Welcome to the next installment in my series on binary vs. quaternary pumps for liquid chromatography!
In my previous post, I discussed how gradients are created as well as the importance of having accuracy and precision in your gradient. Today I am going to talk about what happens after all that — specifically, mixing.
After chromatographic instrument’s pump or the proportioning valve delivers a gradient, the system then needs to mix the solvents to ensure that a homogenous solution gets delivered to the column. Pumps currently fall into two categories: High pressure mixing (traditionally binary pumps) and low pressure mixing (traditionally quaternary pumps).
What is the difference between these two mixing modes and does it matter?
You can ask 100 different LC users and you will get 100 different answers. This is an eternal debate that will probably go on forever, like which movie is better: Star Trek or Star Wars (obviously, we know which one is better — it’s Star… oh wait I’m not supposed to put my personal opinions into the Waters blog!). However, I can at least provide you information of how each works, and you can decide if you are pro-high pressure mixing (HPM) or pro-low pressure mixing (LPM).
Impact of Mixing Type
The biggest impact these two mixing modes have on chromatography is on the system volume, also known as dwell volume. Dwell volume (or system volume) is measured from where the mobile phase begins mixing to when it reaches the column. The larger this volume, the longer it takes for the new mobile phase conditions to arrive at the column.
High Pressure Mixing (HPM)
In the case of a HPM system (typically binary pumps), the mix-point occurs at the mixing chamber, which is located past the pumps. This is characterized with a lower dwell volume for the system, as the volume that is measured is from the mixer to the head of the column (Figure 1). This lower dwell volume characteristic allows a binary LC system to deliver changes in the gradient much more rapidly to the column, allowing for faster re-equilibration between samples.
Low Pressure Mixing (LPM)
In the case of a LPM system (typically quaternary pumps), the point where mixing of the mobile phase begins is right after the proportioning valves. So when accounting for the dwell volume in a LPM system, all these volumes must be taken into consideration:
- The volume from the valves to the pump head
- The volume of the pump heads
- The volume from the pump heads to the column
- Additional injection or holding loops
- Any additional mixers that are added to facilitate further mixing
This leads to a larger dwell volume for a LPM system, which can lead to a longer time for re-equilibration conditions to reach a column after each sample is completed (Figure 2). Another concern with the extra dwell volume in an LPM system is that the volume can create an additional amount of isocratic hold if it is not accounted for during a method transfer. Fortunately, there are a lot of tools available to help scientists account for dwell volume when they are transferring methods.
Do I have a choice?
This question has a yes-and-no answer. You do have a choice in that you can pick a binary or quaternary system, but in the end the system defines which mixing you will get:
- If you use a binary system, you will be in a high pressure mixing environment, as there is no real way to create a low pressure mixing system using a binary pump
- With a quaternary system, you will either be in a pure low pressure mixing environment or a hybrid environment that does low pressure mixing, but will add a mixer after the pumps to facilitate better homogenization of the gradient packets
In the next post (chapter 4), I will discuss how you can you can decide which pump is right for you.
Chapters in this series on LC system operation: