Liquid Chromatography - The Mobile Phase Supply System 1
The Mobile Phase Supply System
The mobile phase supply system consists of number of reservoirs (200 ml to 1,000 ml in capacity). At least two reservoirs would be necessary and are usually constructed of glass or stainless steel and contain an exit port open to air. Stainless steel, however, is not considered satisfactory for mobile phases buffered to a low pH and containing certain materials that can cause corrosion. Each reservoir is usually fitted with a gas diffuser through which helium can be bubbled. Many solvents and solvent mixtures (particularly aqueous mixtures) contain significant amounts of dissolved nitrogen and oxygen from the air. These gasses can form bubbles in the chromatographic system that cause both serious detector noise and loss of column efficiency. As helium is very insoluble in most solvents, it purges the oxygen and nitrogen from the solvent but does not produce bubbles in the system itself. Applying a vacuum to the reservoir is not a permanent solution to dissolved air as, on releasing the vacuum to allow the solvent to pass to the pump, air again dissolves in the solvent. The solvent is filtered through a stainless steel or sintered glass filter to remove any solid contaminants. Depending on the type of solvent programmer that is employed, the supply from each reservoir may pass either to a pump or to a valved blending device. Solvent reservoirs are not usually thermostatted but, when necessary, the solvent can be brought to the column temperature by the use of an appropriate heat exchanger. The solvent containers are often situated in an enclosure that protects the user from toxic solvent vapors such as chloroform or aromatic hydrocarbons. Such enclosures also isolate the solvents from atmospheric moisture.
The Gradient Programmer
The High Pressure Programmer
There are two basic types of solvent programmer. In the first, the solvent mixing occurs at high pressure and in the second the solvents are premixed at low pressure and then passed to the pump. The high pressure programmer is the simplest but most expensive as each solvent requires its own pump. Theoretically, there can be any number of solvents involved in a mobile phase program, however, most LC analyses require only two solvents, nevertheless, up to four solvents can be accommodated. The layout of a high pressure gradient system is shown in figure 2 and includes, as an example, provision for three solvents to be mixed by appropriate programming.