Ion Chromatography - The Sample Valve
The Sample Valve
Ideally, LC sample valves must sustain column inlet pressures up to 10,000 p.s.i., in practice however, they generally do not operate at pressures much above 3,000 p.s.i.. The higher the pressure the tighter the valve seating surfaces must be forced together to ensure no leak. Consequently, any abrasive material, however fine, (such as dust, packing particles etc.) that passes into the valve can result in the valve seating being scored each time it is rotated which eventually lead to leaks.
The net effect will be that the sample size will vary between samples, which will degrade the accuracy of the analysis. Thus, all solid material must be carefully removed from all samples before filling the valve. A diagram of a typical sample valve is shown in figure 16.
Courtesy of Rheodyne Instruments Inc.
Figure 16. The Modified External Loop Sample Valve
The difference between this type of valve and the common external loop valve is the introduction of an extra port at the front of the valve. This port permits the injection of the sample using a syringe directly into the front of the sample loop. Position (A) is depicted as the inject position. Injection into the port causes the sample to flow directly into the sample loop. The needle tip passes through the rotor seal and, during injection, is in direct contact with the ceramic stator face. Note the needle is chosen so that it's diameter is too great to enter the hole that connects to the actual sample volume. To place the sampleon the column, the valve is rotated to position (B) and (as seen from the diagram) the mobile phase flushes the sample directly onto the column.
The sample is actually flushed from the beginning of the sample loop directly onto the column and, thus, the sample does not need to flow through the entire length of the loop before entering the column. This type of system is very suitable for quantitative LC, and is probably the more popular injection system in use today. Due to the high pressures that must be employed, LC sample valves are normally fabricated from stainless steel. However, for biochemical applications where the construction materials may need to be bio-compatible, the valves can be made from titanium or other appropriate bio-compatible material. In the case of ion chromatography the valve must also be fabricated from suitably inert materials The construction of a sample valve varies from one manufacturer to another but all simple modifications of the basic sample valve shown in figure 16. A diagram of tone type of valve assembly is depicted in figure 17.
Courtesy of Valco Instruments Inc.
Figure 17. A Simple Form of the LC Sample Valve
The valve consists basically of five parts. Firstly there is a control knob or handle that permits the valve to be rotated and, thus, determines the load and sample positions. Secondly, there is a connecting device that communicates the rotary movement to the rotor. Thirdly, there is the valve body that contains the different ports that connect to the mobile phase supply, the column, the sample loop, the sample injection port and finally a port to waste. Next there is the rotor that actually selects the operation mode and contains slots that can connect the alternate ports in the valve body and, thus provides loading and sampling functions. Finally, there is a pre-load assembly that provides adequate pressure between the faces of the rotor and valve body to ensure a leak tight seal.