Solid phase microextraction (SPME) is a convenient and efficient extraction method that involves using a thin polymer film coating on a fine silica fiber to adsorb analytes of interests from a sample matrix. A theoretical model is proposed to deal with the dynamic adsorption process of SPME. In this model, mass diffusion from the matrix to the SPME polymer film is considered as the rate-determining step in reaching an adsorption equilibrium, and a steady-state diffusion is assumed for SPME in an effectively agitated sampling medium. Mathematical treatment of the adsorption process generates an expression that can describe experimental adsorption time profiles of the SPME process. The expression also provides a directly proportional relationship between the amount of analyte adsorbed by the SPME fiber and its initial concentration in the sample matrix. This relationship indicates that SPME quantification is feasible before an adsorption equilibrium is reached, once the agitation condition and the sampling time are held constant. The theoretical model is fitted with experimental data, and there is a very good agreement between them. Data plots of the adsorbed amount vs the initial concentration showed excellent linearity, with a sampling time much shorter than that required for reaching an adsorption equilibrium.
Solid phase microextraction (SPME) has remarkable advantages in headspace chemical analysis over conventional static headspace and purge-and-trap techniques in terms of its ease of use and superior reproducibility. A theoretical treatment of the headspace SPME process is proposed in this report that focuses on the mass transfer at the two interfaces, condensed phase/gas phase and gas phase/SPME polymer. The rate-determining step of the SPME process can be either the analyte evaporation from the condensed phase to its headspace or the analyte diffusion from the SPME polymer film surface into its inner layers. Mass transfer in the gas phase is considered a fast process. The mathematical solution provides an expression that correlates the amount of extracted analyte with its initial concentration in the condensed phase. A directly proportional relationship exists between them in all cases of headspace SPME, and this relationship indicates that SPME quantification is feasible before reaching a partition equilibrium once the SPME conditions and the sampling time are held constant. Experimental extraction−time profiles fit the theoretical predictions. Data plots of the extracted amount vs the initial concentration showed excellent linearity with a sampling time much shorter than that required to reach a partition equilibrium.
Previously proposed models (Ai, J. Anal. Chem. 1997, 69, 1230-6; 3260-6) dealing with the dynamics of solidphase microextraction (SPME) were based on the assumption that a steady-state mass transfer is in effect throughout the SPME extraction process. In reality, a steady-state mass transfer may not be established at the initial stage of headspace SPME, which involves mass transfer across two interfaces. The rate of analyte transfer at the condensed phase/headspace interface may not be equal to the extraction rate at the headspace/SPME polymer film interface. An improved model is proposed in this report to handle the situation of the non-steadystate mass transfer for the headspace SPME. A mathematical solution is obtained for the dynamic process of the non-steady-state mass transfer by correlating the variation of the analyte concentration in the headspace with the analyte extraction rate. This solution provides an equation relating the extracted amount of analyte to the extraction time a little more complicated than the previous models. It also gives a better description of experimental observations. Solid-phase microextraction (SPME) developed by Pawliszyn and co-workers 3,4 has great advantages for analyzing organic chemicals in the headspace over a condensed phase. During a static headspace analysis, chemicals with high affinity toward the SPME polymer film are concentrated in the film 5-10 and the extraction results in a higher sensitivity than conventional static headspace analysis. SPME is a partition process if the polymer film acts like a liquid phase. Once the partition equilibrium is attained, the extracted amount of analyte can be expressed as follows:Here n ∞ is the amount of analyte extracted by the SPME polymer film when partition equilibrium is attained. K fh and K hs are equilibrium partition constants for the analyte between the headspace and the polymer film and between the condensed phase and its headspace, respectively. V f , V h , and V s are volumes of the SPME polymer film, the headspace, and the sample matrix. C 0 is the initial concentration of the analyte in the sample matrix.The dynamic process of the SPME before partition equilibrium has also been investigated. Pawliszyn and co-workers used a diffusion equation to describe the SPME extraction process. 11-13 Based on the boundary conditions of SPME, the analytical solution is very complicated. 13 Direct application of the model to the experimental results is a difficult task. A simple dynamic model 2 with the focus on the mass transfer at the two interfaces was proposed to deal the dynamic process of headspace SPME. This model avoided the mathematical treatment for the second-order partial differential equation, and a simple analytical solution was obtained:Here n is the amount of analyte extracted by the SPME polymer film before partition equilibrium, t is the extraction time, and a h is a complicated parameter that determines how fast the equilibrium can be reached. The extracted amount before partition equilibrium is proportional to...
The thermal degradation of calcium oxalate monohydrate is studied by using diffuse reflectance Fourier transform infrared spectroscopy combined with quadrupole mass spectrometry (DRIFTS/MS). DRIFTS/MS results are correlated with thermogravimetric analyses to verify the thermal properties of samples studied. Infrared spectral evidence for CaC2O4H2O dehydration and infrared spectra for three different CaC2O4 phases are given. In addition, the significance of a band at 1085 cm−1 assigned to CO22− which appears during the decomposition of CaC2O4 to CaCO3 is discussed.
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