Is a 1 - 2 minute retention time acceptable?
This question was recently posted in an on-line chromatography forum. There were many responses, but too many of the "experts" who replied did not provide a good answer. In my opinion, many got it wrong! Why?
This question is about the "void time" in chromatography. Because of the physical volume of mobile phase (or carrier gas) inside the column, there is a certain minimum time required just to get from the injection point to the detector - this is called the void time. Nothing can elute from the column before this time, and anything that elutes at or just after this time did not spend enough time on the stationary phase to create any separation from other components. As a result, you cannot get any useful data from these peaks. Your retention time should be at least three times your void time.
So, how do you know what your void time is? It will depend on the size of your column and your flow rate. You can estimate the void volume (Vm) in HPLC from this equation:
Vm = 0.5 * length * diameter * diameter
The void time (tm) is then calculated from Vm and your flow rate (Fc):
tm = Vm / Fc
For some typical LC columns, void times are estimated in the following table.
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In GC, we usually set the linear velocity (u, in cm/sec) rather than the flow rate, so you can calculate the void time from the column length and velocity:
tm = Length / Linear Velocity
The following table gives estimated void times for some typical GC columns.
So, back to the original question - is a 1 - 2 minute retention time acceptable? The answer is - it depends!
For many "typical" columns under typical operating conditions, the answer is NO. Such retention times are at or near the void volume.
If you are using a short column, and/or high flow rates (fast LC, UHPLC, fast GC, etc.), a 2 minute retention time may actually represent a reasonable retention time.
Take Home Message: Know your Void Time, and adjust your settings so retention time is at least three times the void time. (Important note: you cannot make this change by adjusting the flow rate, since the void time will also change.)
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