Assessment of aldehyde contributions to PTR-MS m/z 69.07 in indoor air measurements

Proton transfer reaction-mass spectrometry (PTR-MS) has been widely used for monitoring outdoor and indoor volatile organic compounds. For outdoor air, mass-to-charge-ratio m/z 69.07 is usually assigned to isoprene. Isoprene is also a major component of human breath and therefore abundant in occupied indoor environments. Mass 69.07 as an indicator of indoor isoprene can suffer interference resulting from fragmentation of aldehydes [V. Ruzsanyi, et al., Multi-capillary-column proton-transfer-reaction time-of-flight mass spectrometry, J. Chromatogr. A, 2013, 1316, 112-118], which are also abundant indoors, especially when ozone is elevated [C. J. Weschler, Roles of the human occupant in indoor chemistry, Indoor Air, 2016, 26, 6-24]. As part of the Indoor Chemical Human Emission and Reactivity (ICHEAR) campaign we examined this effect in human-occupied chamber studies, in the absence and presence of ozone. We find that such interferences do occur when ozone reacts with both human skin oil and cotton-based clothing. In the presence of humans and 35 ppb ozone, PTR-mass 69.07 was three times higher than the isoprene mixing ratio measured independently by GC-MS. To investigate this effect, we measured the fragmentation patterns of aldehydes and examined the contribution of different aldehydes to m/z 69.07 in the ICHEAR experiments. Nonanal, and its contribution to m/z 69.07, could be quantified reliably for clothing and human dermal emissions under the experimental conditions. In contrast, decanal is difficult to quantify, since compounds other than decanal fragment to m/z 157.16, its MH+ peak, which also makes it difficult to estimate decanal's contribution to m/z 69.07.

Automated summary

Proton transfer reaction-mass spectrometry (PTR-MS) is commonly used for monitoring volatile organic compounds in indoor and outdoor environments. However, the use of mass-to-charge ratio m/z 69.07 as an indicator of indoor isoprene can be affected by the presence of aldehydes, especially when ozone levels are high.