High-pressure valve injector

On-line coupling of SFC with gas chromatography has sporadically been used for the investigation of volatiles from aromatic herbs and spices. The requirements for instrumentation regarding the pumps, the restrictors, and the detectors are similar to those of SFE-GC. Additional parts of the device are the separation column and the injector, to introduce the sample into the mobile phase and successively into the column. The most common injector type in SFC is the high-pressure valve injector, similar to those used in HPLC. With this valve, the sample is loaded at ambient pressure into a sample loop of defined size and can be swept into the column after switching the valve to the injection position. The separation columns used in SFC may be either packed or open tubular... 

Sample Introduction. The small internal diameters of SFC capillary columns place stringent requirements on saoiple introduction in order to avoid band-broadening due to too large of an injector volume. These requirements have been met using a 0.2- jL internal volume high pressure valve operated in a splitting mode (12). Split ratios of 3 1 to 5 1 are usual. Recently, valves designed specifically for use with capillaries have become available (Valeo Instruments Co., Houston, Texas). These valves offer internal volumes as low as 60-nL and show promise for use in capillary SFC. 

The high-pressure picoliter injector also consists of a moving monolithic plug similar to that used for valving, thus significantly reduced in size. The injection volume is controlled by pressure difference between sample and mobile phase channels and by time. For example, a 0.25 s long injection afforded a sample volume of 180 pL. 

Surfactants are used in fuel applications to prevent fuel injector deposits, intake valve and exhaust valve deposits, combustion chamber deposits [2,3], injector deposits of direct gasoline injector systems [4,5] and high-pressure diesel injector systems [6], and low-temperature intake valve deposits [7]. 

Today, the most common sample valves operate in the loop filling mode and are called high-pressure sample injectors. This means that the sample is transferred at atmospheric pressure from a syringe into a sample loop. By means of valving action the sample loop is then connected to the high-pressure mobile phase stream, which carries the sample onto the column (Fig. 6). The valving action can be carried out either manually or automatically by electric or pneumatic actuators. 

For liquid chromatography, a sample of the mixture solution is injected through a loop injector which allows a quantity of the solution to be placed in a small tubular loop at atmospheric pressure. By manipulating a valve, the high-pressure flow of solvent to the column is diverted through the loop, carrying the sample with it (Figure 35.5). 

For higher pressures, stopped flow injection has been used in which a valve releases the pressure in the column to atmosphere, at which point syringe injection at the top of the column (without septum) takes place. There has, however, been a trend in recent years to use valve injectors because of their generally high pressure capabilities and their ability to deliver large... 

Injectors introduce the sample into the mobile phase under high pressure. There are several approaches to injection in HPLC, such as syringe injection via septum, a combination of a septum and syringe or a valve injection. Valve injection is the method of preference in up-to-date HPLC instrumentation. 

Autosampler or injector—replace rotor seal in injector valve or high-pressure needle seal pack, sampling needle, and sampling syringe. 

The operation of septumless syringe injectors involves inserting a syringe needle into the injector and tightening an inert seal around the needle in order to obtain a pressurized fit. A valve is then adjusted and the injection is made at the operating pressure and flow-rate of the column. The syringe must be suitable for high-pressure injection. 

Figure 4.11 High-pressure injection valves. (A) The universal injector (Waters, Inc.). A known volume of sample is injected, displacing a corresponding volume from the sample loading loop shown in lop diagram. After sample loading plug is replaced, the loading loop is placed in line, and the eluent from pump moves sample onto column.

The D4 content of the samples was determined by reverse-phase high-pressure liquid chromatography (HPLC) with a Varian 5500 liquid chromatograph. A DuPont Zorbax ODS (Cis) column was used with a Wilmad infrared detector set at 12.45 xm to monitor the Si-CHa vibration. The mobile phase was an 83 17 mixture of acetonitrile and acetone at a flow rate of 0.8 mL/min. A Rheodyne injector valve operating on compressed air was used with a 10-p,L sample loop for reproducible injection volumes. Ethyl acetate was used to dissolve the samples for analysis. 

The dwell volume is the volume within the HPLC system from the point where the solvents are mixed to the entrance of the column. In a low-pressure system it consists of the volumes of the proportioning valve, the mixer, the pump head, the injector and the connecting capillaries. In a high-pressure system it is smaller because only the mixer, injector and the capillaries add their volumes. 

To avert high-pressure blow-outs of plumbing and equipment, pressure gauges and pop-off valves should be installed in the following places (1) in the condenser to sense product pressure, (2) near the base of the stripping and rectifying column sections to sense internal column pressure, and (3) in the reboiler steam chamber or the steam injector nozzle to sense steam pressure being applied. 

---