Restrictor

The restrictor controls the backpressure and the SF flow rate which circulates through the vessel and, moreover, the SF is depressurized within the restrictor. When the SF passes through, the fluid decreases the pressure from the high pressure inside the extraction chamber to atmospheric pressure, passing the fluid from supercritical conditions in the extraction vessel to atmospheric conditions. Three main types of restrictors exist  

The narrow orifices of restrictors can be prone to plugging. Heating the restrictors helps to alleviate (although not eliminate) plugging problems. 

The extracted analytes are collected after the depressurized step. The analytes travel through the restrictor, where the SF decompresses, and analyte deposits in some type of trapping device. Analyte trapping after the extraction step can be carried out with either a small amount of collection solvent (an appropriate solvent placed in a cooled vial) or in an adsorbent trap (solid surface cryogenically cooled by means of liquid nitrogen). The trapping system is selected depending on the nature of extracted analyte. 

There are two common ways to operate SFE, in online mode or offline mode. In the online mode, the outlet of the SFE instrument is directly hnked to an analytical instrument. Direct coupling with different chromatographic techniques allows simultaneous analyte preparation, separation and determination. The main drawback of the online extraction method is the limited sample size. In the offline mode, the extracted analytes are trapped and later, the extracted analytes can be analyzed by means of different chromatographic techniques. The offline collection system is chosen according to the extracted analyte characteristics. The use of online methods reduces possible errors and sources of contamination. Studies on trapping methods have been performed to optimize the corresponding parameters.  

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Vapor Pressure. The Shiley Infusaid implantable infusion pump utilizes energy stored in a two-phase fluorinated hydrocarbon fluid. The pump consists of a refillable chamber that holds the dmg and a chamber that holds the fluid. The equiUbrium vapor pressure of the fluid, a constant 60 kPa (450 mm Hg), compresses the bellows, pumping the dmg through a bacterial filter, a capillary flow restrictor, and an infusion cannula to the target body site (56,116). 

FIGURE 13.22 Sur ctants on the 6 column set, using a photodiode array (PDA) detector. The Span and Tween 80 can be distinguished from each other very nicely in a mix. THF was the solvent used at 50°C. (A restrictor after the detector minimizes bubbles.)... 

The system can be made more sophisticated through the addition of fused silica restrictors acting as bleeds, thus preventing back diffusion of analytes towards the solenoid valves. Additional pre-concentration stages can also be provided between the primary and secondary columns to reduce the peak widths introduced to the secondary column. 

Figure 6.5 Schematic diagram of a typical interface used for on-line SFE-GC coupling (from ref. 40) 1, extraction cell 2, restrictor 3, on-column injector 4, heater 5, oven 6, ayogenic module 7, column.

Figure 6.12 Schematic diagram of the interface used for direct SFE-CEST coupling without a sample pre-concenti ation step 1, micro-LC pump 2, heated restrictor 3, six-port valve 4, direct by-pass to the CE unit 5, three-port valve 6, CE instmment. (from ref. 58).

J. T. B. Strode and L. T. Taylor, Supercritical fluid exti action employing a vai iable restrictor coupled to gas chi omatography via a sample pre-concentration cap , ]. High Resolut. Chromatogr. 19 651-654 (1996). 

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

Figure 14.10 Schematic diagram of the aromatics analyser system BP, back-pressure regulator CF, flow controller CP, pressure controller Inj, splitless injector with septum purge V, tliree-way valve column I, polar capillary column column 2, non-polar capillary column R, restrictor FID I, and FID2, flame-ionization detectors.

The term bladed refers to raised, continuous flow restrictors with a standoff distance from the bit body of more than 1.0 in. In most cases cutters are affixed to the blades so that the cutter arrangement may also be described as bladed. The term ribbed refers to raised continuous flow restrictors with a standoff distance from the bit body of 1.0 in. or less. Cutters are usually affixed to most of the ribs so that the cutter arrangement may also be described as ribbed. The term open face refers to nonrestricted flow arrangements. Open face flow designs generally have a more even distribution of cutters over the bit face than with bladed or ribbed designs. 

Externally pressurized gas journal bearings have the same principle of operation as hydrostatic liquid-lubricated bearings. Any clear gas can be used, but many of the design charts are based on air. There are three forms of external flow restrictors in use with these bearings pocketed (simple) orifice, unpocketed (annular) orifice, and slot. 

Capillary restrictor tube Capillary restrictor tube ... 

Control flow Contains adjustable matching inserts, no restrictor bar Has restrictor bar or flow dividers in each polymer channel but with blown film dies control is by individual extruder speed or gearboxes... 

Layer uniformity Individual layer thickness correction of 10 percent Restrictors and manifold can meet 5 percent... 

Lost-wax Also called RP molding, fusible-core. A bar (or any shape) of wax is wrapped with RP. After the RP is cured (bag molding, etc.) in a simplified restrictor mold to keep the RP-wax shape, the wax is removed by drilling a hole or removing the end caps by applying a low temperature so that the RP is not effected (review in this chapter INJECTION MOLDING, Modified... 

The liquid enters the micro channel device via a large bore that is connected to a micro channel plate via a slit (Figure 5.2). The slit acts as a flow restrictor and serves for equipartition of the many parallel streams [1, 3, 4]. The liquid streams are re-collected via another slit at the end of the micro structured plate and leave the device by a bore. The gas enters a large gas chamber, positioned above the micro channel section, via a bore and a diffuser and leaves via the same type of conduit. 

SEE is an instrumental approach not unlike PLE except that a supercritical fluid rather than a liquid is used as the extraction solvent. SFE and PLE employ the same procedures for preparing samples and loading extraction vessels, and the same concepts of static and dynamic extractions are also pertinent. SFE typically requires higher pressure than PLE to maintain supercritical conditions and, for this reason, SFE usually requires a restrictor to control better the flow and pressure of the extraction fluid. CO2 is by far the most common solvent used in SFE owing to its relatively low critical point (78 atm and 31 °C), extraction properties, availability, gaseous natural state, and safety. 

Blcrameters) by using a prepacked colunn as a restrictor down stream from the e tty column blank to be packed [159]. 

Figure 6.13 Flow restrictors of different design A, linear B, tapered C, integral and D, frit. On the right side is shown a modified high pressure cell for UV detection using open tubular columns.

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