Principles and Practice of Chromatography - The Basic Column Chromatograph > The Sampling System > Page 54

Courtesy of Valco Instruments Inc.

Figure 25 The External Loop Valve


The sampling position is shown by the diagram on the right. On rotating the valve, the sample loop is interposed between the column and the mobile phase supply by connecting port (3) and (4) and ports (5) and (6) and the sample is swept onto the column. In the sampling position, the third rotor slot connects the syringe port to the waste port. After sampling, the rotor can be returned to the loading position, the system washed with solvent and the sample loop loaded in readiness for the next injection. For analytical applications, the sample loop can have a volume ranging from 1 to 20 ml, but for preparative work, loops with sample volumes of 1 ml or more can be placed on a preparative column.

Modern liquid chromatographs that are used for routine analysis usually include an automatic sampling device. This involves the use of some type of a transport mechanism that may take the form of a carousel or some form of belt conveyor system. The transporter carries a series of vials that alternately contain sample and washing solvent. The sampling can involve a complex sequence of operations that are controlled by a microprocessor. The syringe plunger is operated pneumatically and the syringe is first washed with solvent, then rinsed with the sample, reloaded with the sample and the contents discharged into the column. In routine analytical laboratories, which often have very sophisticated LC assemblies, there may also be a sample-preparation robot which will automatically carry out such procedures as extraction, concentration, derivatization, etc. which can be considered as part of the sampling system. The robot is usually programmable, so that a variety of separation procedures can be carried in a sequence that is unique for each sample. In laboratories that have a high throughput of samples, an automatic sampling device is often essential for the economic operation of the laboratory.