Rapid Vapor-Collection Method for Vapor Pressure Measurements of Low-Volatility Compounds

Abstract

Dynamic vapor microextraction (DVME) is a new method that enables rapid vapor pressure measurements on large molecules with state-of-the-art measurement uncertainty for vapor pressures near 1 Pa. Four key features of DVME that allow for the rapid collection of vapor samples under thermodynamic conditions are (1) the use of a miniature vaporequilibration vessel (the "saturator") to minimize the temperature gradients and internal volume, (2) the use of a capillary vapor trap to minimize the internal volume, (3) the use of helium carrier gas to minimize nonideal mixture behavior, and (4) the direct measurement of pressure inside the saturator to accurately account for overpressure caused by viscous flow. The performance of DVME was validated with vapor pressure measurements of n-eicosane (C 20 H 42 ) at temperatures from 344 to 374 K. A thorough uncertainty analysis indicated a relative standard uncertainty of 2.03−2.82% for measurements in this temperature range. The measurements were compared to a reference correlation for the vapor pressures of n-alkanes; the deviation of the measurements from the correlation was ≤2.85%. The enthalpy of vaporization of n-eicosane at 359.0 K was calculated to be Δ vap H = 91.27 ± 0.28 kJ/mol compared to Δ vap H(corr) = 91.44 kJ/mol for the reference correlation. Total measurement periods as short as 15 min (3 min of thermal equilibration plus 12 min of carrier gas flow) were shown to be sufficient for high-quality vapor pressure measurements at 364 K.

Vapor Pressure Measurements on Linalool Using a Rapid and Inexpensive Method Suitable for Cannabis-Associated Terpenes

“…The measurement uncertainties for linalool p sat are higher than our previous measurements for n-eicosane, which ranged from 0.0203•p sat to 0.0282•p sat at temperatures from 344 to 374 K. 1 One reason for the difference is that, with n-eicosane, more carrier gas flow (240 to 800 scc) was required to obtain The values of p sat were calculated with eqs 1−4 from the mass of helium carrier gas (m 1 ), the mass of trapped linalool vapor (m 2 ), and the pressure in the saturator vial (p saturator ). All measurements were made with a flow rate of 6.00 sccm and a thermal equilibration period of 10 min.…”

“…The relatively high p sat of linalool raised concerns about the vapor-trapping efficiency. Consequently, we increased the capillary length to 1.5 m (from 1.0 m used in the n-eicosane study 1 ), and a thermoelectric cooling plate was installed to cool the capillary vapor trap to 263 K (Figure 1). Sufficient trapping efficiency was verified in triplicate at 354 K with a total flow of 240 scc and a flow rate of 10 sccm�more demanding conditions than any of the p sat measurements.…”

“…The vial was capped, and the beads were shaken to ensure an even coating of linalool. The cap was then removed and replaced with a clean cap; this was done to prevent physical transfer of linalool from the septum cap to the capillary vapor trap . A fresh linalool saturator was prepared for each day of measurements.…”

“…The working equations for DVME are shown in eqs – , p sat = p saturator · y 2 / x 2 p saturator = p ambient + p overpressure y 2 = ( m 2 / M 2 ) / ( false( m 1 / M 1 false) + false( m 2 / M 2 false) ) x 2 = 1 − x 1 − x impurities …”

“…septum cap to the capillary vapor trap. 1 A fresh linalool saturator was prepared for each day of measurements. To remove the water impurity in the linalool, the saturator was pretreated by flowing 700 scc of warm (354 K) helium through it at 10 sccm before making p sat measurements (see the Supporting Information, Section S1 and Figure S1).…”