Variable Restrictors

Such devices can be implemented within an analytical-scale SFE to control either pressure (need a pressure transducer) or flow rate (need a flow transducer) the fluid pump control system would be used to control the complementary parameter. The two control systems allow the decoupling of flow rate and pressure. The Hewlett-Packard model 7680 T contains just such a device, and similar devices are becoming available through other commercial sources. With properly implemented devices, the restrictor can become transparent to the user - providing settable control of parameters and operation that is highly reproducible and more robust to wider ranges of extractable components. 

In SFC, in early years of experimentation [17, 18], often the on-line detectors were UVWisible (spectroscopy) tjfpes where it was useful, and even necessary, to maintain supercritical or at least high density conditions through and beyond the detector. Expansion into a large unswept zone was not a parameter that needed to be considered or controlled 

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Variable restrictor (programming of density and pressure, independent of flow-rate)... 

Dimensions of restrictor Variable restrictor nozzle, with electronic control of dimensions. Automatically achieves/controls to pressure selected per extraction step. 

A needle valve (variable restrictor) mounted in the chromatograph. 

Even the sample loop should be controlled to the same pressure as the column side to minimize pressure upsets when switching the valve. Frazer et al. (9) found that pressure and flow regulation on all inlets as well as the appropriate micrometering valves (variable restrictors) used to dynamically balance the carrier gas flow and pressure allowed them to switch a 20 foot by 1/8-inch column in or out of a multivalve, multicolumn system with a resulting change in the baseline signal of less than 5 iV. 

Supercritical fluid extraction system - Hewlett Packard Model 7680A totally automated system with unlimited-capacity reciprocating pump, specially designed extraction chamber with safety interlocks, a variable restrictor nozzle and analyte collection trap. The operation of the extractor is controlled by a personal computer which is a Microsoft Windows-based system. An animated status screen provides real-time monitoring of the extraction process. Table II gives the SFE conditions for the HP extractor. 

Thimble Volumes. For these experiments the thimble volumes and the extraction times were held constant. To accomplish this, the mass flow of the system had to be varied by changing the flow rate at the pump head for each density step. Controlling the mass flow rate allowed the linear/volumetric flow to be consistent throughout the experiments. This is different from the paprika experiments in which the mass flow was held constant and the extraction times were changed, to keep thimble volumes constant for each extraction step. Flow control is one of the major advantages of variable restrictor based SFE units. 

The variable restrictor allows the user to control both the mass flow of the system and the density independently. 

Its function is to expand the pressurized solution to separate the "solvent gas" from dissolved extracted components. If a fixed restrictor is used, the mass flow rate of the fluid changes as a function of pressure (density) mass flow can increase by a factor of 25 as pressure is increased from 80 to 400 bar (24). Not only are the pressure and flow coupled, the coupling is via a static conduit whose dimensions are imprecisely controlled during an extraction (partial plugging by particulates and precipitated components, temperature) and from component to component during maintenance replacements. This results in a lack of control in operating parameters (density) and timed sequences (via flow rate and time). A variable restrictor whose dimensions are set and adjusted by an electronic feedback control loop is an alternative solution. 

J. T. B. Strode and L. T. Taylor, Supercritical fluid extraction employing a variable restrictor coupled to gas chromatography via a sample pre-concentration trap , J. High Resolut. Chromatogr. 19 651-654 (1996). 

A column consisting of a deactivated silica-based stationary phase is used for the packed-column mode. A packed column allows larger volumes of sample solvent to be injected, thus improving sensitivity. Generally, the column dimensions are 1 x 100-250 mm and the particle size is 5 / m. Commercial SFC instruments are also available that will handle the classical 4.6 x 150-mm or 250-mm columns. With the introduction of electronically controlled variable restrictors to control the back pressure, the packed columns are becoming increasingly more popular. This feature allows the independent flow and pressure control of mobile phases, thus helping in rapid optimization of selectivities. Some of the commonly used packed columns are as follows ... 

Fig. 6.11. Schematic diagram of a manual variable restrictor for coupling to an extraction system. (A) Side view. (B) Upper view. IX. in. OD stainless steel tubing, 2 adjustable parts, 3 restrictor body, 4 outlet part. (Reproduced with permission of Elsevier.)...

In some systems, the restriction apparatus is a simple transfer tube restriction used for low-flow experiments. In others, a variable restrictor or back-pressure regulator is used between the outlet and the source. In the configuration used by Pinkston and Baker [7], the restriction apparatus is a series of tees to add a coaxial flow of nebulizing gas, electrospray buffer sheath flow, and another to introduce liquid from a syringe pump to regulate mobile-phase pressure. In Fig. lb, a direct interface is shown in which a split directs a fraction of the effluent flow toward the API source while the remainder is sent to waste through a back-pressure regulator or some controlled restriction to maintain system pressure. 

Separations (Allentown, PA) and Isco (Lincoln, NE) have their own versions of variable restrictors, and these are included with their commercial SFE instruments. The cost of variable restrictors can range from hundreds to thousands of dollars. 

Since its introduction in the 1960s, SFC has experienced several ups and downs in its development. Either a gas or a Uquid above its critical temperature and pressure is used as the mobile phase for SFC. In most cases, COj is used because of its favorable critical parameters (i.e., a critical temperature of 31 °C and a critical pressure of 7.3 MPa). Moreover, CO2 is cheap, nontoxic, and nonflammable. A high-pressure pump delivers the mobile phase through either a packed (pSFC) or capillary column (cSFC) to the detector. The mobile phase is maintained under supercritical or subcritical conditions via an electronic controlled variable restrictor that is positioned after detection (pSFC) or via a fixed restrictor positioned before a gas-phase detector (cSFC). The retention characteristics of the analytes are influenced by the properties of the stationary phase and by the polarity, selectivity, and density of the CO2 mobile phase. The density is controlled by variation of the temperature and pressure of the supercritical medium. Furthermore, the elution of very polar compounds under high densities can be achieved with a precolumn addition of polar modifiers such as methanol. Nowadays, pSFC formats use the same injector and column configurations as LC methods. Consequently, pSFC formats are considered to be more useful for routine operation than cSFC. The most remarkable... 

During the methods optimisation work, approximately 1100 samples were run on three instruments in 21 working days. Diu-ing that time, no problems with plugging of the variable restrictors were encountered. The summary of the recoveries and precisions is presented in Table 3 the recommended SFE conditions are summarised in Table 4. 

Switch the valve to the BACKFLUSH position and adjust the variable restrictor to give the same column vent flow set in 11.3.1. This is necessary to minimize flow changes when the valve is switched. 

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