HPLC pumps, maintenance

Common maintenance tasks that can be easily performed by the user are the replacement of solvent filters (sinkers), in-line filters, check valves, and piston seals. Most modern HPLC pumps are designed for easy maintenance, with front panel access to many internal components. Figure 10.1 is a diagram of a slide-out pump unit showing the two pump heads with check valves, the purge valve, the in-line filter, and other components. Procedures for replacing some consumable items are summarized below ... 

Traditionally maintenance parts are replaced on a time base. For example, an HPLC pump seal every 2 months, a detector s lamp every 3 months or so. This is not economical for the laboratory and not environmentally friendly because frequently a replacement of the parts would not yet be necessary. A better way is to exchange maintenance parts on a usage basis. The user can enter limits for the lamp, the solvent pumped through and the number... 

Monoliths Low backpressure, suited for conventional HPLC Higher separation efficiency by column coupling Rugged against delay volume and extra-column band broadening fast column re-equilibration Reduced maintenance on pumps and injector seals Reduced need for sample pre-treatment... 

Other problems with pneumatic intensifier pumps include the fact that access to the high pressure seals for inspection and maintenance is usually quite restricted, by nature of their design. Because of the way they operate, the flow they produce is inherently highly pulsatile in nature and they also tend to be extremely noisy in use. For these reasons, pumps of this type are not used in general analytical HPLC. However, pneumatic intensifier pumps have found a niche in the packing of HPLC columns, where the intermittent nature of the function and their ability to deliver very high pressures compensate somewhat for their shortcomings in the analytical field. 

While it is possible to use a number of volatile solvents as the mobile phase for SCF chromatography, the most commonly used mobile phase is carbon dioxide. However, carbon dioxide is not a good solvent for polar compounds so it is common to add a small amount of some additional polar organic liquid such as an alcohol or even water as a modifier. However, the modifier needs to be miscible with carbon dioxide. Much of the other technology associated with either GC or HPLC in terms of sample inlets and types of pumps are adapted to specific applications but the key attribute of the SCF-type chromatography is the maintenance of T, P) conditions near the critical point of the mobile phase. A selection of columns is available just as for GC or HPLC. 

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