Resolution, Efficiency and the van Deemter Curve
Resolution compares peak spacing with peak widths. The van Deemter model relates plate height to mobile-phase speed. Retention, selectivity and efficiency help distinguish useful separation changes from changes that only increase analysis time.
What a learner can do afterwards
- Calculates resolution using baseline peak widths and explains the approximate 1.5 criterion
- Names the basic van Deemter contributions and describes their speed dependence
- Finds the best measured plate height and distinguishes it from an exact curve minimum
- Chooses selectivity or efficiency changes and explains the benefits and costs of increasing column length
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Resolution Rs compares peak spacing with width. For baseline widths, Rs = 2(tR2 - tR1)/(w1 + w2). Use the same units for all times and widths. For similar, roughly Gaussian peaks, Rs about 1.5 is a common near-baseline separation target. Unequal or tailing peaks may require a different target.
Peaks at 20 and 25 seconds have baseline widths of 4 and 4 seconds. Their gap is 5 seconds and their summed width is 8 seconds. Rs = 2 × 5/8 = 1.25.
Plate height H = L/N measures efficiency per column length; lower H means better efficiency. The basic van Deemter model is H = A + B/u + Cu, where u is mobile-phase linear speed. In a packed column, A represents different flow paths and is treated as constant. Longitudinal diffusion gives B/u, which falls with speed. Mass-transfer resistance gives Cu, which rises with speed. An ideal open-tubular column has no packed-path A term.
For positive diffusion and mass-transfer terms, the model's minimum H gives an optimum speed. Diffusion raises H at low speed; mass-transfer resistance raises it at high speed. Among measured settings, choose the lowest H. A few measurements do not locate the exact minimum between them.
Nearly equal retention factors suggest changing the phase or mobile phase to alter selectivity. If retention and selectivity are suitable but efficiency is low, extra column length may help. At fixed H, N grows with length; at fixed retention factor and selectivity, resolution grows roughly with the square root of N. Longer columns also increase retention times and pressure demand. Peaks need not become narrower in seconds.
Specify the width convention, seek low plate height, and distinguish a best tested speed from an exact optimum. Extra length can improve resolution without narrowing peaks in seconds.
Original lesson, questions and flashcards written for this dataset.
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