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

摘要

Here we compare volatile organic compound (VOC) measurements using a standardproton-transfer-reaction quadrupole mass spectrometer (PTR-QMS) with a newproton-transfer-reaction time of flight mass spectrometer (PTR-TOF) duringthe Uintah Basin Winter Ozone Study 2013 (UBWOS2013) field experiment in anoil 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 fullmass spectra with typical 0.1 s–1 min integrated acquisition times. Thesensitivity of the PTR-QMS in units of counts per ppbv (parts per billion by volume) is about a factor of10–35 times larger than the PTR-TOF, when only one VOC is measured. Thesensitivity of the PTR-TOF is mass dependent because of the massdiscrimination caused by the sampling duty cycle in theorthogonal-acceleration region of the TOF. For example, the PTR-QMS on mass33 (methanol) is 35 times more sensitive than the PTR-TOF and for massesabove 120 amu less than 10 times more. If more than 10–35 compounds aremeasured with PTR-QMS, the sampling time per ion decreases and the PTR-TOFhas higher signals per unit measuring time for most masses. For UBWOS2013 thePTR-QMS measured 34 masses in 37 s and on that timescale the PTR-TOF ismore sensitive for all masses. The high mass resolution of the TOF allows forthe measurements of compounds that cannot be separately detected with thePTR-QMS, such as oxidation products from alkanes and cycloalkanes emitted byoil and gas extraction. PTR-TOF masses do not have to be preselected,allowing for identification of unanticipated compounds. The measured mixingratios of the two instruments agreed very well (R2 ≥ 0.92 and within20%) for all compounds and masses monitored with the PTR-QMS.