Proton Transfer Reaction Mass Spectrometry (PTR-MS) for Ambient and (Compliance) Source Testing Discussion

摘要

Proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS) is a continuous monitoring technique primarily used for ambient environmental measurements. It is an extremely sensitive and real-time instrument capable of measuring into the part per billion (ppb) and parts per trillion (ppt) levels for many organic and inorganic compounds. In recent years, its abilities were investigated to be used in identifying, quantifying, and monitoring stationary sources, typically after dilution. Although regulatory acceptance for PTR-TOF-MS is sparse, there is an accepted instrument for use with modified EPA Method 8265 for vapor intrusion and sewer gas studies. Anecdotal discussions with regulatory personnel indicate a reluctance to embrace PTR technology due to its cost and general availability to industry. A recent example of this phenomenon is Fourier Transform Infrared (FTIR) spectrometry. There was an initial reluctance to embrace the technology; however, after roughly a decade, it became fully integrated as a compliance source testing tool. Cost and availability of the PTR technology to industry for testing are quickly being addressed and acceptable methodologies are being developed for United States Environmental Protection Agency (EPA) consideration. The need for real-time, ultra-low-level measurements is real as can be demonstrated with the recent ethylene oxide risk assessment reviews. Real-time or near real-time (<15 min per data point) detection limits of ethylene oxide in ambient air have already been demonstrated down to 100 ppt. A draft ASTM method is also being developed, "Standard Guide for Selection of Real-Time and Near Real-Time Mass Spectrometer-Based Technologies for Online Measurements of Volatile Organic Compounds in Ambient Air." Additionally, placement of a gas chromatograph (GC) or "fast GC" in front of the PTR, with the PTR-TOF-MS acting as the detector, fits the definition of EPA Method 18 direct interface, "Measurement of gaseous organic compound emissions by gas chromatography."