Temperature dependence of Henry's law constants: An automated, high-throughput gas stripping cell design coupled to PTR-ToF-MS

Abstract

a b s t r a c tLiquid-air partition coefficients (Henry's law constants, HLCs) of eight flavour compounds (volatile organic compounds, VOCs) were determined in water, over a temperature range of 4 • C to 85 • C. The HLCs were derived by using nitrogen to strip a dilute solution of a VOC and then determining the decrease in concentration of the VOC in real-time in the stripped gas using proton-transfer-reaction time-of-flight mass-spectrometry (PTR-ToF-MS). This approach provided HLCs of improved accuracy (small 95% standard deviation) over a large temperature range, especially for low volatility VOCs (HLC > 2 mol/(m 3 Pa)). The outstanding features of this approach are: (i) it is applicable for VOCs over a large range of volatility; (ii) it can be used over a wide temperature range (4 • C to 85 • C); (iii) it is automated (high-throughput); (iv) it does not require calibration or knowledge of the initial concentration of the analyte; and (v) the experimental temperature can be controlled very precisely ( T better than ±0.1 • C). The eight flavour compounds analysed in water were: (E)-␤-damascenone, 2,3-butanedione, 2-ethyl-3,5-dimethylpyrazine, 2-methylfuran, 3-methylbutanal, acetaldehyde, ethyl-3-methyl butanoate and guaiacol. Based on the measured HLCs at five fixed temperatures (4 • C, 25 • C, 45 • C, 65 • C and 85 • C), accurate non-linear analytical expressions for the temperature dependence of HLCs were derived, which were then used to calculate thermodynamic constants.

pH dependent partitioning behaviour of food and beverage aroma compounds between air‐aqueous and organic‐aqueous matrices

“…Mean signed, mean absolute, and root mean squared errors (MSE, MAE, and RMSE, respectively) are only ‐0.01, 0.22, and, 0.29 pK a units. Similarly good performance was also found for the K aw (n = 19, r = 0.9998, p < 10 ‐30 , m = 0.927 ± 0.004, b = 0.004 ± 0.002, MSE/MAE/RMSE = 0.06/0.23/0.28 log 10 K aw units) and K ow (n = 45, r = 0.963, p < 10 ‐25 , m = 0.973 ± 0.041, b = 0.037 ± 0.090, MSE/MAE/RMSE = ‐0.01/0.32/0.38 log 10 K ow units) when compared to experimental data …”

Temperature-dependent Henry's Law constants of 4-alkyl branched-chain fatty acids and 3-methylindole in an oil-air matrix and analysis of volatiles in lamb fat using selected ion flow tube mass spectrometry

“…Henry's Law constant calculations based on the static and direct determination of equilibrium headspace concentrations of volatile compounds at known solution concentrations have considerably better accuracies than other measurement methods as previously mentioned . The major uncertainty in the calculated k H values based on the measurement of equilibrium headspace concentrations via SIFT‐MS is primarily attributed to the measurement uncertainty of the mean concentration of trace gas molecules in the helium carrier gas over a measured time interval.…”

“…The finite time required for the headspace sample to mix with the carrier gas and reach a steady flow, at best, gives only an approximate headspace trace gas concentration which generates uncertainties in the subsequent Henry's Law calculations . Other headspace sampling related uncertainties from equilibrated systems that could potentially contribute to inaccuracies in the subsequent calculation of Henry's Law constants have been discussed by Wieland and co‐workers . Španěl and co‐workers estimated an overall statistically most probable uncertainty of ±12% in measuring and subsequently calculating k H values using headspace concentrations obtained via the SIFT technique .…”

“…The additional parameter (y‐intercept), Δ S , denotes the entropy of phase change. In Equation , Δ H and Δ S are assumed to be independent of the temperature whereas k H is expected to be linearly correlated to the reciprocal of absolute temperature within a small temperature range, usually at Δ T < 20°C . Any resulting curvature or non‐linearity on the plot of the natural logarithm of k H vs 1/ T , which is usually observed over a large temperature range, depicts the temperature dependence of the enthalpy and the entropy of phase change.…”

“…Knowledge of the fundamental physical partitioning process of volatile organic compounds (VOCs) between air and a liquid‐phase matrix is important in understanding the fate, transport, environmental and atmospheric impacts, and sensory perceptions of odorous compounds in many scientific fields and disciplines . The behavior of flavor compounds, which are a delicate mixture of volatile compounds, is important to the food and flavor industry .…”