A comparison of GC-FID and PTR-MS toluene measurements in ambient air under conditions of enhanced monoterpene loading

Abstract

Toluene was measured using both a gas chromatographic system (GC), with a flame ionization detector (FID), and a proton transfer reaction-mass spectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station Thompson Farm (THF) in rural Durham, NH during the summer of 2004. Simultaneous measurements of monoterpenes, including α- and β-pinene, camphene, Δ <sup>3</sup>-carene, and <i>d</i>-limonene, by GC-FID demonstrated large enhancements in monoterpene mixing ratios relative to toluene, with median and maximum enhancement ratios of ~2 and ~30, respectively. A detailed comparison between the GC-FID and PTR-MS toluene measurements was conducted to test the specificity of PTR-MS for atmospheric toluene measurements under conditions often dominated by biogenic emissions. We derived quantitative estimates of potential interferences in the PTR-MS toluene measurements related to sampling and analysis of monoterpenes, including fragmentation of the monoterpenes and some of their primary carbonyl oxidation products via reactions with H<sub>3</sub>O<sup>+</sup>, O<sub>2</sub><sup>+</sup> and NO<sup>+</sup> in the PTR-MS drift tube. The PTR-MS and GC-FID toluene measurements were in good quantitative agreement and the two systems tracked one another well from the instrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. A correlation plot of the PTR-MS versus GC-FID toluene measurements was described by the least squares regression equation <i>y</i>=(1.13± 0.02)<i>x</i>−(0.008±0.003) ppbv, suggesting a small ~13% positive bias in the PTR-MS measurements. The bias corresponded with a ~0.055 ppbv difference at the highest measured toluene level. The two systems agreed quantitatively within the combined 1σ measurement precisions for 60% of the measurements. Discrepancies in the measured mixing ratios were not well correlated with enhancements in the monoterpenes. Better quantitative agreement between the two systems was obtained by correcting the PTR-MS measurements for contributions from monoterpene fragmentation in the PTR-MS drift tube; however, the improvement was minor (<10%). Interferences in the PTR-MS measurements from fragmentation of the monoterpene oxidation products pinonaldehyde, caronaldehyde and α-pinene oxide were also likely negligible. A relatively large and variable toluene background in the PTR-MS instrument likely drove the measurement bias; however, the precise contribution was difficult to accurately quantify and thus was not corrected for in this analysis. The results from THF suggest that toluene can be reliably quantified by PTR-MS using our operating conditions (drift tube pressure, temperature and voltage of 2.0 mbar, 45 °C and 600 V, respectively) under the ambient compositions probed. This work extends the range of field conditions under which PTR-MS validation studies have been conducted

Eddy covariance emission and deposition flux measurements using proton transfer reaction – time of flight – mass spectrometry (PTR-TOF-MS): comparison with PTR-MS measured vertical gradients and fluxes

“…The fragment ion from para-cymene accounts for about 80 % of total ions from para-cymene based on our E/N ratios for both instruments (Tani et al, 2003;Ambrose et al, 2010). A few studies have claimed that toluene can be directly emitted from plants (Heiden et al, 1999;White et al, 2009) from orange trees (Ciccioli et al, 1999).…”

Annual cycle of volatile organic compound exchange between a boreal pine forest and the atmosphere

“…There were also other controversial discoveries such as net emissions of m/z 93. A compound at m/z 93 is usually connected with toluene but it might be a fragmentation product of p-cymene as well (Ciccioli et al, 1999;Heiden et al, 1999;White et al, 2009;Ambrose et al, 2010;Park et al, 2013). We found a dependency between the m/z 93 fluxes and E/N where E is the electric field and N the number density of the gas in the drift tube.…”

Comparison of VOC measurements made by PTR-MS, adsorbent tubes–GC-FID-MS and DNPH derivatization–HPLC during the Sydney Particle Study, 2012: a contribution to the assessment of uncertainty in routine atmospheric VOC measurements

“…These include α-and β-pinene, p-cymene and several C 9 aromatics (ethyltoluenes, 1,2,3-trimethylbenzene), all of which are known to produce fragment ions at m/z 93 in PTR-MS Maleknia et al, 2007;Ambrose et al, 2010;Gueneron et al, 2015). These potential interferent compounds, with the exception of p-ethyltoluene and 1,2,3-trimethylbenzene, were measured in the AT-VOC samples.…”