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 selec- tively 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 uti- lized in laboratory and field experiments using a conven- tional PTR-TOF-MS. Isoprene is ionized via charge trans- fer leading to the major product ion C5H + (> 99 %) (e.g. Spanel and Smith, 1998). 232 MBO undergoes a hydroxide ion transfer reaction resulting in the major product ion chan- nel C5H + (> 95 %) (e.g. Amelynck et al., 2005). We show that both compounds are ionized with little fragmentation (< 5 %) under standard operating conditions. Typical sensi- tivities of 11.1 ± 0.1 (isoprene) and 12.9± 0.1 (232 MBO) ncps ppbv 1 were achieved, which correspond to limit of de- tections of 18 and 15 pptv respectively for a 10 s integra- tion time. Sensitivities decreased at higher collisional ener- gies. Calibration experiments showed little humidity depen- dence. We tested the setup at a field site in Colorado domi- nated by ponderosa pine, a 232 MBO emitting plant species. Our measurements confirm 232 MBO as the dominant bio- genic 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