Gas-phase Concentrations Research Articles

Direct quantification of chemical warfare agents and related compounds at low ppt levels: comparing active capillary dielectric barrier discharge plasma ionization and secondary electrospray ionization mass spectrometry.

A novel active capillary dielectric barrier discharge plasma ionization (DBDI) technique for mass spectrometry is applied to the direct detection of 13 chemical warfare related compounds, including sarin, and compared to secondary electrospray ionization (SESI) in terms of selectivity and sensitivity. The investigated compounds include an intact chemical warfare agent and structurally related molecules, hydrolysis products and/or precursors of highly toxic nerve agents (G-series, V-series, and "new" nerve agents), and blistering and incapacitating warfare agents. Well-defined analyte gas phase concentrations were generated by a pressure-assisted nanospray with consecutive thermal evaporation and dilution. Identification was achieved by selected reaction monitoring (SRM). The most abundant fragment ion intensity of each compound was used for quantification. For DBDI and SESI, absolute gas phase detection limits in the low ppt range (in MS/MS mode) were achieved for all compounds investigated. Although the sensitivity of both methods was comparable, the active capillary DBDI sensitivity was found to be dependent on the applied AC voltage, thus enabling direct tuning of the sensitivity and the in-source fragmentation, which may become a key feature in terms of field applicability. Our findings underline the applicability of DBDI and SESI for the direct, sensitive detection and quantification of several CWA types and their degradation products. Furthermore, they suggest the use of DBDI in combination with hand-held instruments for CWAs on-site monitoring.

Emission of phthalates and phthalate alternatives from vinyl flooring and crib mattress covers: the influence of temperature.

Emissions of phthalates and phthalate alternatives from vinyl flooring and crib mattress covers were measured in a specially designed chamber. The gas-phase concentrations versus time were measured at four different temperatures, that is, 25, 36, 45, and 55 °C. The key parameter that controls the emissions (y0, gas-phase concentration in equilibrium with the material phase) was determined, and the emissions were found to increase significantly with increasing temperature. Both the material-phase concentration (C0) and the chemical vapor pressure (Vp) were found to have great influence on the value of y0. The measured ratios of C0 to y0 were exponentially proportional to the reciprocal of temperature, in agreement with the van't Hoff equation. A emission model was validated at different temperatures, with excellent agreement between model calculations and chamber observations. In residential homes, an increase in the temperature from 25 to 35 °C can elevate the gas-phase concentration of phthalates by more than a factor of 10, but the total airborne concentration may not increase that much for less volatile compounds. In infant sleep microenvironments, an increase in the temperature of mattress can cause a significant increase in emission of phthalates from the mattress cover and make the concentration in the infant's breathing zone about four times higher than that in the bulk room air, resulting in potentially high exposure.

Source apportionment of atmospheric PAHs and their toxicity using PMF: Impact of gas/particle partitioning

24-h PM2.5 samples were simultaneously collected at six sites in a subtropical city of South China during November–December, 2009. Particle-phase concentrations of polycyclic aromatic hydrocarbons (PAHs) and organic tracers such as hopanes for vehicular emissions (VE), levoglucosan for biomass burning (BB) and picene for coal combustion (CC) were determined. Meanwhile, their gas-phase concentrations were calculated from gas/particle (G/P) partitioning theory using the particle-phase concentrations. The 4 ring PAHs (fluoranthene to chrysene) had lower particle-phase fractions (10%–79%) than other species. Estimated BaPeq and lifetime cancer risk for particle-only (P-only) vs gas + particle (G + P) data sets showed similar values, indicating PAHs with 5–7 rings dominated the carcinogenicity of PAHs. Positive Matrix Factorization (PMF) was applied on both P-only and G + P data sets to estimate the source contributions to PAHs and their toxicity. Three common sources were identified: VE, BB and CC, with CC as the most significant source for both particulate (58%) and total (G + P, 40%) PAHs. While CC exhibited consistent contributions to BaPeq for P-only (66%) vs G + P (62%) solutions, VE and BB contributions were under- and overestimated by 68% and 47%, respectively by the P-only solution, as compared to the G + P solution. The results provide an insight on the impact of G/P partitioning on the source apportionment of PAHs and their toxicity.

Sources and fate of polycyclic aromatic hydrocarbons in the Antarctic and Southern Ocean atmosphere

AbstractPolycyclic aromatic hydrocarbons (PAHs) are a geochemically relevant family of semivolatile compounds originating from fossil fuels, biomass burning, and their incomplete combustion, as well as biogenic sources. Even though PAHs are ubiquitous in the environment, there are no previous studies of their occurrence in the Southern Ocean and Antarctic atmosphere. Here we show the gas and aerosol phase PAHs concentrations obtained from three sampling cruises in the Southern Ocean (Weddell, Bellingshausen, and South Scotia Seas), and two sampling campaigns at Livingston Island (Southern Shetlands). This study shows an important variability of the atmospheric concentrations with higher concentrations in the South Scotia and northern Weddell Seas than in the Bellingshausen Sea. The assessment of the gas‐particle partitioning of PAHs suggests that aerosol elemental carbon contribution is modest due to its low concentrations. Over the ocean, the atmospheric concentrations do not show a temperature dependence, which is consistent with an important role of long‐range atmospheric transport of PAHs. Conversely, over land at Livingston Island, the PAHs gas phase concentrations increase when the temperature increases, consistently with the presence of local diffusive sources. The use of fugacity samplers allowed the determination of the air‐soil and air‐snow fugacity ratios of PAHs showing that there is a significant volatilization of lighter molecular weight PAHs from soil and snow during the austral summer. The higher volatilization, observed in correspondence of sites where the organic matter content in soil is higher, suggests that there may be a biogenic source of some PAHs. The volatilization of PAHs from soil and snow is sufficient to support the atmospheric occurrence of PAHs over land but may have a modest regional influence on the atmospheric occurrence of PAHs over the Southern Ocean.

Missing SO<sub>2</sub> oxidant in the coastal atmosphere? – observations from high-resolution measurements of OH and atmospheric sulfur compounds

Abstract. Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in NE Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected-ion chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 104 cm−3 at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analyzed in conjunction with the condensational sink for both compounds derived from 3 nm to 10 μm (aerodynamic diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed, leading to estimated atmospheric lifetimes on the order of 7 and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal, evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s), its ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated H2SO4 concentrations were consistently lower than the measured concentrations by a factor of 4.7 ± 2.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J(O1D), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis and photooxidation of biogenic iodine compounds. As to the identity of the atmospheric SO2 oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on {ab initio} calculations. Nevertheless, IO could contribute significantly to the observed CIMS background signal. A detailed analysis of this CIMS background signal in context with recently published kinetic data currently suggests that Criegee intermediates (CIs) produced from ozonolysis of alkenes play no significant role for SO2 oxidation in the marine atmosphere at Mace Head. On the other hand, SO2 oxidation by small CIs such as CH2OO produced photolytically or possibly in the photochemical degradation of methane is consistent with our observations. In addition, H2SO4 formation from dimethyl sulfide oxidation via SO3 as an intermediate instead of SO2 also appears to be a viable explanation. Both pathways need to be further explored.

Inverse gas chromatography for identification of poly(ethylene glycol adipate) and tripropionitrilamine on the surface of polymeric sorbent

Identification of the adsorption of poly(ethylene glycol adipate) (PEGA) and tripropionitrilamine (TPA) on styrene-divinylbenzene porous polymer was performed with the use of the inverse gas-chromatography method in finite concentration. PEGA and TPA were applied in quantities ω = 10−6−10−3 mass fractions. As test sorbates n-propanol, n-butanol, and n-hexane used. The adsorption isotherm and the values of specific retention volumes were calculated via the Glueckauf method. It was found that the dependence of the retention volumes on the sorbate concentration in gas phase of the studied samples is well approximated by a power function corresponding to the Freundlich isotherm. The possibility of detecting PEGA and TPA on the surface of sorbent through the A coefficients of a power function was shown, and a linear dependence between the A values and negative logarithm of ω was also found, making it possible to quantitatively determine TPA and PEGA on the surface of a sorbent.

Gas/particle partitioning of PCDD/F compounds in the atmosphere of Istanbul

Gas/particle partitioning of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) compounds in the ambient atmosphere were investigated at three different sites (urban-industrial, urban and sub-urban) in İstanbul. Average gas and particle phase concentrations were measured as 133fgm−3 and 1605fgm−3, respectively. Gas phase concentrations of polychlorinated dibenzo-p-dioxin/furan (PCDD/F) compounds were determined to be 128fgm−3, 50fgm−3, 153fgm−3 during summer season and 204fgm−3, 164fgm−3, 154fgm−3 during winter season for the respective three sampling sites. Particle phase concentrations were determined to be 287fgm−3, 176fgm−3, 160fgm−3 during summer and 6586fgm−3, 2570fgm−3 and 1861fgm−3 during winter season for those three sampling sites. Chlorination level and molecular weight of congeners affected gas/particle partitioning of PCDD/F compounds. Gas phase percentages of 2,3,7,8-TCDD and OCDD concentrations were determined to be 47% and 1% respectively. A relatively high correlation was found between total particle matter (TPM) and particle phase PCDD/F concentration during winter season.

Cavity collapse in a bubbly liquid

The effect of an ambient liquid state on a spherical cavity dynamics under atmospheric hydrostatic pressure p and extremely low initial gas pressure p(0) inside was investigated. The equilibrium bubbly (cavitating) medium with sound velocity C as the function of gas phase concentration k was considered as the ambient liquid model. The cavity dynamics is analyzed within the framework of Herring-equation for the following diapasons of main parameters : k = 0—5%, р(0) = 0.02—10(−6) atm. Numerical analysis has shown that the deceleration C by two order does not have an influence neither on an asymptotic value of collapsed cavity radius nor on the acoustical losses under its collapse. It means than in the whole the integral acoustical losses remain invariable. However the collapse cavity dynamics and the radiation structure are essentially changed: from numerous pulsations with a decreasing amplitudes up to a single collapse and from a wave packet up to a single wave, correspondingly. It has turned out that the acoustic corrections in the Herring-equation don’t influence practically on the cavity dynamics if the term of equation with dH/dt is absent. Naturally, the deceleration C exerts essential influence on an empty cavity dynamics. The graphs of dR/Cdt values as a function of R/Ro for different C values are located higher the data of classical models of Herring, Gilmore and Hunter. So the value M = 1 is reached at R/Ro = 0.023 for k = 0, and at the value 0.23 when k = 5 %. [Support RFBR, grant 12-01-00314.]

Inhaled diesel emissions generated with cerium oxide nanoparticle fuel additive induce adverse pulmonary and systemic effects.

Diesel exhaust (DE) exposure induces adverse cardiopulmonary effects. Cerium oxide nanoparticles added to diesel fuel (DECe) increases fuel burning efficiency but leads to altered emission characteristics and potentially altered health effects. Here, we evaluated whether DECe results in greater adverse pulmonary effects compared with DE. Male Sprague Dawley rats were exposed to filtered air, DE, or DECe for 5 h/day for 2 days. N-acetyl glucosaminidase activity was increased in bronchial alveolar lavage fluid (BALF) of rats exposed to DECe but not DE. There were also marginal but insignificant increases in several other lung injury biomarkers in both exposure groups (DECe > DE for all). To further characterize DECe toxicity, rats in a second study were exposed to filtered air or DECe for 5 h/day for 2 days or 4 weeks. Tissue analysis indicated a concentration- and time-dependent accumulation of lung and liver cerium followed by a delayed clearance. The gas-phase and high concentration of DECe increased lung inflammation at the 2-day time point, indicating that gas-phase components, in addition to particles, contribute to pulmonary toxicity. This effect was reduced at 4 weeks except for a sustained increase in BALF γ-glutamyl transferase activity. Histopathology and transmission electron microscopy revealed increased alveolar septa thickness due to edema and increased numbers of pigmented macrophages after DECe exposure. Collectively, these findings indicate that DECe induces more adverse pulmonary effects on a mass basis than DE. In addition, lung accumulation of cerium, systemic translocation to the liver, and delayed clearance are added concerns to existing health effects of DECe.

Assessing worker exposure to inhaled volatile organic compounds from Marcellus Shale flowback pits

Natural gas drilling sites employing hydraulic fracturing present a potential source of inhalation exposure to volatile organic compounds (VOCs) via the use of flowback pits. These open-air pits are used as a means of storing flowback water, a waste product of hydraulic fracturing, and represent an understudied source of VOC exposure for workers. The objective of this study was to assess this worker exposure and the resulting health risks for 12 VOCs present in flowback water stored in such an open reservoir on a drilling site. Flowback pit VOC mean, 2.5 percentile, and 97.5 percentile concentrations were used to model aqueous phase concentrations, and two models of volatilization were applied to estimate flux to the gas phase. A mass-balance approach was used to estimate gas phase concentrations that were, in turn, used to estimate worker exposure. A literature review was performed to determine VOC health effects, exposure limits, and worker protection methods. Neither model demonstrated an increased risk of adverse effects due to subchronic exposure at the 2.5 percentile and mean concentration values for the 12 VOCs as indicated by hazard quotients, hazard indices, or excess lifetime cancer risks; however, 97.5 percentile hazard indices approached 1 in one model and did demonstrate unacceptable risks in the evaluation of limitations. Either model may apply to worker health assessment depending upon industry practice; however, differing weather conditions, industry practice, and the small number of VOCs evaluated necessitate further research regarding worker risks and health effects.

CFD Simulation of a Bubbling Fluidized Bed Gasifier Using a Bubble-Based Drag Model

Wood gasification in a fluidized bed reactor is regards as a promising technique to efficiently exploit the energy from biomass, which has attracted great attention. To further understand complex fluid mechanics and thermal behaviors of wood during the gasification process, a multiphase reactive model is presented, and a computational fluid dynamics (CFD) simulation of wood gasification in a bubbling fluidized bed gasifier is implemented. To account for the impact of bubble-structure on the gas–solid interaction, a revised bubble-based energy minimization multiscale (EMMS) drag model is developed and incorporated into a two-fluid model. Flow behaviors and reactive characteristics in a bubbling fluidized bed gasifier are analyzed. Profiles of temperature and concentration of gas and solid phase are also obtained. By comparisons of the outlet gas species concentrations, the prediction by a revised bubble-based EMMS drag model agrees better with experimental results than that by the conventional drag model.

Formaldehyde emission behavior of building materials: On-site measurements and modeling approach to predict indoor air pollution

The purpose of this paper was to investigate formaldehyde emission behavior of building materials from on-site measurements of air phase concentration at material surface used as input data of a box model to estimate the indoor air pollution of a newly built classroom. The relevance of this approach was explored using CFD modeling. In this box model, the contribution of building materials to indoor air pollution was estimated with two parameters: the convective mass transfer coefficient in the material/air boundary layer and the on-site measurements of gas phase concentration at material surfaces. An experimental method based on an emission test chamber was developed to quantify this convective mass transfer coefficient. The on-site measurement of gas phase concentration at material surface was measured by coupling a home-made sampler to SPME. First results had shown an accurate estimation of indoor formaldehyde concentration in this classroom by using a simple box model.

Modeling global persistent organic chemicals in clouds

A cloud model was implemented in a global atmospheric transport model to simulate cloud liquid water content and quantify the influence of clouds on gas/aqueous phase partitioning of persistent organic chemicals (POCs). Partitioning fractions of gas/aqueous and particle phases in clouds for three POCs α-hexachlorocyclohexane (α-HCH), polychlorinated biphenyl-28 (PCB-28), and PCB-138 in a cloudy atmosphere were estimated. Results show that the partition fraction of these selected chemicals depend on cloud liquid water content (LWC) and air temperature. We calculated global distribution of water droplet/ice particle–air partitioning coefficients of the three chemicals in clouds. The partition fractions at selected model grids in the Northern Hemisphere show that α-HCH, a hydrophilic chemical, is sorbed strongly onto cloud water droplets. The computed partition fractions at four selected model grids show that α-HCH tends to be sorbed onto clouds over land (source region) from summer to early fall, and over ocean from late spring to early fall. 20–60% of α-HCH is able to be sorbed to cloud waters over mid-latitude oceans during summer days. PCB-138, one of hydrophobic POCs, on the other hand, tends to be sorbed to particles in the atmosphere subject to air temperature. We also show that, on seasonal or annual average, 10–20% of averaged PCB-28 over the Northern Hemisphere could be sorbed onto clouds, leading to reduction of its gas-phase concentration in the atmosphere.

Latex paint as a delivery vehicle for diethylphthalate and di-n-butylphthalate: Predictable boundary layer concentrations and emission rates

The description of emission processes of volatile and semi-volatile organic compounds (VOCs and SVOCs) from building products requires a detailed understanding of the material and the air flow conditions at the surface boundary. The mass flux between the surface of the material and air depends on the mass transfer coefficient (hm) through the boundary layer, the gas phase concentration of the target compound immediately adjacent to the material (y0), and the gas-phase concentration in bulk air (y(t)). In the present study emission experiments were performed in two chambers of quite different sizes (0.25m3 and 55m3), and, in the larger chamber, at two different temperatures (23°C and 30°C). The emitting material was latex wall paint that had been doped with two plasticizers, diethylphthalate (DEP) and di-n-butylphthalate (DnBP). The phthalate content in the paint was varied in the small chamber experiment to evaluate the impact of the initial concentration in the bulk material (C0) on the emission rate. Boundary layer theory was applied to calculate hm for the specific phthalates from the Sherwood number (Sh) and the diffusion coefficient (Dair). Then y0 was determined based on the bulk gas-phase concentration at steady state (y¯). For both, DEP and DnBP, the y0 obtained was lower than the respective saturation vapor pressure (Ps). Furthermore, for both phthalates in latex paint, the material/air partition coefficient (C0/y0) was close in value to the octanol/air partition coefficient (KOA). This study provides a basis for designing phthalate emitting reference materials that mimic the emission behavior of common building materials.

The effect of ventilation on indoor exposure to semivolatile organic compounds.

A mechanistic model was developed to examine how natural ventilation influences residential indoor exposure to semivolatile organic compounds (SVOCs) via inhalation, dermal sorption, and dust ingestion. The effect of ventilation on indoor particle mass concentration and mass transfer at source/sink surfaces, and the enhancing effect of particles on mass transfer at source/sink surfaces are included. When air exchange rate increases from 0.6/h to 1.8/h, the steady-state SVOC (gas-phase plus particle phase with log KOA varying from 9 to 13) concentration in the idealized model decreases by about 60%. In contrast, for the same change in ventilation, the simulated indoor formaldehyde (representing volatile organic compounds) gas-phase concentration decreases by about 70%. The effect of ventilation on exposure via each pathway has a relatively insignificant association with the KOA of the SVOCs: a change of KOA from 10(9) to 10(13) results in a change of only 2-30%. Sensitivity analysis identifies the deposition rate of PM2.5 as a primary factor influencing the relationship between ventilation and exposure for SVOCs with log KOA = 13. The relationship between ventilation rate and air speed near surfaces needs to be further substantiated.

Chemical vapor infiltration of activated carbon with tetramethylsilane

Chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) at 200hPa total pressure and a gas phase concentration of 15 (mol-)% TMS in nitrogen is studied. The influence of temperature on the infiltration process is discussed in detail. Up to 873K, the infiltration is performed in the kinetically controlled regime resulting in high loadings up to around 42 (wt.-)%. The modified materials show high values for BET-surface and pore volume indicating a sufficient adoption of the infiltrated silicon layer to the surface morphology of the carbon substrates. Low oxidation resistance of the infiltrated material and EDX measurements give rise to the assumption that the infiltrated material is silicon. At higher infiltration temperatures above 873K, particles are formed which have the shape of cylindrical nanostructures. EDX measurements reveal that silicon carbide is produced at these temperatures.

Comparative Modeling and Analysis of the Mass Transfer Coefficient in a Turbulent Bed Contactor using Artificial Neural Network and Adaptive Neuro-Fuzzy Inference Systems

This study deals with predicting the gas film volumetric mass transfer coefficient (Koga) in a turbulent bed contactor (TBC), using both artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS) techniques. The networks have been trained and evaluated with the experimental data available in the literature. Input variables to the networks are process variables such as gas and liquid phase concentration, gas and liquid superficial velocities and also specific area of packings. The results obtained the ability of developed ANN and ANFIS for prediction of Koga. Although it was observed that both ANN and ANFIS models provided a good statistical prediction in terms of coefficient of determination (R2), mean relative error (MRE) and root mean square error (RMSE), the accuracies of ANFIS predictions were better than those of ANN predictions.

Use Horizontal Aeration Method to Treat Organic Pollution in Soil

This design adopts the method of horizontal arrangement to set the aerator pipes because it’s more effective than an upright way. The pipes are divided into two vertical levels. Firstly there should be an excavation of foundation pit or grooves to put the pipes in two levels. Then the pipes in two levels can be set at vertical directions. After burying the pipe the earth should be backfilled into the grooves. When backfilling the lower level groove, we put some certain microorganism to help cleaning the organic pollution. While working, the pipes above are mainly used to manufacturing gas phase concentration difference, letting the organic volatilize and goes into the atmosphere. The pipes in lower level are mainly used to provide oxygen to microorganisms in the soil, so that they can decompose organic and make them into inorganic which is no harm to the soil and ground water.

ADSORPTION OF CR-(VI) FROM AQUEOUS ENVIRONMENT USING NEEM LEAVES POWDER

This study is aimed at utilizing Neem Leaves powder as low cost adsorbent material for removal of Cr-(VI) from aqueous solution. Adsorption is an important process used for removal of colour, odour, turbidity, metal ions and reduction of COD. In adsorption, the solute present in dilute concentration in liquid or gas phase is extracted by contacting with suitable solid adsorbent so that the transfer of the component first takes place on the surface of solid and then into the pore of the solid. Neem Leaves powder Adsorbent used in present study is prepared at Laboratory scale which is very effective to remove chromium from its aqueous solution. This study describes the detailed experimental procedure to obtain Adsorption Equilibrium and the Effect of various parameters such pH, agitation time, initial metal ion concentration & adsorbent doses on batch technique.

Simulation of the breakthrough behavior of volatile organic compounds against sorbent tube sampler as a function of concentration level and sampling volume

The breakthrough (BT) properties of Tenax TA sorbent were challenged by gaseous standards containing a suite of 13 volatile organic compounds (VOC): (1) aromatic hydrocarbons: benzene (B), toluene (T), p-xylene (p-X), and styrene (S), (2) aldehydes: acetaldehyde (AA), propionaldehyde (PA), butyraldehyde (BA), isovaleraldehyde (IA), and valeraldehyde (VA), (3) ketones: methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK), and (4) two others: isobutyl alcohol (i-BuAl) and butyl acetate (BuAc). To this end, 1–3L of standards (10–50ppb) were loaded on the two sorbent tubes (ST) connected in series at 100mLmin−1. The front ST-1 was used for calibration purposes, while the ST-2 for breakthrough (recovery criterion of <1% with p-xylene as the key datum point). Although aromatic hydrocarbons generally met such criterion, benzene was readily distinguishable with the maximum BT. The BT for the aldehydes exhibited ∼100% (AA)≥85% (PA)≥45% (BA)≥30% (VA and IVA). There is good correlation between ST-2 recovery vs. carbon number for >CO entity (aldehydes, ester, and ketones). As such, BT is essentially concentration independent and relatively predictable across different functional groups and between the homologues. However, the BT behavior of ppb level VOCs is no longer consistent for certain species (like benzene or MEK) relative their ppm counterparts. This variation is explained by the Langmuir equation in which the 1/BTV is proportional to analyte gas-phase concentration, if the gas-phase/sorbent partition coefficient is large.