PTR-QMS versus PTR-TOF comparison in a region with oil and natural gas extraction industry in the Uintah Basin in 2013

Abstract

Abstract.Here we compare volatile organic compound (VOC) measurements using a standard proton-transferreaction quadrupole mass spectrometer (PTR-QMS) with a new proton-transfer-reaction time of flight mass spectrometer (PTR-TOF) during the Uintah Basin Winter Ozone Study 2013 (UBWOS2013) field experiment in an oil and gas field in the Uintah Basin, Utah. The PTR-QMS uses a quadrupole, which is a mass filter that lets one mass to charge ratio pass at a time, whereas the PTR-TOF uses a time of flight mass spectrometer, which takes full mass spectra with typical 0.1 s-1 min integrated acquisition times. The sensitivity of the PTR-QMS in units of counts per ppbv (parts per billion by volume) is about a factor of 10-35 times larger than the PTR-TOF, when only one VOC is measured. The sensitivity of the PTR-TOF is mass dependent because of the mass discrimination caused by the sampling duty cycle in the orthogonal-acceleration region of the TOF. For example, the PTR-QMS on mass 33 (methanol) is 35 times more sensitive than the PTR-TOF and for masses above 120 amu less than 10 times more. If more than 10-35 compounds are measured with PTR-QMS, the sampling time per ion decreases and the PTR-TOF has higher signals per unit measuring time for most masses. For UBWOS2013 the PTR-QMS measured 34 masses in 37 s and on that timescale the PTR-TOF is more sensitive for all masses. The high mass resolution of the TOF allows for the measurements of compounds that cannot be separately detected with the PTR-QMS, such as oxidation products from alkanes and cycloalkanes emitted by oil and gas extraction. PTR-TOF masses do not have to be preselected, allowing for identification of unanticipated compounds. The measured mixing ratios of the two instruments agreed very well (R 2 ≥ 0.92 and within 20 %) for all compounds and masses monitored with the PTR-QMS.

A high-resolution time-of-flight chemical ionization mass spectrometer utilizing hydronium ions (H<sub>3</sub>O<sup>+</sup> ToF-CIMS) for measurements of volatile organic compounds in the atmosphere

“…The fill time for each canister was 3-15 s depending on aircraft altitude. A brief description of the iWAS system was provided in recent publications (de Gouw et al, 2015;Warneke et al, 2016). A few compounds were measured by both H 3 O + ToF-CIMS and the iWAS system, including benzene, toluene and C8 aromatics.…”

Photochemical aging of volatile organic compounds associated with oil and natural gas extraction in the Uintah Basin, UT, during a wintertime ozone formation event

“…The ratio of each VOC to benzene was then determined. Benzene is a well-calibrated compound with few interferences on the PTR-MS; additionally, good agreement with other instrumentation (GC-FID and PTR-ToF) at the site affords a high level of confidence in this measurement (Warneke et al, 2015).…”

“…10). Other measurement and modeling studies of the Uintah Basin suggest that unquantified secondary species are responsible for a large part of this gap: many additional oxygenated VOCs were detected, but not quantified, by PTR-time-of-flight (PTR-ToF) mass spectrometry (Warneke et al, 2015), and MCM modeling indicates that carbonyl groups formed from higher-weight species (e.g., aromatics) were not only abundant but major drivers of ozone formation.…”

“…The instrument is not able to distinguish between isomers; e.g., a measurement of C8 aromatics comprises the sum of ethylbenzene, o-, m-, and p-xylenes. Comparison with a high-resolution PTR-time-of-flight spectrometer operated by the University of Wyoming (Warneke et al, 2015) indicates negligible contribution from isobaric compounds (e.g., benzaldehyde).…”

Observations of VOC emissions and photochemical products over US oil- and gas-producing regions using high-resolution H<sub>3</sub>O<sup>+</sup> CIMS (PTR-ToF-MS)

“…The Ionicon PTR-ToF-MS instrument relies on the same measurement principle as the SONGNEX H 3 O + ToF-CIMS and has similar mass resolution. The PTR-ToF-MS Uintah Basin measurements have been previously described by Warneke et al (2015), and a comparison between the H 3 O + ToF-CIMS and PTR-ToF-MS instruments is given by Yuan et al (2016a). Finally, we include H 3 O + ToF-CIMS headspace measurements (at 25 • C) of a crude oil sample.…”