The operating pressure of an HPLC is sufficiently high that we cannot inject the sample into the mobile phase by inserting a syringe through a septum (as we can do in gas chromatography). Instead, we inject the sample using a loop injector, a diagram of which is shown here.
In the load position a sample loop—which is available in a variety of sizes ranging from 0.5 μL to 5 mL—is isolated from the mobile phase (shown in green) and open to the atmosphere. The sample loop is filled using a syringe (shown in blue) with a capacity several times that of the sample loop, with excess sample exiting through the waste line. After loading the sample, the injector is turned to the inject position, directing the mobile phase through the sample loop and onto the column.
