A

TIME

TIME

TIME

TIME

Figure 4.10 Representation of two procedures used to obtain multiple samples for analysis. (A) Several identical reaction mixtures are prepared, and the enzyme will be added to each to start the reaction. Each tube is sampled only once during the incubation. (B) Only one reaction mixture is prepared, and the enzyme is added to start the reaction. The incubation mixture is sampled repeatedly during the course of the reaction. Note that the volume of the reaction mixture in arrangement (B) is usually greater than in arrangement {A).

and injected. Thus, with this type of injector, the volume to be injected is determined by the volume drawn into the syringe.

In the second type, the injector unit contains a device called a "loop," which is a coiled tube of precise volume. The loop is loaded with sample, and once filled, its contents can be injected onto the column. It is always necessary to overfill such a loop to ensure complete loading. To overfill, the loop is loaded with sample until it overflows; the excess sample spills out one of the injection valve vents. Although injectors differ in the amount of overfilling required, experience leads us to recommend overfilling by as much as 30%, since it is critical to load the injector loop completely. Partially filled loops will result in abnormal chromatographic profiles. There is waste with injectors of the type shown in Figure 4.11B, and if material is in short supply or expensive, the type of injector (Fig. 4.1L4) might be preferred.

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