Just want to have a clear picture what is the difference between high presure and low pressure mixer. What should we practically consider when you choose them? Thanks.

High pressure mixers mix on the outlet side of the pump, while low pressure mixing systems mix on the inlet side of the pump.

Since you will often be mixing orgainc with aqueous mixing on the low pressure side can cause outgassing. Low pressure mixing systems need to be used with a vacuum degasser or sparging.

Since the internal volume of the pump is upstream of the point of mixing in a high pressure mixing system, these systems tend to have a lower dwell volume. This can be an important factor if you are using narrow ID columns or low flow rates. Also having mixed systems can cause transfer problems with gradient methods because the dwell volume does affect gradient selectivity. You need to know how to adjust for this.

For most methods either system will work. For very demanding gradient work I might choose a high pressure mixing system. For day to day work I like the flexibility of quatanary systems, these tend to mix on the low pressure side.

High-pressure mixers mix the two eluents after the pumps. This requires, obviously, two pumps. The system dead volume is smaller, because the gradient is formed close to the column. So the programmed gradient times match the chromatogram better. The shorter distance means less turbulent flow, so some eluents may fail to mix. If the eluent mixture is going to outgas, this will happen at the detector, and may cause detector problems. High-pressure mixing systems are often used for microbore columns, which need the precision gradient and large prep scale LC which needs the high capacity.

Low-pressure mixers use a proportioning valve to mix eluents. They only need one pump. The proportioning valve adds dead volume, because the pump's dead volume is now added to the time it takes the gradient to reach the column. The movement of the proportioning valve, and the dead volume helps the eluents mix. If eluents are going to outgas, this will occur at the proportioning valve or the pump inlet, and may cause flow to stop. These are typical workhorse systems for general use. They often have 4 reservoir proportioning valves so method development can be done with a variety of mixtures. Not many people have 4 high-pressure pumps connected to some 5-way tubing to achieve the same flexibility.

Just to add a few comments:

High pressure mixing works by altering the flow rate of the two components to achieve the desired mix (but still keep the same overal flow rate). Low pressure works by just altering the actual mix of the two components, as the other posters indicate.

Anonyymous indicates that two pumps are needed. Some vendors supply binary pumps which have two sets of pump heads to perform the same task (which may work out cheaper than separate pumps, but is more expensive than a quaternary pump). This (may) also make for easier gradient programming, since you just put in the % changes, rather than having to determine flow rate changes - the pump does the rest for you (some vendors using separate pumps have software/firmware that will do this too).

How much benefit you will see depends on the vendor of the pumps and your methods. At least one of our gradient methods does not work on an Agilent Quaternary pump (part of an 1100 system), but did work on a Waters Quaternary pump (part of an Alliance 2690). In order to get separation with Agilent, we had to use their binary pump.

Tim,
Are you saying that the Agilent 1100 and the Alliance 2690 are not low-pressure mixing systems?

A1100 comes in 2 styles, Binary (High Pressure) and Quaternary (low pressure). 2690 is a low pressure mixing system.