Abstract
Abstract. Recent studies have demonstrated that organic compounds can partition from the gas phase to the walls in Teflon environmental chambers and that the process can be modeled as absorptive partitioning. Here these studies were extended to investigate gas–wall partitioning of organic compounds in Teflon tubing and inside a proton-transfer-reaction mass spectrometer (PTR-MS) used to monitor compound concentrations. Rapid partitioning of C8–C14 2-ketones and C11–C16 1-alkenes was observed for compounds with saturation concentrations (c∗) in the range of 3 × 104 to 1 × 107 µg m−3, causing delays in instrument response to step-function changes in the concentration of compounds being measured. These delays vary proportionally with tubing length and diameter and inversely with flow rate and c∗. The gas–wall partitioning process that occurs in tubing is similar to what occurs in a gas chromatography column, and the measured delay times (analogous to retention times) were accurately described using a linear chromatography model where the walls were treated as an equivalent absorbing mass that is consistent with values determined for Teflon environmental chambers. The effect of PTR-MS surfaces on delay times was also quantified and incorporated into the model. The model predicts delays of an hour or more for semivolatile compounds measured under commonly employed conditions. These results and the model can enable better quantitative design of sampling systems, in particular when fast response is needed, such as for rapid transients, aircraft, or eddy covariance measurements. They may also allow estimation of c∗ values for unidentified organic compounds detected by mass spectrometry and could be employed to introduce differences in time series of compounds for use with factor analysis methods. Best practices are suggested for sampling organic compounds through Teflon tubing.
Highlights
Teflon tubing is widely used for sampling organic gases in field and laboratory studies, primarily because it is chemically inert and flexible
In the study presented here, we quantified delays observed when a set of organic compounds with a range of volatilities were sampled through Teflon tubing for analysis in a PTRMS and developed a model that applies the principles of gas chromatography and gas–wall partitioning in Teflon environmental chambers to predict the delays measured for different tubing lengths and diameters, flow rates, and organic functional groups
This can be explained by noting that the criteria for gas–wall partitioning equilibrium is that the rates of absorption and desorption are equal throughout the system, so for tubing this condition is everywhere given by Eq (10): ka [G] = kd[W ]
Summary
Teflon tubing is widely used for sampling organic gases in field and laboratory studies, primarily because it is chemically inert and flexible. These properties make Teflon the material of choice for environmental “smog” chambers, most of which are constructed using fluorinated ethylenepropylene (FEP) or perfluoroalkoxy (PFA) Teflon film (Hallquist et al 2009). In the study presented here, we quantified delays observed when a set of organic compounds with a range of volatilities were sampled through Teflon tubing for analysis in a PTRMS and developed a model that applies the principles of gas chromatography and gas–wall partitioning in Teflon environmental chambers to predict the delays measured for different tubing lengths and diameters, flow rates, and organic functional groups. The results quantify the potential effects of tubing on measurements of organic gases and enable better design of sampling systems, in particular when fast instrument response is needed
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