Supercritical fluid extraction dynamic mode

Supercritical fluid extraction can be performed in a static system with the attainment of a steady-state equilibrium or in a continuous leaching mode (dynamic mode) for which equilibrium is unlikely to be obtained (257,260). In most instances the dynamic approach has been preferred, although the selection of the method probably depends just as much on the properties of the matrix as those of the analyte. The potential for saturation of a component with limited solubility in a static solvent pool may hinder complete recovery of the analyte. In a dynamic system, the analyte is continuously exposed to a fresh stream of solvent, increasing the rate of extraction from the matrix. In a static systea... 

Static/Dynamic Selection Valve. This valve is the key feature of our design in that it eliminates the use of a restrictor. Restrictors are the most common means of controlling the pressure or density of a supercritical process. With no restriction, flow is dead-ended (i.e. restricted) via a switching valve in our invention. Supercritical fluid extractions are then conducted in a static (no flow) mode. 

Influence of matrix on the supercritical fluid extractability of cocaine using COj, COj + MeOH and COj + HjO/TEA (85 15 v/v). SFE conditions 400 atm, 110 C, 10 min static, 15 min dynamic mode 100 pL modifier. (From Morrison, J. F., MacCrehan, W. A., Selavka, C. M., submitted. With permission from the National Institute of Standards and Technology.)... 

Depending on the way the sample and extractant are brought into contact, supercritical fluid extraction can be implemented in two different operational modes, viz. static and dynamic. 

In the static addition mode, a preset volume of modifier highly dependent on sample size is directly added to the sample, held in the cell, prior to extraction. The principal disadvantage of this mode is that, as the supercritical fluid begins to circulate through the sample, the modifier is swept from the extraction cell, so the matrix is brought out of contact with the modified supercritical fluid. Although dynamic addition is usually more effective than static addition — the modifier is continuously passed through the sample in the former — Ashraf and Taylor [35,36] found the static addition of methanol... 

Neither MAE nor ASE is currently in a configuration that would readily lead to the automation of sample preparation. Supercritical fluid extraction can be used as online system that can then be connected to the chromatographic and detection systems. Connected online with the GC/MS, SFE was successfully used for the determination of PAHs in marine sediments. Using either CO2 alone or modified with toluene or MeOH in the extraction, the PAHs were cryofocused in the accumulation cell of the GC and then directly chromatographed. For the study of PAHs in marine sediments, a new extraction technique, which consists of the combination of ASE (dynamic and static mode) and SFE (dynamic mode), was developed, with an extraction time longer than in ASE but shorter than in SFE, and... 

The mode of extraction for PAHs is highly dependent on the matrix. For solid-based matrices such as food samples, sediments, soil, marine organisms, etc. extraction methods such as Soxhlet extraction with nonpolar solvent [35 6], hollow fiber membrane solvent microextraction (HFMSME) [10], pressimzed hquid extraction (PLE) [37,38], sonication extraction [3], microwave-assisted extraction (MAE) [3], supercritical fluid extraction, (SEE) [39], accelerated solvent extraction (ASE) [40], cold extraction [41], soxtec extraction [42], microwave-assisted alkaline saponification (MAAS) [43], dynamic microwave-assisted extraction (DMAE) [44], add-induced cloud point extraction (ACPE) [45], methanolic saponification extraction (MSE) [7], etc. are employed. Of all these, Soxhlet extraction is the most common for solid samples and has achieved excellent extraction with high-level recovery but its setback is the high consmnption of solvent and time associated with it. 

There are two basic modes of supercritical fluid extraction static and dynamic. Both will be discussed in the following sections. The basic instrumentation required for both modes of SFE is similar. Figure 11.28 illustrates the minimum... 

Extractions can be carried out in dynamic, static, or combination modes. In the dynamic mode, the supercritical fluid continuously flows through the sample in the extraction vessel and out the restrictor to the trapping vessel. In the static mode, the supercritical fluid circulates in a loop containing the extraction vessel for some period of time before being released through the restrictor to the trapping vessel. In the combination mode, a static extraction is performed for some period of time, followed by a dynamic extraction. 

Once the extraction is complete, the static/dynamic selection valve is repositioned to the dynamic mode to allow flow. Subsequently, pressure and density are rapidly reduced to prevent significant losses of the supercritical fluid from the syringe pump tank and the extraction effluent, which is being transferred for collection. With a non-re-stricted transfer, the flow of supercritical fluid effluent is rapid. This desire for rapid depressurization led to the development of a delivery nozzle which would ensure collection of the extracted solutes without losses. Details of this delivery system can be found in the next section. 

Delivery of Extract Delivery Nozzle. Delivery of the extraction effluent is conducted via the six port static/dynamic valve while in the dynamic mode. Generally, extractions are conducted at a high density in the static mode. Once the extraction is complete, the valve is re-positioned into a dynamic evacuation, pressure or density is reduced rapidly to prevent significant losses of the supercritical fluid and the extraction effluent is transferred for collection. The extract leaves through the heated static/dynamic valve to the heated lines then to the delivery nozzle(s). Figure 8 shows a diagram of the delivery nozzle and its components. 

The instrumentation should allow for two extraction modes one dynamic and the other static. In the dynamic mode, the extraction vessel is constantly supplied with fresh supercritical fluid, while the recovery container is continually supplied with the solute. In the static mode, the outlet of the extraction vessel is closed and extraction takes place without the regeneration of supercritical fluid. On completion of the extraction, the vessel is rapidly rinsed by the supercritical fluid in order to permit the recuperation of the solute. At this point we should return to our discussion of the four components of the instrumentation. 

In the dynamic extraction mode, the extractant is pumped through the sample into the collection system or interface when the extractor is connected on-line to a chromatograph or detector. In this way, the supercritical fluid is passed through the sample once before it is driven to the restrictor. 

Which extraction mode is the better remains a controversial issue. While the static mode provides longer contact between the sample and solvent, swells the matrix and facilitates penetration of the extractant in its interstices — thereby increasing its efficiency — the dynamic mode allows the analyte to be continuously exposed to the pure (clean) solvent, thus favouring displacement of the analyte s partitioning equilibrium to the mobile phase. Most SFE methods use both modes a static step is employed to ensure close contact between the sample and supercritical fluid without consuming much extractant that is followed by a dynamic step where the extracted analytes are driven to the restrictor and equilibrium is allowed to complete. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Extractions can be performed in static, dynamic, or recirculating modes. In the static mode, the extraction cell is pressurized with supercritical fluid and allowed to equilibrate before the analyte is removed for collection. In the dynamic mode, the supercritical fluid is passed through the extraction cell and the analyte is collected continuously. In the recirculating mode, the same fluid is pumped through the sample then, after some time, it is repeatedly pumped... 

An Sl E system can be operated in one of two ways. In the dynamic extraction mode, the valve between the extraction cell and the restrictor remains open so that the sample is continually supplied with fresh supercritical fluid and the extracted material flows into the collection vessel where depressuriz-ation occurs. In the static extraction mode, the valve between the extraction cell and the restrictor is closed and the extraction cell is pressurized under static conditions. After a suitable period, (he exit valve is opened and the cell contents arc iransferred through the rcsiricior by a dynamic flow of fluid from the pump. The dynamic mode is more widciv used than the static mode. 

Like HSGC, SFE has two modes of extraction dynamic and static (102). In the dynamic mode, the sample matrix is continuously flushed with fresh supercritical fluids, which pass through the sample matrix, solvate the analytes, and carry them to a trap where the analytes are collected. 

Extraction can be performed in static, dynamic or recirculation mode. In static mode, the cell containing the sample is filled with the supercritical fluid, pressurized and allowed to equilibrate. In dynamic mode, the supercritical fluid is continuously passed through the extraction cell. In the recirculation mode, the same supercritical fluid is repeatedly pumped through the sample until it is finally pumped out to the collection system. 

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