Fast online characterization of novel VOC permeation tubes by TD-GC-FID: Direct quantification via the effective carbon number concept

Abstract. In atmospheric measurements, reliable calibration of analytical instruments used to monitor the concentrations of volatile organic compounds (VOCs) is achieved by means of traceable calibration standards. Permeation technology provides a flexible and continuous source of gaseous standards at trace levels; however, its application is not widespread for atmospherically-relevant VOCs, and the characterization remains limited to time-consuming gravimetric methods.

A comprehensive on-line and rapid thermodesorption-gas chromatography-flame ionization detector (TD-GC-FID) method was adapted after coupling with a dynamic generation system that housed four permeation tubes (PTs) containing benzene, (E)-2-hexenal, 2-methyl-1-butanol, and D-limonene–three of which are used for the first time in permeation. Emission rates (ERs) were determined for each PT over a total temperature range between 25 and 120 °C. At 35 °C, the ERs were 461±70 (±2σ) ng min⁻¹, 1721±74 ng min⁻¹, 723±103 ng min⁻¹, and 3.4±0.4 ng min⁻¹ for benzene, (E)-2-hexenal, 2-methyl-1-butanol, and D-limonene. Experimental ERs varied between 288–1076 ng min⁻¹ (25–50 °C) for benzene, between 1214–2022 ng min⁻¹ (25–40 °C) for (E)-2-hexenal, between 346–1022 ng min⁻¹ (25–45 °C) for 2-methyl-1-butanol, and between 3.4–1628 ng min⁻¹ (35–120 °C) for D-limonene. The effective carbon number (ECN) approach was employed to calculate the response factors on FID for the simple and fast quantification of the generated concentrations. Generated concentrations for an overall total dynamic flow of zero-air between 200- and 536.4-mL min⁻¹ were in the ranges of high ppb, 150–1358 ppb, 496–2134 ppb, 158–1091 ppb, for benzene, (E)-2-hexenal, 2-methyl-1-butanol, and ranging from low to high ppb (1.2–1018 ppb) for D-limonene, under their respective investigated temperature conditions. The ER-temperature dependence was fully established and explained based on Antoine’s vapor pressure law and Fick’s law of diffusion with maximum correlation R²>0.998 over the investigated temperature ranges. Apparent permeabilities were estimated for each PT VOC-membrane system, with values ranging between 10⁻¹³–10⁻⁹ mol m⁻¹ s⁻¹ Pa⁻¹, equivalent to magnitudes of permeability coefficients. Finally, the proposed method was extensively compared with other conventional gravimetric, offline chromatographic, and spectroscopic methods.