Pneumatic column switching

In alternative forms of these systems, the applications program is stored in the processor the analyzer microprocessor digitizes only the detector signal and transmits the digitized values to the data processor. Applications program event commands are received in real lime from the data processor and convened at the analyzer to electrical and pneumatic signals for sample valve actuation, column switching, sample conditioner conirul. and so on. 

A schematic diagram of a heart-cut LC-LC system is depicted in Figure 5.4. The column switching technique was developed by employing two high-pressure four-way pneumatic valves inserted before and after the precolumn (39). The front-cut and the end-cut of the sample eluted from the first column were vented to waste. The valves were manipulated to transfer only the heart-cut of the analyte of interest to the analytical column. The detailed operational conditions for the four-step sequence of this system can be described as follows ... 

The hardware employed in preparative liquid chromatography typically has the capability for feedback control of the pumps, automatic column switching, column backflushing, recycling, gradient mixing, online pressure, flow rate, UV-absorbance, and temperature monitoring, and automatic pneumatic actuation of fraction collection valves based on time, volume, or UV-absorbance thresholds. These capabilities are afforded by computer control. 

Valve Actuation Gas—This test method permits the use of any type of valve switching or valve actuation. When pneumatic vtdves are used, air of any grade that will result in no water condensation or will not introduce oil or other contaminates in the switching valves may be used. Air from a piston operated compressor equipped with a water and oil separator has been found suitable. Column switching valves that do not require air to operate do not have this air requirement. 

The design has been well proved in quality assurance and origin control of flavours and fragrances. A double-oven system is shown in the Fig. 17.3, with two independent temperature controls and two detectors (DM 1, DM 2). A live switching coupling piece is used to switch the effluent flow to either the first detector or the chiral column. With optimum pneumatic adjustment of the MDGC system, certain fractions are selectively transferred onto the chiral main column as they are eluted from the precolumn (heart-cutting technique) [15]. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Multiple-column systems were previously explored in the petroleum industry and some process-control situations. In the former case, typical petrochemical samples share some similarities with biochemical samples in terms of complexity while the GC column typically receives a total sample, only certain portions of it may be of interest. Thus, selected parts of a column effluent can be pneumatically switched over to a second column for an optimum analysis, while the residual uninteresting substances (heavy ends) are being rapidly removed through backflushing. In the case of process GC analysis, such backflushing is essential to the speed of analysis required from these industrial analyzers indeed, a similar situation is often found in a clinical laboratory. 

The Varian Model 8500 pump (Fig. 5) is the most sophisticated syringe-type pump. There are two gas solenoid valves, the first of which pressurizes the solvent container to re-fill the chamber and the second actuates a pneumatic valve downstream of the pressure transducer on the column tubing. The operating controls are selected automatically by depressing push-button switches on the front panel of the pump controller. These switches control the opening of the... 

Column heads of semi-technical plants are made up of separate condenser and reflux units. The valves are operated mechanically and, more commonly, electro-magnetically. Their operation is similar to that in laboratory column heads (Figs. 306, 247). The valves are controlled automatically by means of electromagnets, electromotors or pneumatic drives (Fig. 248). For the automatic division of liquid, however, the swinging funnel is chiefly used (Fig. 142) which is actuated by an electromagnet placed outside the column wall. With the magnet switched on the distillate is taken off laterally. 

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