Modeling the effect of temperature on solid-phase microextraction of volatile organic compounds from air by polydimethylsiloxane coating using finite element analysis.

A development of analytical methods based on solid-phase microextraction (SPME) is a very time- and labor-consuming task. The finite element methods have found a wide application in SPME modeling for faster and more accurate optimization of analytical methods. In this work, a computational model for predicting the effect of temperature on extraction of VOCs from air onto SPME coating based on polydimethylsiloxane (PDMS) has been developed using COMSOL Multiphysics® (CMP) software. Most suitable methods and models for estimating the diffusion coefficients of analytes in air and coating, and coating-air distribution constants of the analytes at different extraction temperatures were chosen. The Fuller method was chosen for calculating diffusion coefficients of analytes in air due to its simplicity and reliability. Coating-air distribution constants at different temperatures were estimated using van't Hoff equation. A combination of inverse gas chromatography on a capillary column with a similar stationary phase for estimating diffusion coefficients and linear temperature programmed retention indices (LTPRI) for estimating coating-air distribution constants at initial temperature were chosen for modeling purposes because in most cases it provided lowest values of root-mean-square difference from experimental extraction profiles from 125 mL bulb at 25 and 40 °C. The developed model can be recommended for faster and simpler optimization of the methods of air sampling using PDMS SPME fiber. It can also be used for obtaining extraction profiles at fluctuating temperatures.

Determination of transformation products of unsymmetrical dimethylhydrazine in water using vacuum-assisted headspace solid-phase microextraction.

A new, sensitive and simple method based on vacuum-assisted headspace solid-phase microextraction (Vac-HSSPME) followed by gas chromatography-mass-spectrometry (GC-MS), is proposed for the quantification of rocket fuel unsymmetrical dimethylhydrazine (UDMH) transformation products in water samples. The target transformation products were: pyrazine, 1-methyl-1H-pyrazole, N-nitrosodimethylamine, N,N-dimethylformamide, 1-methyl-1Н-1,2,4-triazole, 1-methyl-imidazole and 1H-pyrazole. For these analytes and within shorter sampling times, Vac-HSSPME yielded detection limits (0.5-100 ng L-1) 3-10 times lower than those reported for regular HSSPME. Vac-HSSPME sampling for 30 min at 50 °C yielded the best combination of analyte responses and their standard deviations (<15%). 1-Formyl-2,2-dimethylhydrazine and formamide were discarded because of the poor precision and accuracy when using Vac-HSSPME. The recoveries for the rest of the analytes ranged between 80 and 119%. The modified Mininert valve and Thermogreen septum could be used for automated extraction as it ensured stable analyte signals even after long waiting times (>24 h). Finally, multiple Vac-HSSME proved to be an efficient tool for controlling the matrix effect and quantifying UDMH transformation products.