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

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 C₠H₈⁺ (>99%) (e.g. Spanel and Smith, 1998). 232 MBO undergoes a hydroxide ion transfer reaction resulting in the major product ion channel C₠H₉⁺ (>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.