High-pressure mixing

The basic instrument required for packed-column unified chromatography is shown schematically in Figure 7.9. This is essentially a two-pump HPLC instrument utilizing high-pressure mixing with just a few new components. At least one pump must... 

In high pressure mixing devices each solvent is piunped separately in the proportions required by the gradient into a mixing chamber before being delivered to the column. Solvent compressibility and thermodynamic volume changes on mixing may Influence the accuracy of the composition delivered to the column. 

Cross A high-pressure, mixed-phase, thermal process for cracking petroleum, introduced in the United States in 1924 by the Cross brothers, further developed by the MW Kellogg Company, and widely used in the 1920s and 1930s. Eventually, 130 units were built in the United States and abroad. 

High pressure mixing to allow minimum gradient delay to the column... 

Another way to classify pumping systems is based on the achievement of solvent blending i.e., under low- or high-pressure mixing conditions. 

The Type II system comes in two flavors. They vary by the type of gradient pumping system they contain low-pressure mixing or high-pressure mixing. The rest of the system is the same injector, variable detector, and computer-based data acquisition and control. Autosamplers would allow 24-hr operation, but most university research laboratories find graduate students to be less expensive. 

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... 

The most common job for these systems is the fast-running isocratic separation. They could be built up from the QC isocratic, but dial-a-mix isocratic is faster and more convenient since they switch easily from job to job. These systems come in the same two flavors as the research gradient, low- and high-pressure mixing, but replace the manual injector with an autosampler, allowing 24-hr operation. For thermally labile samples that need to be held for a period of time before being injected, there are autosampler chillers available. 

Figure 9.5 High-pressure mixing gradient system.

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. 

Low Pressure High Pressure Figure 3.11 Typical high-pressure mixing configuration. 

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