Gradient controller high pressure mixing

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. 

In high pressure mixing, two (rarely more than two) independent high pressure pumps each deliver a mobile phase component to a central mixer. To control the final mobile phase compositions delivered to the column, the volumetric flow rate of each pump is controlled via computer or microprocessor. Thus, as earlier, an isocratic mobile phase of 75% water and 25% methanol delivered to the column at 1 mL/min would be obtained by having Pump A deliver water at 0.75 mL/min and Pump B deliver methanol at 0.25 mL/min. To effect the gradient as before, the flow rates of Pump A would be smoothly and continuously reduced from... 

High pressure systems provide one high pressure pump for each solvent the output from each pump is regulated by an electronic control, and the combined streams are mixed and sent to the sample valve. Although this system is more costly because of the need for two pumps, it produces more precise mixtures at the extremes of a gradient. 

Gradient Devices. Gradient devices can also be divided into two types high pressure and low pressure. In a typical low pressure system (depicted in Figure 9.22), various proportions of up to three solvents are mixed before they enter the high pressure pump. The proportions are regulated by valves that are in turn controlled electronically with microprocessors. 

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