Selective measurements of isoprene and 2-methyl-3-buten-2-ol based on NO⁺ ionization mass spectrometry

Biogenic VOC emissions are often dominated by 2-methyl-1,3-butadiene (isoprene) and 2-methyl-3-buten-2-ol (232 MBO). Here we explore the possibility to selectively distinguish these species using NO⁺ as a primary ion in a conventional PTR-MS equipped with an SRI unit. High purity of NO⁺ (>90%) as a primary ion was utilized in laboratory and field experiments using a conventional PTR-TOF-MS. Isoprene is ionized via charge transfer leading to the major product ion C₅H₈⁺ (>99%) (e.g. Spanel and Smith, 1998). 232 MBO undergoes a hydroxide ion transfer reaction resulting in the major product ion channel C₅H₉⁺ (>95%) (e.g. Amelynck et al., 2005). We show that both compounds are ionized with little fragmentation (>5%) under standard operating conditions. Typical sensitivities of 11.1 ± 0.1 (isoprene) and 12.9 ± 0.1 (232 MBO) ncps ppbv⁻¹ were achieved, which correspond to limit of detections of 18 and 15 pptv respectively for a 10 s integration time. Sensitivities decreased at higher collisional energies. Calibration experiments showed little humidity dependence. We tested the setup at a field site in Colorado dominated by ponderosa pine, a 232 MBO emitting plant species. Our measurements confirm 232 MBO as the dominant biogenic VOC at this site, exhibiting typical average daytime concentrations between 0.2-1.4 ppbv. The method is able to detect the presence of trace levels of isoprene at this field site (90-250 ppt) without any interference from 232 MBO, which would not be feasible using H₃O⁺ ionization chemistry, and which currently also remains a challenge for other analytical techniques (e.g. gas chromatographic methods).