Dwell volume and dead volume can effect your chromatography. What’s the difference between the two?
Let me make a couple of clarifications here. Instead of using the term “dead volume,” we prefer the term “extra column volume.” Extra column volume indicates the volume outside of the column, which we aim to minimize.
The Importance of Extra Column Volume
So, what exactly do I have control over? It’s the tubing I use to connect the column. Every piece of tubing used to connect the injector to the column and the column to the detector affects your extra column volume. This, in turn, influences your peak shapes, making your peaks broader, not skinnier. Therefore, minimizing extra column volume is crucial for all our LCs, as it affects our efficiency and the skinniness of our peaks.
Exploring Dwell Volume
Now, let’s talk about dwell volume. Dwell volume is defined as the volume between where we form a gradient and where it actually touches the column. The gradient takes some time to get from the pump to the head of the column. This involves physical components such as tubing and fittings in the instrument, and the mobile phase has to travel from where it’s formed to the head of the column. Depending on your instrument’s generation, the dwell volume might be 1 milliliter, which means it takes one milliliter of volume from when you instruct the instrument to change the mobile phase to when it actually reaches the column.
The Shower Analogy
My analogy for this is thinking about your shower at home. Do you ever step into the shower and then turn on the water? Of course not. Why? Because we know there will be cold water initially. There’s a volume between our hot water heater and the shower head, which is akin to the dwell volume. It represents how long it takes for the mobile phase to reach the column. This is crucial because, as we switch between instruments, it will affect the chromatography.
Advice for Handling Dwell Volume
For most, my advice is not to overthink this. It’s a function of your instrument and will impact you if you transfer a method from one instrument to another. You might notice differences in retention time, especially at the beginning of the gradient. If it affects the separation, then adjust the conditions accordingly.
If you would like to go deeper into this topic, feel free to send me an email for more information about dwell volume, including how to measure it.