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directly to port (4) and the column to port (5). This arrangement, causes the mobile phase to flow directly through the column. At the same time, the ends of the sample loop are also connected to ports (3) and (6). The sample loop is charged by a syringe through port (1), and by way of the rotor slot to the sample loop which is connected to port (6). The third slot in the rotor connects the sample loop exit to waste at port (2). On rotating the valve (see diagram lower right) the sample loop is placed between the column and the solvent supply by connecting port (3) and (4) and ports (5) and (6). The mobile phase then sweeps the sample onto the column. At the same time, the third rotor slot connects the syringe port to the waste port. After sampling, the rotor is returned to the loading position, the external sample tube washed with solvent and the sample loop filled with the next sample for analysis. As the sample loop can be made of any length or radius, a wide range of sample

of the interstitial volume. The Sampling System Gas samples are generally placed on a GC column using an external loop sampling valve but liquid samples are usually injected onto the column by a syringe via a heated injector. Sample are placed on an LC column directly with either an internal or external loop sample valve the valve being connected directly to the column. The external loop sample system, employing six ports, is depicted in figure 25. The external loop sample valve has three slots cut in the rotor so that any adjacent pair of ports can be connected. In the loading position, shown on the left, the mobile phase supply is connected by the rotor slot between port (4) and port (5) directly through to the column. In this position, the sample loop is connected across ports (3) and (6). Sample passes either from a syringe or other sample device into port (1) through the rotor slot to the sample loop at port (6) and the third slot in the

the packed loop and allows it to b washed with either water or normal saline. A  syringe is used to fill the open loop which may be a water sample, blood, tissue dispersion etc.. If the sample is not of biological origin the samples displaced it into the packed tube by water from the reservoir. If the sample is blood, or, a cell suspension the sample is displace by saline to ensure that no protein is precipitated in the packed tube. In general, a volume equivalent to twice that of the open loop is pumped through the packed tube to waste. If saline is used to wash the packed tube free of particulate matter then the tube is washed free of saline with water. The water wash is also important as it not only washes the packed tube free of saline, it also washes lines from the solvent selection valve through the sampling system free of saline. After the water wash, the packed loop now contains the sample for separation, focused as a sharp band at the front of the packing in contact with

the valve seating to become scored each time it is rotated which will ultimately lead to leaks. This will cause the sample size to vary between samples and eventually affect the accuracy of the analysis. It follows that any solid material must be carefully removed from any sample before filling the valve. The two basic types of LC sample valve have been discussed in Principles and Practice of Chromatography. In LC however, there is a third type of valve which is similar to the external loop valve but contains an extra loading port and behaves like an internal loop valve. a diagram of which is shown in figure 13. The basic difference between this type of valve and the normal external loop sample valve is the introduction of an extra port at the front of the valve. This port allows the injection of a sample by a syringe directly into the front of the sample loop. Position (A) shows the inject position. Injection in the front port causes the sample to flow into the sample loop.

then the packed column would be the column of choice if high accuracy and precision are required. LC Sample Valves LC sample valves must operate at high pressures (sometimes as great as 10,000 p.s.i.) which demands high mechanical strength and precise methods of construction to ensure a leak proof system. A diagram of a high pressure LC sampling valve and its mode of action is shown in figure 6. The valve system can take two basic forms, the internal loop sampling valve and the external loop sampling valve. A diagram of each type of valve is included in figure 6. The sample volume of an internal loop valve is situated in the connecting slot of the valve rotor and can be used only for relatively small sample volumes. Internal sample loop valves provide samples with volumes ranging from 0.1 ml to about 0.5 ml. Valve operation is shown in figure 6. The left-hand side diagram shows the load position. The sample occupies the rotor slot and has been filled by passing the sample from

The procedure is as follows; a mobile phase mixture programmed to provide a solvent concentration profile is formed over a period of time and pumped into the gradient storage column. During the process of loading the gradient into the storage column, the solvent content of the storage vessel is passed to waste. When the complete solvent program is contained in the storage column, the flow is arrested. The sample is then charged into the sample loop (an internal sample valve loop should be used). The loop is then placed in line with the column and the gradient is discharged at full flow rate through the sample loop and column. An example of the rapid separation of a thirteen component mixture in just over 20 seconds is shown in figure 28. J. Chromatogr.,253(1982)159 Column Length 2.5 cm, column I.D. 2.6 mm, packing, C18 reversed phase, particle size 3 mm, solvent program 25% v/v acetonitrile in water to 100 % acetonitrile, flow