Introducing a compact GC-PTR-TOF-MS: A novel method for the long-term monitoring of volatile organic compounds

Long-term, routine monitoring of volatile organic compounds (VOCs) is needed to provide insight into emission sources and patterns, oxidation processes including photochemistry and radical cycling, and the formation of tropospheric ozone and secondary organic aerosol. However, instrumentation that can provide high quality data both in terms of temporal resolution and molecular speciation has historically been cost-prohibitive, and requires advanced users for field deployment, operation, and data analysis.

While the use of proton transfer reaction mass spectrometry, paired with time-of-flight technology (PTR-TOF-MS), has been utilized for the detection of VOCs in a wide variety of environments and measurement platforms; this technique cannot provide molecular structure information and suffers from detection interferences such as fragmentation, cluster formation, and mixed ionization schemes that complicate data analysis and interpretation. Combining in situ gas chromatography (GC) with PTR-TOF-MS is a remedy, offering isomer level resolution and the ability to easily quantify product ion distributions to account for complex detection schemes. The addition of chromatographic pre-separation not only enhances the accuracy of species typically reported by PTR methods, it can also be used to broaden the scope of VOCs quantified using PTR methods, while maintaining low limits of detection (typically 1 ppt) without losing high time resolution data and negating the need for high mass resolving power.

Here, we present a new instrument package for the online, long-term detection of VOCs consisting of an in situ GC system equipped with an integrated thermal desorption (TD) system coupled to a compact PTR TOF-MS. The entire instrument is contained in a 50 x 65 x 55 cm footprint, weighs < 70 kg, and consumes < 600 W for typical operation. Coupling the GC system with the PTR-TOF allows the automated acquisition of both direct, high-time resolution data (e.g. 1 s) and pseudo-continuous molecular speciation data for isomer separation and improved quantification of the direct-PTR data.

A description of the instrument, including its utilization of a new VUV PTR ion source, will be presented along with > 4 weeks of continuous ambient data acquired in Spring 2025 in Thun, Switzerland. This data demonstrates the stability of the system and the value of continuous VOC detection with regular molecular speciation. This compact, easier to operate and maintain system makes it feasible to deploy this technology in many locations, including those with access limitations, for greater spatial resolution to acquire coinciding datasets to elucidate local, regional, and global VOC trends. 

The compact chemical ionization time-of-flight mass spectrometer (CI-TOF-MS) described, here utilizing proton transfer reaction (PTR) ionization, was developed with support from the Beckmann Foundation.