Appropriate Mixer Volume for HPLC and UHPLC Applications

For gradient analysis, most analytical scale HPLC (UHPLC) systems incorporate a solvent mixer which is designed to balance the requirements of moderate dwell volume, low noise and good mixing efficiency. Depending on the method run, the ideal mixer's volume may in fact be completely different than the one installed in your chromatography system. A high-pressure mixing Binary pump can often work well with a slightly lower volume mixer than a low-pressure mixing ternary or quaternary pumping system (because the high pressure mixing gives you a head start), but both pump types benefit from additional mixing.

• Be sure to also consider the volume of any pulse dampener used too as these often have large internal volumes and act as mixers. Some pulse dampeners also incorporate the pressure transducer and/or mixer. These types of combination modules may limit the types of modifications which can be made to optimize the mixing and reduce the dwell volume.

• Don't forget to address the dwell volume contribution of the autosampler, injector loop, interconnecting tubing (extra column volume) and detector flow cell too when optimizing the flow path of your HPLC system.

Here are some general guidelines to help you determine the appropriate mixer volume for your own HPLC system. Note: Since many types of mixer designs exist (static, dynamic, shear...), these are guidelines only. There are some commercially available, high efficiency, low-volume mixers available which can reduce the need for a large volume mixer. Your specific application should be taken into account to determine which size is best.

HPLC System Mixer Volume Choices - Size Matters ("Mixer Volume")

SMALL: Fast or ultrahigh speed separations using low volume, small particle columns. These types of applications depend on a low dwell volume mixer for gradient analysis. To achieve this, your HPLC system should be plumbed with narrow bore capillary tubing (example: 0.005" ID; 0.12mm ID) and include a gradient mixer with a volume of less than 100 ul for low flow rates (example: ~35 ul is rather common size). 

LARGE: High Sensitivity Analysis: Gradient analysis where sensitivity is key, benefit from larger volume mixers to minimize contributions of any UV absorbing additives (e.g. TFA) and turbulence in the flow. Traditional 300 to 750 ul mixers often work well in these applications, provided that the column volumes are also large. Smaller column volumes will require smaller mixer volumes to reduce the added dwell effect.

MEDIUM: Routine HPLC Analysis: Typical analytical separations using 3 to 5 mm ID columns (x 100 mm or longer) usually benefit from modest sized mixers within a range of 200 to 400 ul volume. For these applications, I often start with a recommendation to use a mixer which has 10% of the columns volume as a starting point. For a typical 4.6 x 250 mm, 5 micron porous support column, which has about 3 mLs of internal volume, a 300 ul volume mixer usually provides enough mixing volume for routine gradient analysis.  


Additional Info:

Back in the 1980's we often related mixer volume to intended flow rate/column dimensions. For example: A mixer size of 25 ul was suggested for 50 ul/min flow rates (commonly used with 1 mm ID columns). A mixer size of 200 ul was suggested for 200 ul/min flow rates (commonly used with 2.1 mm ID columns) and 350 ul mixer volume for 1.000 ml/min flow rates (commonly used with 4.6 mm ID columns). Note: Mixers such as these, with large volumes relative to the column volume contributed to large gradient delay times, but this was, and still is, of less concern for isocratic methods.

As mentioned before, the type of mixer, column volume, flow rate and mobile phase characteristics will help suggest the most applicable volume for your application. When in doubt, select a larger mixer volume for isocratic analysis (less baseline noise, better for gradients) and a smaller one if reducing gradient analysis delay volume is critical.