The typical operating pressure of an HPLC is sufficiently high that it is impossible to inject the sample in the same manner as in gas chromatography. Instead, the sample is introduced using a loop injector (Figure 12.28). Sampling loops are inter- changeable, and available with volumes ranging from 0.5 μL to 2 mL.
In the load position the sampling loop is isolated from the mobile phase and is open to the atmosphere. A syringe with a capacity several times that of the sampling loop is used to place the sample in the loop. Any extra sample beyond that needed to fill the sample loop exits through the waste line. After loading the sample, the in- jector is turned to the inject position. In this position the mobile phase is directed through the sampling loop, and the sample is swept onto the column.