High pressure gradient systems

Choice between low pressure and high pressure gradient systems is based on specific analytical requirements. 

High pressure gradient systems employ two pumps whose output is usually controlled by a microprocessor. These systems usually require a mixing chamber which allows accurate and reproducible gradients to be generated. It should be noted that complete mixing of the mobile phases is important to minimise excessive detector noise. 

A few years ago I would have always recommended the high-pressure mixing system, even though it was more expensive performance merited the difference in price. Today, it depends on the applications you anticipate running. If you plan on running 45-min gradients to separate 23 different components, some of them as minor amounts such as with PTH amino acids, then... 

My preference today would be for a high-pressure mixing system if I had to run very complex mixtures on a routine basis because these systems give the best reproducible gradients as a rule. As a routine research instrument or a methods development system, I would prefer a low-pressure, four-solvent dynamically mixed system using a dual-headed pump. I would use the deoxygenation apparatus (Fig. 6.4) to degas my solvents with helium and run them under a helium demand valve to conserve helium. 

Figure 4.2 Low-pressure gradient system (top) and high-pressure system (bottom).

Flush the pump with open drain valve at high flow rate. Degas the mobile phase or use an on-line degasser. (This is absolutely necessary with low-pressure gradient systems.) Clogged inlet filter of the pump ... 

Tolson, D. Organ, A. Shah, A. Development of a High-pressure Gradient Pumping System for Parallel Liquid Chromatography/Mass Spectrometry for the Analysis of Combinatorial Libraries, Rapid Commun. Mass Spectrom. 15, 1244-1249 (2001). 

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