Low-pressure mixing system

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.)... 

High Pressure Figure 3.12 Low-pressure mixing system. 

Figure 4.2. Examples of modular and integrated HPLC systems. Modular systems (a) Agilent 1100 Series, (b) Jasco low-pressure mixing system. Integrated systems (c) Waters Alliance, (d) Shimadzu 2010.

Figure 4.4. Schematic diagrams of a low-pressure mixing system (a) using a single pump and a four-port proportioning valve, and a high-pressure mixing system (b) using two separate pumps and a controller to blend solvents under high pressure.

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. 

Low-pressure mixing systems require advanced software control for the opening and closing of the valves. This is mainly because LC pumps are designed to deliver a flow that is as constant as possible. The suction (input) flow, however, is extremely variable. For the delivery stroke to be smooth, the suction (refill) stroke has to be rapid. Yet, it is in this inlet flow stream that mixtures and gradients are being generated. 

Figure 2.6 High-pressure mixing systems use two or more independent pumps to generate the gradient. Low-pressure mixing systems use a singie pump with a proportioning vaive to controi composition. The advantages of high-...

---