Abstract
Abstract. Proton transfer reactions between hydronium ions (H3O+) and volatile organic compounds (VOCs) provide a fast and highly sensitive technique for VOC measurements, leading to extensive use of proton-transfer-reaction mass spectrometry (PTR-MS) in atmospheric research. Based on the same ionization approach, we describe the development of a high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) utilizing H3O+ as the reagent ion. The new H3O+ ToF-CIMS has sensitivities of 100–1000 cps ppb−1 (ion counts per second per part-per-billion mixing ratio of VOC) and detection limits of 20–600 ppt at 3σ for a 1 s integration time for simultaneous measurements of many VOC species of atmospheric relevance. The ToF analyzer with mass resolution (m∕Δm) of up to 6000 allows the separation of isobaric masses, as shown in previous studies using similar ToF-MS. While radio frequency (RF)-only quadrupole ion guides provide better overall ion transmission than ion lens system, low-mass cutoff of RF-only quadrupole causes H3O+ ions to be transmitted less efficiently than heavier masses, which leads to unusual humidity dependence of reagent ions and difficulty obtaining a humidity-independent parameter for normalization. The humidity dependence of the instrument was characterized for various VOC species and the behaviors for different species can be explained by compound-specific properties that affect the ion chemistry (e.g., proton affinity and dipole moment). The new H3O+ ToF-CIMS was successfully deployed on the NOAA WP-3D research aircraft for the SONGNEX campaign in spring of 2015. The measured mixing ratios of several aromatics from the H3O+ ToF-CIMS agreed within ±10 % with independent gas chromatography measurements from whole air samples. Initial results from the SONGNEX measurements demonstrate that the H3O+ ToF-CIMS data set will be valuable for the identification and characterization of emissions from various sources, investigation of secondary formation of many photochemical organic products and therefore the chemical evolution of gas-phase organic carbon in the atmosphere.
Highlights
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere
As higher proton affinities are expected for these heavier oxygenated VOCs (OVOCs) species, the humidity dependence curves for these heavier OVOCs should lie in the shaded area filled by patterns in Fig. 5
The new H3O+ ToF-CIMS has sensitivities in the range of 100–1000 cps ppbv−1 for many VOCs of interest and the 1 s detection limits are in the range of 20–500 ppt, depending on product ion masses and their instrument backgrounds
Summary
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere. The oxidation of VOCs contributes to formation of ozone (Atkinson, 2000) and secondary organic aerosol (SOA) (Hallquist et al, 2009). A high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) utilizing hydronium ions (H3O+) to PTR-MS was developed based on the commercial Aerodyne ToF-CIMS (Lee et al, 2014). This instrument contains two quadrupole ion guides to transport ions from the drift tube reaction region to the time-of-flight mass analyzer. The axial voltage gradients for SSQ and BSQ were tuned using the Thuner software (Tofwerk AG) by maximizing signals of the protonated product ions of several VOC species (acetone, benzene and isoprene), minimizing VOC signals from charge transfer reactions with O+2 and maximizing mass resolution of the ToF analyzer before the campaign. All of the signals are reported as the ToF duty cycle corrected signals (Icorr), unless otherwise noted
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