Extra Column Dispersion - LC Sample Valves > Page 17
After allowing a short period of time (a few seconds) for the sample to enter the column, the valve is rotated back to the load position. The internal loop valve is employed for columns where the peak dispersion is constrained to a few microliters so that the resulting dispersion will not degrade the column performance. This type of sample valve might also be used with short columns, perhaps 4.6 mm in diameter and 3 cm long packed with support particles only 3 m in diameter or, with small microbore columns (e.g., 10 to 50 cm long and 1 mm I.D.) .
For columns providing peaks of larger volume, the external loop valve with six ports can be used. In this valve, three slots are cut in the rotor in a manner that will allow any two adjacent pair of ports to be connected. In the loading position (shown on the left of the lower diagram), the mobile phase supply is connected directly to port (4) and the column to port (5). This arrangement, causes the mobile phase to flow directly through the column. At the same time, the ends of the sample loop are also connected to ports (3) and (6). The sample loop is charged by a syringe through port (1), and by way of the rotor slot to the sample loop which is connected to port (6). The third slot in the rotor connects the sample loop exit to waste at port (2).
On rotating the valve (see diagram lower right) the sample loop is placed between the column and the solvent supply by connecting port (3) and (4) and ports (5) and (6). The mobile phase then sweeps the sample onto the column. At the same time, the third rotor slot connects the syringe port to the waste port. After sampling, the rotor is returned to the loading position, the external sample tube washed with solvent and the sample loop filled with the next sample for analysis. As the sample loop can be made of any length or radius, a wide range of sample volumes can be chosen.