Pressurized matrix dependency

On-line SFE-SFC method development for validated quantitative analysis of PP/(Irganox 1010/1076, Tinuvin 327) has been reported [93]. SFE conditions required optimisation of extraction time and pressure, matrix type (particle or film) and matrix parameters (particle size, film thickness, sample weight). About 30% of extracts were lost during collection. Very poor recoveries (20-25 %) were reported from ground samples (particle size 100 p,m dependent recoveries of 45-70% for 30-p.m-thick films. Biicherl... 

Accelerated solvent extraction (ASE) is also known as pressurized fluid extraction (PFE) or pressurized liquid extraction (PLE). It uses conventional solvents at elevated temperatures (100 to 180°C) and pressures (1500 to 2000 psi) to enhance the extraction of organic analytes from solids. ASE was introduced by Dionex Corp. (Sunnyvale, CA) in 1995. It evolved as a consequence of many years of research on SFE [45], SFE is matrix dependent and often requires the addition of organic modifiers. ASE was developed to overcome these limitations. It was expected that conventional solvents would be less efficient than supercritical fluids, which have higher diffusion coefficients and lower viscosity. However, the results turned out to be quite the opposite. In many cases, extraction was faster and more complete with organic solvents at elevated temperature and pressure than with SFE. Extensive research has been done on the extraction of a variety of samples with ASE. ASE was approved by EPA as a standard method in 1996. 

There are many factors involved in optimizing static headspace extraction for extraction efficiency, sensitivity, quantitation, and reproducibility. These include vial and sample volume, temperature, pressure, and the form of the matrix itself, as described above. The appropriate choice of physical conditions may be both analyte and matrix dependent, and when there are multiple analytes, compromises may be necessary. 

On a geological time scale the conservation of mass for the solid phase is determined by the deformation of the solid matrix and the accompanying time-changes of porosity, and the dislocation of the solid matrix in space. The deformation and dislocation of the solid matrix depend on the changing stress, groundwater pressure, temperature and chemical conditions. 

Method development in SFE is not so simple since several parameters have to be optimized, including temperature and pressure of die SF, extraction time, flow rate, addition of cosolvent (type of solvent and amount) and finally collection mode (e.g. in a solvent, in an empty vessel or on a solid-phase trap). Furthermore, the methods are generally matrix-dependent, i.e. a method developed for a particular target-molecule(s) cannot be directly applied to other types of samples than the one(s) it was optimized for. 

It may be that in light of advances made using both desorption electrospray ionization (DESI) [20,21] and direct analysis in real time (DART) [22,23] that the use of IT-SIMS protocols may be supplemented in some applications. These techniques differ in that they sample surface contaminants at atmospheric pressure and do not necessarily require collection of a sample, or transporting the sample into the vacuum environment of the mass spectrometer. However, preliminary experiments have shown that these techniques are tremendously matrix dependent, even more so than SIMS, and thus the generation of even semiquantitative data for environmental surfaces like soil samples remains challenging. Comparisons of ultimate detection limits between the techniques have not yet been accomplished. 

For non-Newtonian flows, the stiffness matrix depends on shear rate and temperature and thus on pressure. The algebraic system is non-linear and it should be solved iteratively. 

One limitation of carbon dioxide as an extractant is its polarity. In its supercritical state and at low densities, CO2 has a polarity close to that of hexane. Even at extremely high pressures the solubility parameter may not approach that which is required to solubilize and extract polar analytes. This limitation can be overcome by the use of another extraction fluid, which is more polar, or by adding a polar modifier to the CO2. The most commonly used modifier with CO2 has been methanol. Increased solubilities and recoveries of polar analytes have been reported when a polar modifier is added to a less polar supercritical fluid (66-68). The ability of the supercritical fluid to dissolve a particular analyte is not the only factor, which affects extraction efficiency. The degree to which the analyte partitions into the supercritical fluid fi om the solid-sample matrix depends greatly on the sorptive and active sites on the solid matrix and the polarity of the solute (64,69). The addition of a polar modifier or entrainer, such as methanol, to a supercritical fluid such as CO2, not only increases the solubility of polar analytes in the supercritical fluid, but also may help block sorptive sites on the surface of the sample matrix. 

The thermoplastic semifinished part production can be divided into the following processes impregnating, consolidating, and transitioning to the solid state. During processing, the input materials (fiber and matrix) depend on the process-required control variables Temperature T, time t, and pressure p, which will then determine the resulting properties of the semifinished parts [2]. 

Direct thermal desorption provides a rapid technique for the qualitative analysis of solid samples with little or no sample preparation. Volatiles are thermally desorbed from the sample and concentrated directly onto the head of a GC column. Similar chromatographic profiles may be obfained using DTD relative to P T. Since the purge and desorption times are concurrent in the DTD method, analysis times are shorter. The relative purge efficiencies are compound and matrix dependent. DTD in some cases may provide a greater amount of material for detection. This is especially true for low-boiling compounds with higher vapor pressures. 

All the experimental teats described so far have been confined to binary mixtures, but of course it is also desirable to know whether flux relations adequate in binary mixtures are still successful in mixtures with more than two components. Even in the case of ternary mixtures the form of explicit flux relations is very complex, and a complete investigation of the various matrix elements, in their dependence on both pressure and composition, would be a forbidding undertaking. Nevertheless some progress in this direction has beet made by Hesse and Koder [55] and by Remick and Geankoplis [56]. 

In general, tests have tended to concentrate attention on the ability of a flux model to interpolate through the intermediate pressure range between Knudsen diffusion control and bulk diffusion control. What is also important, but seldom known at present, is whether a model predicts a composition dependence consistent with experiment for the matrix elements in equation (10.2). In multicomponent mixtures an enormous amount of experimental work would be needed to investigate this thoroughly, but it should be possible to supplement a systematic investigation of a flux model applied to binary systems with some limited experiments on particular multicomponent mixtures, as in the work of Hesse and Koder, and Remick and Geankoplia. Interpretation of such tests would be simplest and most direct if they were to be carried out with only small differences in composition between the two sides of the porous medium. Diffusion would then occur in a system of essentially uniform composition, so that flux measurements would provide values for the matrix elements in (10.2) at well-defined compositions. 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

For gas-liquid solutions which are only moderately dilute, the equation of Krichevsky and Ilinskaya provides a significant improvement over the equation of Krichevsky and Kasarnovsky. It has been used for the reduction of high-pressure equilibrium data by various investigators, notably by Orentlicher (03), and in slightly modified form by Conolly (C6). For any binary system, its three parameters depend only on temperature. The parameter H (Henry s constant) is by far the most important, and in data reduction, care must be taken to obtain H as accurately as possible, even at the expense of lower accuracy for the remaining parameters. While H must be positive, A and vf may be positive or negative A is called the self-interaction parameter because it takes into account the deviations from infinite-dilution behavior that are caused by the interaction between solute molecules in the solvent matrix. 

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