Low pressure gradient systems

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 ... 

If a quaternary low-pressure gradient system and a six-column selector are available, a two-eluent-six-column combination can be tested overnight For the four mixer chamber inlets use, e.g., the following MeCN, acidic water, basic water, rinsing liquid, e.g. ... 

Purge pump A with water and pump B with methanol. Dial-a-mix 20% B and equilibrate the column at 1.5mL/min (six column volumes or a stable baseline at 254 nm, about 10 min). (Lab note If the gradient system is a low-pressure mixing system, solvents must be degassed by purging or running under helium.)... 

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

Also known as gradient delay volume, system dwell volume is the liquid holdup volume of the HPLC system from the point of solvent mixing to the head of the column. This includes the additive volumes of the injector, the sample loop, all fluidic connection tubing, and any internal pump volumes of a low-pressure mixing system. The typical dwell volume of a modem HPLC is 0.5-2 mL, but can be as high as 5-7 mL in older systems. 

In low pressure systems the gradient is generated prior to the pump and is delivered to the colunm in pulses determined by the dead volume of the pump head. Their main advantage is the relatively low cost. A simple system designed to generate a low pressure gradient has... 

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