all the studied HOM. Overall, the CI-Orbitrap is an extremely promising instrument, and it provides a much-needed extension to on-going research on HOM, with potential to impact also many other fields within atmospheric chemistry.

Structures and reactivity of peroxy radicals and dimeric products revealed by online tandem mass spectrometry

“…In general, dimers appear to bind more strongly to NO 3than do monomers ( Supplementary Figs. 2c and 3c) 44 . In Fig.…”

“…Negligible particle formation (<100 cm −3 ) was observed by a condensation particle counter (TSI CPC 3772) during all experiments. A Q Exactive Orbitrap mass spectrometer (Thermo Scientific, US) coupled to an atmospheric pressure CI inlet 44 was used for online analysis of ozonolysis products. The Orbitrap has been used with an automatic gain control (AGC) and maximum injection time set to 1 × 10 5 and 3000 ms, respectively.…”

“…In this study, we report the in-situ structural characterization of gas-phase HOMs, RO 2 radicals, and dimeric products of αpinene and limonene ozonolysis based on online MS/MS using an ultrahigh-resolution Orbitrap mass spectrometer equipped with a NO 3chemical ionization (CI) source 44,45 . Using online MS/MS data, aided by known structure-specific fragmentation patterns from existing offline MS/MS literature, we are able to infer the most plausible isomers of different compounds.…”

Orbitool: A software tool for analyzing online Orbitrap mass spectrometry data

“…Computational approaches, including ion deconvolution procedures, are required to partly resolve this limitation in order to extract the maximum possible information content (Cubison and Jimenez, 2015;Meija and Caruso, 2004;Zhang et al, 2019aZhang et al, , 2019b). To overcome these limitations, we have coupled a high-resolution mass spectrometer (Orbitrap, Eliuk and Makarov 2015) with a CI source (Riva et al, 2019a(Riva et al, , 2020 and an extractive ESI (EESI) inlet (Lee et al, 2020) for online analysis. Similarly, Zuth et al (2018) have combined an atmospheric pressure chemical ionization (APCI) interface to an Orbitrap and have shown that such a technology can achieve much greater mass resolving power at similar detection limit for the real-time measurement of organic aerosols.…”

“…The demand for a new software tool for analyzing Orbitrap data comes along with applying an Orbitrap in long-term continuous online measurements. The commercial software, Xcalibur TM that has been used in our initial studies (Lee et al, 2020;Riva et al, 2019aRiva et al, , 2020, provides an interface for basic data analysis e.g., reading single or averaged scans and exporting the time evolution of selected signal peaks. However, more complicated data analysis is usually needed for investigating the complexity and chemical processes occurring within the atmosphere.…”