Injection, sample valve stopped-flow

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 3.8 — (A) Biosensors used in different FI manifolds to perform reaction-rate measurements (I) stopped-flow manifold (II) iterative flow-reversal system (III) open-closed configuration S sample B buffer P pump IV injection valve PC personal computer IMEC immobilized enzyme cell D detector W waste SV switching valve. (B) Types of recordings obtained by using the three types of biosensors and measurements to be performed on them in order to develop reaction-rate methods. (Reproduced from [50] with permission of Elsevier Science Publishers).

The stop-flow technique is the simplest and least expensive method of injection. The flow of the mobile phase is first stopped, either by turning an on—off valve in the line before the column (with constant-pressure systems), or by stopping the pump (with constant-flow systems). The column then returns rapidly to atmospheric pressure, and the sample can be injected directly on to the column with a normal low-pressure syringe. 

On column. One on-column sample application method involves stop-flow techniques. In this instance, the pump is stopped and isolated by a three-way valve from the column, the sample is then loaded via a syringe through an injection port which does not contain a septum. The pump is then restarted, the flow restored by switching the valve, and the sample is rapidly flushed onto the column. There is no apparent loss in efficiency but inaccuracies in retention measurement occur due to the finite time required for flow to be established and therefore this technique is redundant. 

Septum injection, pneumatic high-pressure syringe injection and v.alve injection are the usual kinds of sampling devices. The stop-flow injection technique is also used. Automated sampling devices based on high-pressure syringe injection or valve injection are already available. 

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