Extra Column Dispersion - LC Sample Valves > Page 16
The success of this method depends on there being a significant difference between the boiling points of the solutes and any solvent in which they are dissolved. An alternative procedure, called the 'solute focusing method' is more effective, but requires more involved and expensive equipment. The injector is designed to have two consecutive, independently heated and cooled zones located at the front of the column. A diagram of the solute focusing system is shown in figure (5). The second zone is now heated, using an appropriate temperature program, and the separation developed in the normal manner. This technique is more flexible than the 'retention gap method' but the apparatus and the procedure is more complex. Sample splitting does not occur in packed columns and thus, if the sample is tractable to separation on such columns, then the packed column would be the column of choice if high accuracy and precision are required.
LC Sample Valves
LC sample valves must operate at high pressures (sometimes as great as 10,000 p.s.i.) which demands high mechanical strength and precise methods of construction to ensure a leak proof system. A diagram of a high pressure LC sampling valve and its mode of action is shown in figure 6. The valve system can take two basic forms, the internal loop sampling valve and the external loop sampling valve. A diagram of each type of valve is included in figure 6. The sample volume of an internal loop valve is situated in the connecting slot of the valve rotor and can be used only for relatively small sample volumes. Internal sample loop valves provide samples with volumes ranging from 0.1 ml to about 0.5 ml. Valve operation is shown in figure 6. The left-hand side diagram shows the load position. The sample occupies the rotor slot and has been filled by passing the sample from an appropriate syringe through the rotor slot to waste. While loading the sample, the mobile phase supply is passed through the valve directly to the column. To place the sample onto the column, the valve is then rotated and the valve slot containing the sample is now placed between the solvent supply and the column. As a result, the sample is passed into the column by the flow of solvent.
Courtesy of Valco Instruments Inc.
Figure 6. The Internal and External Loop LC Sample valves