JWD: There are a variety of different answers to this question. Let me post just a couple, but first, let’s make sure we understand what is meant by high- and low-pressure mixing.

   Figure 1 shows a schematic of a high-pressure mixing system. Such systems usually are limited to two solvents, each of which is pumped by a separate pump into the mixer, which is located on the high-pressure side of the pump. The mobile phase mixture is controlled by the relative flow rate of the two pumps. The flow rate is determined by the combined flow rate of both pumps.

   Figure 2 is a diagram of a low-pressure mixing system. Most low-pressure systems have the ability to deliver any combination of up to four different solvents (for clarity, only two are shown in Figure 2). Mixing takes place on the low pressure side of the pump and a single pump is operated at a constant flow rate.

   For method development work, low-pressure mixing systems may have the edge in some cases. Typically, low-pressure systems are configured to allow you the use of 4 solvents in any combination. This means that you could put 2 different pH buffers plus methanol (MeOH) and acetonitrile (ACN) in the 4 reservoirs. Now you could program scouting gradients for two different pH values with MeOH or ACN (4 possibilities) or any blend of MeOH and ACN (limitless possibilities). That gives you a lot of flexibility for unattended method development. High-pressure mixing systems usually are limited to two solvents. However, some high-pressure systems include a solvent selection valve on the pumps that allow you to switch from one solvent to another (between runs), so you could have the A-pump plumbed for buffer 1 or buffer 2, and the B-pump for MeOH or ACN. This configuration should give you almost the same possibilities for either high- or low-pressure mixing.

   For LC-MS work, small, 2.1x50 mm columns usually are used, often with gradient elution. When this is the case, it is best to keep the dwell volume as small as possible. The dwell volume, of course, is the volume from the point the solvents are mixed until they hit the column. High-pressure mixing systems have dwell volumes in the 1.5-3.0 mL region, whereas low-pressure mixers add a milliliter or so for 2.5 – 4.0 mL of dwell volume. The nice thing about high-pressure mixing and LC-MS is that the mixer can easily be replaced with an aftermarket mixer for a smaller dwell volume. For example, in our lab we replaced the conventional high-pressure mixer with a micromixer and reduced the dwell volume from 2.3 mL to 0.4 mL – not bad for a $75 part! Low-pressure mixing systems are not easily converted, but they can be purchased with small-volume mixers for LC-MS applications.

   So the bottom line is that either type of system has its advantages and disadvantages. Fortunately, the differences are small, so either type of system usually can give equivalent results. It’s kind of like which brand of car do you want to buy – there are many choices, but most manufactures have an equivalent model that will get you from point A to point B in similar comfort and with similar economy.

John Dolan is best known as one of the world’s foremost HPLC troubleshooting authorities. His monthly column, LC Troubleshooting, has been a mainstay in LCGC magazine for the last 25 years, with more than 275 installments. He is also known for his ongoing research with Lloyd Snyder, resulting in more than 100 technical publications and three books. Contact John at TechTips@sepscience.com