Identifying an unknown compound in flue gas of semiconductor industry - Forensics of a perfluorocarbon

This study investigated the source of a false positive signal in the measurement of total non-methane organic compounds (TNMOCs) by an on-line analyzer based on flame ionization detection (FID) in the flue gas released from a semiconductor fabrication plant. Since no release of volatile organic compounds (VOCs) into the waste gas stream in acid/base ventilation was assured by the plant authority, the positive detection of VOCs became a subject of dispute. In addition to the TNMOC analysis of 5 samples, the investigation used the method that coupled thermal desorption (TD) with gas chromatography mass spectrometry (GC/MS), dubbed TD-GC/MS, to identify the substance that produced the FID signals of TNMOCs. The waste gas was collected with sampling canisters and analyzed by in-laboratory TD-GC/MS. However, mass scanning from 45 to 250 m/z to remove interference from air matrix of O₂, N₂ and CO₂ forbid detecting any ion fragments smaller than 45 m/z and, thus, led to poor match in mass (MS) library search. As a result, a highly retentive porous layer open tubular (PLOT) capillary column was employed to separate the unknown away from the air peak. The success of acquiring all key ion fragments of 31, 50, 69, and 131 m/z resulted in an excellent match with octafluorocyclobutane (C₄F₈) in the NIST database. A gas standard was then prepared and injected to confirm the identity of C₄F₈ by the identical mass spectrum and GC retention time. The concentrations of C₄F₈ found in the 5 flue gas samples varying from 3.32 to 6.21 ppmv were very close to the NMOC range of 3.48-6.62 ppmv as reported by the TNMOC analyzer, proving that the FID signals observed from TNMOC method were mostly produced from C₄F_8. Consequently, the method of TD-GC/MS would be an ideal method to pre-screen the presence of PFCs before a non-distinguishable TNMOC analyzer is applied to approximate the VOC level as part of the integrated effort to monitor VOC in flue gas.