Principles and Practice of Chromatography - The Basic Column Chromatograph > The Sampling System > Page 53
be used then oxygen or air and hydrogen will be needed for the detector and helium or nitrogen must be available as the mobile phase. If an argon ionization detector is to be used then an argon supply will be necessary. Flow controllers are usually supplied to each gas supply, often under microprocessor control and fitted with inline flow controllers. Such a system can provide flow programming facilities if so desired.
The mobile phase supply for a liquid chromatograph usually has a capacity for at least four solvents which are normally housed in a flame proof environment and solvent vapors are removed by a suitable vapor scavenging device. The solvents can be selected by a microprocessor or manually by suitable valves and these pass either directly to a dual piston pump for isocratic development or to a solvent programmer and thence to a pump for gadient elution. Care must be taken to minimize the volume existing between the solvent programmer exit and the sample valve otherwise the gradient will be distorted by the logarithmic decay function 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 rotor connects the exit of the sample loop to waste at port (2).