Resolution, Efficiency and the van Deemter Curve
Two peaks are separated only if the distance between them beats their width. Width comes from three broadening processes with different dependences on flow rate, which is why every column has a best speed rather than a fastest one.
What a learner can do afterwards
- Calculates resolution from retention times and peak widths and states the value needed for a clean separation
- Names the three broadening contributions and says how each varies with flow rate
- Reads a van Deemter curve to find the optimum flow and explains the shape at each end
- Chooses between a longer column and a more selective phase to fix a stated separation, with a reason
1 · Read
Two peaks split only when their gap beats their width. Resolution is two times the gap divided by the sum of the two widths. At 1.5 the peaks just touch baseline, so aim for 1.5 or more for a clean split.
Width grows from three sources. Eddy paths spread molecules whatever the speed. Diffusion rules at slow flows and fades as you push faster. Resistance to hopping between phases rules at fast flows and grows with speed.
Peaks at 100 and 106 seconds with widths 4 and 4 give resolution two times 6 over 8, which is 1.5. The plate height curve dips at one best flow: high on the slow end from diffusion, high on the fast end from hopping lag, lowest between.
Pick the fix by the cause. A near-zero gap is a selectivity problem, so change the phase. A fair gap with wide peaks is an efficiency problem, so run longer at the best flow.
Beat width with gap, run at the curve dip, and fix gap problems with phase.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.