Rotary sample loop injectors

Rotary sample loop injector Figure 13.6 illustrates the most common design of HPLC injector. To introduce the sample as in GC with a syringe through a... 

Injector The sample, typically 5-200 )J,L, is placed in the carrier stream by injection. Although syringe injections through a rubber septum are used, a more common means of injection is the rotary, or loop, injector used in ITPLC and shown in Figure 12.28 of Chapter 12. This type of injector provides reproducible injection volumes and is easily adaptable to automation, a feature that is particularly important when high sampling rates are desired. 

Two types of injectors are frequently employed. For packed column SFC, a standard six port rotary valve with an external sample loop of 1-10 pL has proven to be quite reliable. For capillary column SFC, a similar rotary valve with an internal "loop of 0.2 to 0.5 pL is typically employed. Frequently the rotor is pneumatically actuated in a very rapid fashion to allow only a small fraction of sample to be introduced ("time-split ) this is done to avoid column overload. Alternatively, the flow from the injector is split off in the same fashion as in GC. A disadvantage of the latter mode is the potential for sample discrimination. 

The injection system was selected by comparison of a rotary valve injector with a fixed 10 jjI injection loop and a Waters U6K septumless injection system. The U6K injection system makes the exclusion of oxygen easier than the rotary valve injector. Care must be taken that the vent of the injector loop is at the same level as the point of injection because of syphonic penetration of oxygen into the sample loop. 

In order to accomplish nested introduction, two loop-based rotary valves with coincident movements [64] or a two-section injector—commutator have been used (Fig. 6.13). In the load position, specified in the figure, the first and second loops are simultaneously filled with the sample and with the reagent (or air, if a mono-segmented flow analyser is used). Switching the injector inserts the selected sample volume between two reagent (or air) plugs into the carrier/wash stream, and the complex zone established is directed towards the detector. 

To ensure insertion of an accurate volume of sample into the carrier stream, injectors are used during FIA experiments. Several concepts of injector have been described in the literature. Two injection devices must be detached (a) the rotary valve [21], which two- to six-port models are the most versatile and (b) the proportional injector (manually operated) developed by researchers from CENA/USP-Brazil [22]. Their operation is based on two well-defined positions loading and unloading positions. With the valve in the loading position, a defined volume is filled with sample. In the unloading position, the volume contained in the loop is inserted into the carrier stream. 

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