General Considerations

According to the theoretical model developed by Rijks et al., in 1983, the efficiency of the SDE process increased with the extraction time and with the liquid and vapour flows. The process also depends on analyte-specific parameters related to the activity coefficient (calculated from the water solubility of the analyte at 100°C) and the gas-liquid distribution coefficient of the compound in water at the process temperature (i.e., 100°C for water steam). Not unexpectedly, the recoveries increased with the affinity of the target compounds for the extracting solvent. This theoretical model is applicable only under ideal conditions, which are achieved when all volumes and flow rates remain constant and there is ideal mixing and equilibrium at every stage. In spite of these limitations, the model reflects the effect of several experimental factors on the SDE process. In fact, the different modifications carried out on the SDE devices originally described by Likens and Nickerson in 1964 and by Flath and Forrey in 1977 reveal the influence of several parameters on the recoveries of the target compounds. The modifications were mainly focused on increasing the size of the vapour chamber and/or the condensing surface to allow a more complete mixing of the solvent and steam vapours, as well as on the miniaturization of the system. As a consequence of the changes in design (Figure 1), the efficiency of the extraction was increased, the analysis time reduced and the field of SDE expanded through the analysis of residue levels of less volatile pollutants in environmental samples.

Due to the characteristics of the technique, the feasibility of SDE for the analysis of less volatile compounds depends on their (i) potential for forming azeo-tropes with water and (ii) relative solubility in water and in the extraction solvent. However, the SDE of the target compounds from complex samples can be expected to occur only after destruction or degradation of the main matrix components, which usually entrap the analytes (see below). Therefore, as stated by Nash in 1984, the applicability of the SDE technique to the analysis of this kind of environmental matrices would be limited by the resistance of the investigated compounds to the selected degradation procedure. Alternatively, in some cases, co-distillation solvents have been used to improve the SDE efficiency by reducing the surface tension of the water and by increasing the extraction power (polarity) of the organic solvent. Finally, rather different results have been published about the suitability of adding anti-foam agents in applications involving fatty samples (see Table 3).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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