Generation of Sub-Part-per-Billion Gaseous Volatile Organic Compounds at Ambient Temperature by Headspace Diffusion of Aqueous Standards through Decoupling between Ideal and Nonideal Henry’s Law Behavior

The feasibility and temporal storability of gas phase standards of volatile organic compounds prepared through liquid phase vaporization in polyester aluminum bags at room temperature

Currently, the presence of many classes of volatile organic compounds (VOCs) in indoor air is well recognized. There is an impetus to accurately quantify airborne VOCs for the proper assessment of their human health risks. VOC standards prepared in a solvent are often vaporized in N₂ gas-filled sampling bags for external calibration as the use of grab sampling bags is a common practice for the collection of real ambient air samples. Such practices can nontheless be subject to many sources of biases in their calibration (e.g., VOC chemical reaction with the solvent, adsorption on the bag interior surface, or leakage). The main goal of this work is to measure the temporal stability of 11 VOC targets (benzene, toluene, o-xylene, styrene, propionaldehyde, n-butyraldehyde, isovaleraldehyde, valeraldehyde, acrylonitrile, isoprene, and methyl ethyl ketone) selected in this research over 24 h which started 10 min after the injection and vaporization of liquid-phase standards (all prepared in methanol solvent) into polyester aluminum (PEA) bags containing 1 atm N₂. Although all tested VOCs showed gradual decreases of their concentrations (e.g., >17% in 24 h), the aromatic hydrocarbon VOCs (namely BTXS) yielded the best relative recoveries (e.g., decreases of 11%-30% in 24 h) and relative errors (e.g., relative standard error (RSE) = 2.14-3.59%) in 5 replicate tests. A good linear relationship was established between the 24 h VOC relative recovery and molecular weight (R² > 0.81). The results of this study offers valuable clues to properly reduce the bias in the calibration of gas-phase VOC standards when calibrating the system through the vaporization of liquid-phase VOC standards prepared in a solvent.

Performance comparison of MOF and other sorbent materials in removing key odorants emitted from pigpen slurry

A batch-type dynamic headspace (HS) system was used to generate vapor-phase volatile organic compounds (VOCs) from a pigpen slurry sample. Sorptive removal capability of MOF-199 and other sorbents (zeolite (ZL) and activated carbon (AC)) was assessed against a total of 13 slurry-borne odorants ((methyl ethyl ketone (MEK), isobutyl alcohol (i-BuAl), benzene (B), toluene (T), p-xylene (p-X), m-xylene (m-X), o-xylene (o-X), styrene (S), o-cresol (o-C), phenol (PhAl), p-cresol (p-C), indole (ID), and skatole (SK)). Adsorption capacity of MOF-199 and two sorbents, when assessed for the 13 odorants at a 10% breakthrough volume (BTV), was 22.6  ±  42.3, 0.70  ±  1.08, and 11.0  ±  18.3 μg g(-1), respectively. The adsorption capacity (μg g(-1)) assessed at 10% BTV showed the superiority of MOF-199 towards phenolic and indolic compounds (such as o-C (0.31  ±  0.04), PhAl (61.6  ±  4.98), p-C (140  ±  7.95), ID (27.8  ±  2.23), and SK (63.9  ±  1.55)), demonstrating the feasibility of MOF as sorption media for treating certain nuisance components.

Application of chemical vapor generation systems to deliver constant gas concentrations for in vitro exposure to volatile organic compounds

Exposure to volatile organic compounds from outdoor air pollution is a major public health concern; however, there is scant information about the health effects induced by inhalation exposure to photochemical transformed products of primary emissions. In this study, we present a stable and reproducible exposure method to deliver ppm-ppb levels of gaseous standards in a humidified air stream for in vitro cell exposure through a direct air-liquid interface. Gaseous species were generated from a diffusion vial, and coupled to a gas-phase in vitro exposure system. Acrolein and methacrolein, which are major first-generation photochemical transformation products of 1,3-butadiene and isoprene, respectively, were selected as model compounds. A series of vapor concentrations (0.23-2.37 ppmv for acrolein and 0.68-10.7 ppmv for methacrolein) were investigated to characterize the exposure dose-response relationships. Temperature and the inner diameter of the diffusion vials are key parameters to control the evaporation rates and diffusion rates for the delivery of target vapor concentrations. Our findings suggest that this exposure method can be used for testing a wide range of atmospheric volatile organic compounds, and permits both single compound and multiple compound sources to generate mixtures in air. The relative standard deviations (%RSD) of output concentrations were within 10% during the 4-hour exposure time. The comparative exposure-response data allow us to prioritize numerous hazardous gas phase air pollutants. These identified pollutants can be further incorporated into air quality simulation models to better characterize the environmental health risks arising from inhalation of the photochemical transformed products.

Extent of sample loss on the sampling device and the resulting experimental biases when collecting volatile fatty acids (VFAs) in air using sorbent tubes

Not all volatile organic compounds (VOCs) are suitable for sampling from air onto sorbent tubes (ST) with subsequent analysis by thermal desorption (TD) with gas chromatography (GC). Some compounds (such as C2 hydrocarbons) are too volatile for quantitative retention by sorbents at ambient temperature, while others are too reactive - either for storage stability on the tubes (post-sampling) or for thermal desorption/GC analysis. Volatile fatty acids (VFAs) are one of the compound groups that present a challenge to sorbent tube sampling. In this study, we evaluated sample losses on the inner wall surface of the sorbent tube sampler. The sorptive losses of five VFA (acetic, propionic, n-butyric, i-valeric, and n-valeric acid) were tested using two types of tubes (stainless steel and quartz), each packed with three sorbent beds arranged in order of sorbent strength from the sampling end of the tube (Tenax TA, Carbopack B, and Carbopack X). It showed significantly higher losses of VFAs in both liquid phase and vapor phase when using stainless steel tube samplers. These losses were also seen if vapor-phase fatty acids were passed through empty stainless steel tubing and increased dramatically with increasing molecular weight, e.g., losses of 33.6% (acetic acid) to 97.5% (n-valeric acid). Similar losses of VFAs were also observed from headspace sampling of cheese products. Considering that stainless steel sampling tubes are still used extensively by many researchers, their replacement with quartz tubes is recommended to reduce systematic biases in collecting VFA samples or in their calibration.