Microvolume sampling valves

Although the problems associated with septum injectors can be eliminated by using stop-flow septumless injection, currently the most widely used devices in commercial chromatographs are the microvolume sampling valves (Fig. 8.3) which enable samples to be introduced reproducibly into pressurised columns without significant interruption of the mobile phase flow. The sample is loaded at atmospheric pressure into an external loop in the valve and introduced into the mobile phase by an appropriate rotation of the valve. The volume of sample introduced, ranging from 2 piL to over 100 /iL, may be varied by changing... 

Figure 30.4 Microvolume Sampling Valve Operation of a Sample Loop.

The sample introduction unit, which is usually an inexpensive low-pressure rotary valve that allows reproducible insertion of accurately measured sample microvolumes without the need for halting the flow - as in liquid chromatography (LC). Liquid samples can also be inserted by time-controlled aspiration at a merging point. 

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