Online Monitoring of Residential Combustion Emissions at the Molecular Level by Vacuum Photoionization High-Resolution Orbitrap Mass Spectrometry

Abstract

Online mass spectrometry of combustion emissions is often limited by unresolved isobaric contributions. An Orbitrap Fourier transform mass spectrometer (Orbitrap FT-MS) with in-vacuum resonance-enhanced multiphoton ionization (REMPI, 248 nm) was therefore deployed at a residential stove and benchmarked against a nominal-resolution orthogonal-acceleration time-of-flight mass spectrometer (oaTOF-MS) using the same REMPI scheme. Under different air-supply conditions, beech logwood, fiberboard, wheat bran, a processed wood-based picture frame, and brown coal briquettes were used in successive burns. Total-ion-current profiles of both instruments agreed closely, while the Orbitrap provided a resolving power roughly 90–110 times that of the oaTOF-MS (R = 1300–1600) and separated up to eight components within a single nominal-mass channel. Scan-wise recalibration held the root-mean-square-error of the calibrant masses at 0.20 ppm; molecular formulas were assigned to 740 monoisotopic features within a ±2 ppm window, 93.4% of them within ±0.5 ppm, and 65% of these assignments were compositionally unique under the applied constraints. The O/CH4 exchange (36.4 mDa) was the most frequent mass difference; the smallest retained separation, 2.7 mDa at m/z 253, required R > 90,000, and no within-nominal-mass split was resolved in the oaTOF-MS data. The resolved chemistry qualitatively tracked both combustion regime and feedstock: methoxyphenol-type oxygenated aromatics during air-limited beech burning, condensed polycyclic aromatic hydrocarbons during flaming, nitrogen-containing aromatics from wheat bran and fiberboard, and alkylated dibenzothiophene-type series from brown coal. Online REMPI Orbitrap FT-MS thus resolves chemically distinct contributions hidden within nominal-mass signals and enables formula-resolved monitoring of residential combustion emissions.