Fragmentation inside proton-transfer-reaction-based mass spectrometers limits the detection of ROOR and ROOH peroxides

Abstract

Abstract. Proton transfer reaction (PTR) is a commonly applied ionization technique for mass spectrometers, in which hydronium ions (H3O+) transfer a proton to analytes with higher proton affinities than the water molecule. This method has most commonly been used to quantify volatile hydrocarbons, but later-generation PTR instruments have been designed for better throughput of less volatile species, allowing detection of more functionalized molecules as well. For example, the recently developed Vocus PTR time-of-flight mass spectrometer (PTR-TOF) has been shown to agree well with an iodide-adduct-based chemical ionization mass spectrometer (CIMS) for products with 3–5 O atoms from oxidation of monoterpenes (C10H16). However, while several different types of CIMS instruments (including those using iodide) detect abundant signals also at “dimeric” species, believed to be primarily ROOR peroxides, no such signals have been observed in the Vocus PTR even though these compounds fulfil the condition of having higher proton affinity than water. More traditional PTR instruments have been limited to volatile molecules as the inlets have not been designed for transmission of easily condensable species. Some newer instruments, like the Vocus PTR, have overcome this limitation but are still not able to detect the full range of functionalized products, suggesting that other limitations need to be considered. One such limitation, well-documented in PTR literature, is the tendency of protonation to lead to fragmentation of some analytes. In this work, we evaluate the potential for PTR to detect dimers and the most oxygenated compounds as these have been shown to be crucial for forming atmospheric aerosol particles. We studied the detection of dimers using a Vocus PTR-TOF in laboratory experiments, as well as through quantum chemical calculations. Only noisy signals of potential dimers were observed during experiments on the ozonolysis of the monoterpene α-pinene, while a few small signals of dimeric compounds were detected during the ozonolysis of cyclohexene. During the latter experiments, we also tested varying the pressures and electric fields in the ionization region of the Vocus PTR-TOF, finding that only small improvements were possible in the relative dimer contributions. Calculations for model ROOR and ROOH systems showed that most of these peroxides should fragment partially following protonation. With the inclusion of additional energy from the ion–molecule collisions driven by the electric fields in the ionization source, computational results suggest substantial or nearly complete fragmentation of dimers. Our study thus suggests that while the improved versions of PTR-based mass spectrometers are very powerful tools for measuring hydrocarbons and their moderately oxidized products, other types of CIMS are likely more suitable for the detection of ROOR and ROOH species.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

“…However, different online instruments may have different sensitivities toward POs. For example, fragmentation could occur for both ROOH and ROOR′ types of POs due to the excess energy of protonated ions coming from the charge-transfer process during the PTR-TOF measurement . The stability of ions with different functionalities in various types of CIMS needs further investigations in the future.…”

“…However, since the high electric field in the ionization region produces protonated ions with high excess energy that are subject to strong decompositions, evidence from both laboratory experiments using PTR-TOF-MS and quantum chemical calculations revealed that the proton-transferring process may induce the decomposition of most POs. 257 Caution should be taken while measuring and interpreting atmospheric POs using PTR-TOF-MS. Recently, combining the NO 3 − chemical ionization source and a high-resolution Orbitrap MS (CI-Orbitrap), Riva et al 258,259 showed a similar sensitivity and selectivity of CI-Orbitrap with CI-APi-TOF using the nitrate ionization scheme, which has a resolution of ≥140,000 at m/z 200.…”

“…Combining PTR-TOF-MS with a cold trap, ISOPOOH was carefully characterized by Liu et al above the Amazon rainforest. However, since the high electric field in the ionization region produces protonated ions with high excess energy that are subject to strong decompositions, evidence from both laboratory experiments using PTR-TOF-MS and quantum chemical calculations revealed that the proton-transferring process may induce the decomposition of most POs . Caution should be taken while measuring and interpreting atmospheric POs using PTR-TOF-MS.…”

Comprehensive detection of analytes in large chromatographic datasets by coupling factor analysis with a decision tree

“…We first provide an overview of the PMF algorithm before describing the overall cataloging approach. In this work, the term "analyte" is used to refer to a chromatographic peak (e.g., a chromatographic "feature") with a unique mass spectrum and retention time whether it has a known definitive identification or not, following the usage of this term in other studies (Amigo et al, 2010;Grace et al, 2019;Isaacman-VanWertz et al, 2017, 2021Li et al, 2022).…”

Selective deuteration as a tool for resolving autoxidation mechanisms in α-pinene ozonolysis

“…For the selectively deuterated samples, we can inspect their purity using PTR mass spectrometer data. As PTR instruments are known to induce isomerisation and fragmentation of the protonated molecules (Tani et al, 2003;Li et al, 2022), we expect that the observed level of deuteration in the PTR mainly provides a lower limit for the purity. The PTR measurements suggested that over 88 % of the measured a-pinene in our chamber contained exactly the number of D-atoms specified, verifying that the samples are very pure and in-line with the observed purity determined using 1 H NMR spectroscopy (see section 2.1).…”