Evaluating the performance of five different chemical ionization techniques for detecting gaseous oxygenated organic species

Abstract

Abstract. The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOC), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from α-pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS). Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new “Vocus” PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim to determine which oxygenated VOC (OVOC) the different methods were sensitive to, and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOC, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies: the PTR instruments were the only ones able to measure the precursor VOC; the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOC) with 3 to 5 oxygen atoms; and the nitrate CI-APi-TOF was mainly sensitive to highly-oxygenated organic (O > 5) molecules (HOM). In addition, the Vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including also a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing non-linear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOC in the atmosphere. While specifically shown for the case of α-pinene ozonolysis, we expect our general findings to be valid also for a wide range of other VOC-oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.

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

“…31,33 As reported in Figure S4, the in-source CID experiments revealed that the least oxidized C10-monomers exhibited the weakest binding energies, which is consistent with the low detection efficiency of such species. 16 Interestingly, the dimers showed very strong binding energies, suggesting that such products are unlikely to undergo declustering in most mass spectrometers. The orange trace represents the spectrum that would be observed using an instrument with mass resolving power of 10 000.…”

“…14,15 In particular, chemical ionization mass spectrometry (CIMS) has emerged as a powerful tool to characterize a wide range of different gaseous compounds. 11,[16][17][18] CI is a soft ionization technique where the analyte is ionized through clustering process with the reagent ions and undergoes only minimal fragmentation. 17,19,20 While CIMS provides very good sensitivities (detection limit as low as 10 4 molecules cm -3 ), 19 they are commonly coupled to TOF mass analyzers having limited mass resolving power.…”

“…This typically ranges from 4000 for a medium-resolution (e.g., the Tofwerk AG HTOF) to 14 000 for a high-performance TOF (e.g., Tofwerk AG LTOF). 16 These resolving powers make identification and quantification of individual organic molecules extremely challenging in cases with multiple overlapping ions, yielding significant uncertainties. 16,21,22 Indeed, "atmospheric" mass spectra acquired using the CIMS technique typically contain a large amount of information, and therefore remain complex in nature, representing a significant challenge for data analysis.…”

“…16 These resolving powers make identification and quantification of individual organic molecules extremely challenging in cases with multiple overlapping ions, yielding significant uncertainties. 16,21,22 Indeed, "atmospheric" mass spectra acquired using the CIMS technique typically contain a large amount of information, and therefore remain complex in nature, representing a significant challenge for data analysis. Computational approaches, including ion deconvolution procedures, are required to solve this limitation in order to extract the maximum possible information content.…”

Bulk Organic Aerosol Analysis by Proton-Transfer-Reaction Mass Spectrometry: An Improved Methodology for the Determination of Total Organic Mass, O:C and H:C Elemental Ratios, and the Average Molecular Formula

“…Most of the soft ionization methods (e.g., iodide chemical ionization, acetate chemical ionization, electrospray ionization) currently used for POM analysis do, however, either not respond to poorly oxidized analytes or exhibit a highly variable compound-specific response. 12,13,29,30 It is thus not possible to quantitatively analyze bulk organic aerosol with these ionization techniques. Here is where PTR-MS with its universal (i.e., independent of n O ) and quantitative ionization via H 3 O + ions fills an important gap.…”