Fragmentation inside proton-transfer-reaction-based mass spectrometers limits the detection of ROOR and ROOH peroxides
Authors: Li Haiyan , Almeida Thomas Golin , Luo Yuanyuan , Zhao Jian , Palm Brett B. , Daub Christopher D. , Huang Wei , Mohr Claudia , Krechmer Jordan E. , Kurtén Theo , Ehn Mikael 
Published: March 2022

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.