A kinetic model was previously developed in our laboratory to predict evaporation of compounds as a function of gas chromatographic retention index (IT). Evaporation rate constants were experimentally determined for compounds in the range IT = 800 – 1400 to define the initial model. While the predictive accuracy was demonstrated, broader application of the model, especially for forensic fire debris applications, requires extension of the IT range to include more volatile compounds. However, such extension requires experimental determination of rate constants, which is challenging due to the explosive hazard and rapid evaporation of volatile compounds. In this work, rate constants of highly volatile compounds were experimentally determined and used to extend the kinetic model to predict evaporation. Prior to experimental evaporations, theoretical calculations were performed to optimize experimental parameters and to ensure that the vapor generated remained below the lower flammability limit for each compound. Compounds were then experimentally evaporated at three different temperatures (10, 20, and 30 °C) and analyzed by gas chromatography-mass spectrometry. The evaporation rate constants for each compound, corrected for condensation, were determined by regression to a first-order rate equation. These rate constants were combined with previously collected data to extend the kinetic model at each temperature. Comparison of predicted and experimentally determined chromatograms of an evaporated validation mixture indicated good model performance, with correlation coefficients ranging from 0.955 – 0.997 and mean absolute percent errors in predicting abundance ranging from 0 – 35%. The kinetic model was originally developed by measuring rate constants of compounds evaporating from water, due to the environmental applications. For the refinement of the model using volatile compounds, evaporations were conducted directly from the surface of a glass dish. In addition to these two substrates, many more surfaces are present in a setting where an intentional fire may originate. The second study presented here investigated the effect of interface on the evaporation rate constants of compounds commonly found in ignitable liquids. Compounds from three homologous series (normal alkanes, alkylbenzenes, and alkyl cyclohexanes) were evaporated from a glass dish, water, cotton fabric, and polyester fabric. Rate constants were determined for the experimental evaporation of these compounds from each interface and class- and substrate-specific models were developed. For all compounds, evaporation was slowest from the glass dish compared to the other substrates, indicating a difference in the chemical interactions between the hydrocarbons and interface or a physical difference, such as porosity or spreading of the liquid. For the remaining three interfaces, rate constants for each compound were similar, although the kinetics were consistently slower from the polyester fabric compared to the cotton fabric. Overall, the models were comparable to both each other as well as the original class-specific models suggesting an applicability of the model across several different substrates without the need for further refinement.
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