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

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 vapor-equilibration 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 (C20H42) 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 ΔvapH = 91.27 ± 0.28 kJ/mol compared to ΔvapH(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.

Dynamic vapor microextraction of ignitable liquid from casework containers.

Dynamic vapor microextraction (DVME) is a headspace concentration method that can be used to collect ignitable liquid (IL) from fire debris onto chilled adsorbent capillaries. Unlike passive headspace concentration onto activated carbon strips (ACSs) that must be eluted with a toxic solvent (carbon disulfide), DVME employs a relatively benign solvent (acetone) to recover the adsorbed IL residue, and each headspace collection is monitored for breakthrough. Here, for the first time, we extend DVME to casework containers while exploring a realistic range of oven temperatures and collection volumes. We investigated metal cans sealed with friction lids (container 1), metal cans sealed within polymer bags (container 2), and glass jars sealed with two-piece lids (container 3). Without additional containment, container 1 was found to leak so excessively that flow through the capillary was unreliable. Therefore, for containers 2 and 3 only, we determined the total number of target compounds collected from 50% weathered gasoline for oven temperatures from 54 °C to 96 °C and collection volumes from 47 standard cubic centimeters (scc) to 90 scc. Only high-volatility species with retention times (tR)< n-decane on a non-polar column were recovered from polymer bags, whereas headspace concentration from glass jars led to the recovery of target compounds across the entire volatility range. DVME at 90 °C from 2-mL containers showed that the presence of polymer bag material leads to IL vapor losses, particularly for low-volatility species with tR> n-decane. DVME was strongly influenced by the casework container, whereas oven temperature and collection volume had a minor influence for the IL samples explored here.

Preservation of vapor samples on adsorbent alumina capillaries and implications for field sampling.

Dynamic vapor microextraction (DVME) is a vapor preconcentration method that employs a capillary trap coated with an adsorbent, followed by solvent elution to recover the sample. DVME has been developed for applications in the laboratory, including highly precise vapor pressure measurements, and in the field. When vapor collection is conducted outside the laboratory, samples must almost always undergo some interval of storage representing the time between collection and analysis. This interval may be hours, days, or longer, depending on the situation. Regardless, in all situations there must be confidence that the integrity of the samples is maintained until processing and analysis. In this paper, we present results of two studies that tested the stability of a 50% weathered gasoline headspace sample on alumina PLOT (porous layer open tubular) capillaries stored at room temperature for periods from 24 h up to 20 wk. We used principal component analysis (PCA) to reduce the dimensionality of the chromatographic and mass spectral data and elucidate trends in stability with respect to the complex sample's range of hydrocarbon classes and molecular weights. Both analyses identified changes over storage periods of six weeks or more. The hydrocarbon class analysis, which used selected ion monitoring (SIM) data as input, proved more sensitive to changes over shorter storage periods. Sample integrity was preserved for at least 24 h, but losses, especially of high-volatility compounds, occurred by 168 h (7 d). Near total loss of sample occurred by 20 wk. These findings, which are specific to the sample, adsorbent, and storage conditions, will guide choices in experimental and instrumental design to ensure that data from future field studies is reliable.

A Distillation Approach to Phase Equilibrium Measurements of Multicomponent Fluid Mixtures.

By building on the Advanced Distillation Curve (ADC) approach to measuring the volatility of fuels and other fluid mixtures, the ADC with Reflux or ADCR technique was developed to address the difficulty of experimentally determining the vapor-liquid equilibrium of fluids containing many components. For fuels and other multicomponent mixtures, the ADCR collects data about the chemical compositions of both liquid and vapor phases across a range of temperatures, elucidating the two-phase region at constant pressure. Two simple mixtures were used to demonstrate the ADCR method: an n-decane/n-tetradecane binary and the Huber-Bruno surrogate, a ternary mixture designed to represent the volatility of an aviation turbine kerosene. These mixtures were chosen to test the method because they have been extensively studied and modeled in previous work. For both test fluids, the ADCR measurements of vapor-liquid equilibrium were in good agreement with model predictions. We conclude that the ADCR is a useful method for determining the T-P-x-y behavior of fluid mixtures with many components. The experimental approach presented may support the development of fuels, design of separations, and forensic sciences that use vapor analysis, especially arson fire debris analysis, by providing quantitative data with well-characterized uncertainty describing the relationships between the vapor and condensed phases of a fuel subjected to thermal weathering.

Occupational exposure to volatile organic compounds and health risks in Colorado nail salons.

Nail salon technicians face chronic exposure to volatile organic compounds (VOCs), which can lead to adverse health outcomes including cancer. In this study, indoor levels of formaldehyde, as well as benzene, toluene, ethylbenzene and xylene, were measured in 6 Colorado nail salons. Personal exposure VOC measurements and health questionnaires (n = 20) were also performed; questionnaires included employee demographics, health symptoms experienced, and protective equipment used. Cancer slope factors from the United States Environmental Protection Agency (US EPA) and anthropometric data from the Centers for Disease Control and Prevention were then used to estimate cancer risk for workers, assuming 20-yr exposures to concentrations of benzene and formaldehyde reported here. Results show that 70% of surveyed workers experienced at least one health issue related to their employment, with many reporting multiple related symptoms. Indoor concentrations of formaldehyde ranged from 5.32 to 20.6 µg m-3, across all 6 salons. Indoor concentrations of toluene ranged from 26.7 to 816 µg m-3, followed by benzene (3.13-51.8 µg m-3), xylenes (5.16-34.6 µg m-3), and ethylbenzene (1.65-9.52 µg m-3). Formaldehyde levels measured in one salon exceeded the Recommended Exposure Limit from the National Institute for Occupational Safety and Health. Cancer risk estimates from formaldehyde exposure exceeded the US EPA de minimis risk level (1 × 10-6) for squamous cell carcinoma, nasopharyngeal cancer, Hodgkin's lymphoma, and leukemia; leukemia risk exceeded 1 × 10-4 in one salon. The average leukemia risk from benzene exposure also exceeded the US EPA de minimis risk level for all demographic categories modeled. In general, concentrations of aromatic compounds measured here were comparable to those measured in studies of oil refinery and auto garage workers. Cancer risk models determined that 20-yr exposure to formaldehyde and benzene concentrations measured in this study will significantly increase worker's risk of developing cancer in their lifetime.

Field portable low temperature porous layer open tubular cryoadsorption headspace sampling and analysis part II: Applications.

This paper details the sampling methods used with the field portable porous layer open tubular cryoadsorption (PLOT-cryo) approach, described in Part I of this two-part series, applied to several analytes of interest. We conducted tests with coumarin and 2,4,6-trinitrotoluene (two solutes that were used in initial development of PLOT-cryo technology), naphthalene, aviation turbine kerosene, and diesel fuel, on a variety of matrices and test beds. We demonstrated that these analytes can be easily detected and reliably identified using the portable unit for analyte collection. By leveraging efficiency-boosting temperature control and the high flow rate multiple capillary wafer, very short collection times (as low as 3s) yielded accurate detection. For diesel fuel spiked on glass beads, we determined a method detection limit below 1 ppm. We observed greater variability among separate samples analyzed with the portable unit than previously documented in work using the laboratory-based PLOT-cryo technology. We identify three likely sources that may help explain the additional variation: the use of a compressed air source to generate suction, matrix geometry, and variability in the local vapor concentration around the sampling probe as solute depletion occurs both locally around the probe and in the test bed as a whole. This field-portable adaptation of the PLOT-cryo approach has numerous and diverse potential applications.

Electron-Transfer Dynamics for a Donor-Bridge-Acceptor Complex in Ionic Liquids.

Intramolecular photoinduced electron transfer from an N,N-dimethyl-p-phenylenediamine donor bridged by a diproline spacer to a coumarin 343 acceptor was studied using time-resolved fluorescence measurements in three ionic liquids and in acetonitrile. The three ionic liquids have the bis[(trifluoromethyl)sulfonyl]amide anion paired with the tributylmethylammonium, 1-butyl-1-methylpyrrolidinium, and 1-decyl-1-methylpyrrolidinium cations. The dynamics in the two-proline donor-bridge-acceptor complex are compared to those observed for the same donor and acceptor connected by a single proline bridge, studied previously by Lee et al. (J. Phys. Chem. C 2012, 116, 5197). The increased conformational freedom afforded by the second bridging proline resulted in multiple energetically accessible conformations. The multiple conformations have significant variations in donor-acceptor electronic coupling, leading to dynamics that include both adiabatic and nonadiabatic contributions. In common with the single-proline bridged complex, the intramolecular electron transfer in the two-proline system was found to be in the Marcus inverted regime.

Analysis of arson fire debris by low temperature dynamic headspace adsorption porous layer open tubular columns.

In this paper we present results of the application of PLOT-cryoadsorption (PLOT-cryo) to the analysis of ignitable liquids in fire debris. We tested ignitable liquids, broadly divided into fuels and solvents (although the majority of the results presented here were obtained with gasoline and diesel fuel) on three substrates: Douglas fir, oak plywood and Nylon carpet. We determined that PLOT-cryo allows the analyst to distinguish all of the ignitable liquids tested by use of a very rapid sampling protocol, and performs better (more recovered components, higher efficiency, lower elution solvent volumes) than a conventional purge and trap method. We also tested the effect of latency (the time period between applying the ignitable liquid and ignition), and we tested a variety of sampling times and a variety of PLOT capillary lengths. Reliable results can be obtained with sampling time periods as short as 3min, and on PLOT capillaries as short as 20cm. The variability of separate samples was also assessed, a study made possible by the high throughput nature of the PLOT-cryo method. We also determined that the method performs better than the conventional carbon strip method that is commonly used in fire debris analysis.