Prediction of gas collection efficiency and particle collection artifact for atmospheric semivolatile organic compounds in multicapillary denuders

A modeling approach is presented to predict the sorptive sampling collection efficiency of gaseous semivolatile organic compounds (SOCs) and the artifact caused by collection of particle-associated SOCs in multicapillary diffusion denuders containing polydimethylsiloxane (PDMS) stationary phase. Approaches are presented to estimate the equilibrium PDMS–gas partition coefficient (K_pdms) from a solvation parameter model for any compound, and, for nonpolar compounds, from the octanol–air partition coefficient (K_oa) if measured K_pdms values are not available. These estimated K_pdms values are compared with K_pdms measured by gas chromatography. Breakthrough fraction was measured for SOCs collected from ambient air using high-flow (300 L min⁻¹) and low-flow (13 L min⁻¹) denuders under a range of sampling conditions (−10 to 25 °C; 11–100% relative humidity). Measured breakthrough fraction agreed with predictions based on frontal chromatography theory using K_pdms and equations of Golay, Lövkvist and Jönsson within measurement precision. Analytes included hexachlorobenzene, 144 polychlorinated biphenyl congeners, and polybrominated diphenyl ethers 47 and 99. Atmospheric particle transmission efficiency was measured for the high-flow denuder (0.037–6.3 μm diameter), and low-flow denuder (0.015–3.1 μm diameter). Particle transmission predicted using equations of Gormley and Kennedy, Pich, and a modified filter model, agreed within measurement precision (high-flow denuder) or were slightly greater than (low-flow denuder) measured particle transmission. As an example application of the model, breakthrough volume and particle collection artifact for the two denuder designs were predicted as a function of K_oa for nonpolar SOCs. The modeling approach is a necessary tool for the design and use of denuders for sorptive sampling with PDMS stationary phase.     

Direct thermal desorption of semivolatile organic compounds from diffusion denuders and gas chromatographic analysis for trace concentration measurement

A novel method for collection and analysis of vapor-phase semivolatile organic compounds (SOCs) in ambient air is presented. The method utilizes thermal desorption of SOCs trapped in diffusion denuders coupled with cryogenic preconcentration on Tenax-TA and analysis by high resolution gas chromatography (GC)-electron-capture detection (ECD). The sampling and analysis methods employ custom-fabricated multicapillary diffusion denuders, a hot gas spike (HGS) apparatus to load known quantities of thermally stable standards into diffusion denuders prior to sample collection, a custom-fabricated oven to thermally desorb SOCs from the diffusion denuder, and a programmable temperature vaporization (PTV) inlet containing a liner packed with Tenax-TA for effective preconcentration of the analytes and water management. High flow rates into the PTV inlet of 750mLmin(-1)during thermal desorption are ca. a factor of ten greater than typically used. To improve resolution and retention time stability, the thermal desorption and PTV inlet programming procedure includes three steps to prevent water from entering the analytic column while effectively transferring the analytes into the GC system. The instrumentation and procedures provide virtually complete and consistent transfer of analytes collected from ambient air into the GC evidenced by recovery of seven replicates of four internal standards of 90.7+/-4.0-120+/-23% (mean+/-95% confidence interval, CI). Retention time based compound identification is facilitated by low retention time variability with an average 95% CI of 0.024min for sixteen replicates of eight standards. Procedure details and performance metrics as well as ambient sampling results are presented.     

Solitary wave solutions to nonlinear evolution equations in mathematical physics

This paper obtains solitons as well as other solutions to a few nonlinear evolution equations that appear in various areas of mathematical physics. The two analytical integrators that are applied to extract solutions are tan–cot method and functional variable approaches. The soliton solutions can be used in the further study of shallow water waves in (1+1) as well as (2+1) dimensions.     

Gas-phase cleanup method for analysis of trace atmospheric semivolatile organic compounds by thermal desorption from diffusion denuders

A novel gas-phase cleanup method was developed for use with a thermal desorption method for analysis of trace semivolatile organic compounds (SOCs) in the atmosphere using diffusion denuder samplers to separate gas-phase from particle-associated fractions. The cleanup selectively removed hydrogen-bonding chemicals from samples, including much of the background matrix of oxidized organic compounds that is present in ambient air samples. Abraham solvation parameters were found to be useful predictors of recovery of compounds through the cleanup method; most compounds with A + B < 0.3 and L ≤ 12.3 were fully recovered through the cleanup method. Addition of the cleanup method successfully produced baseline resolution in air samples and improved method precision. The utility of the method was demonstrated in an investigation of the built environment as a continuing source of semivolatile persistent, bioaccumulative, and toxic chemicals (PBTs) to the atmosphere.