How to use the chromatography resolution calculator
- Enter the dead time, the time for an unretained peak to pass through the column.
- Enter the retention times of the first and second peaks, measured from the injection.
- Enter the base width of each peak in the same time unit.
- Optionally enter the column length in millimeters to get the plate height. Read the resolution, the selectivity, the retention factors and the plate counts.
Formula
Resolution Rs = 2 x (t2 - t1) / (w1 + w2), with w the base widths
Retention factor k = (t - t0) / t0, with t0 the dead time
Selectivity alpha = k2 / k1
Plate count N = 16 x (t / w)^2, and plate height H = column length / N
A resolution of 1.5 or more is commonly used as the sign of baseline separation. A method may set its own requirement, so check yours.
Two peaks at 5.0 and 6.0 minutes
With a dead time of 1 minute, retention times of 5.0 and 6.0 minutes and base widths of 0.5 minutes, the resolution is 2 x 1.0 / (0.5 + 0.5) = 2. The retention factors are 4 and 5, so the selectivity is 5 / 4 = 1.25.
The plate counts are 16 x (5 / 0.5)^2 = 1600 and 16 x (6 / 0.5)^2 = 2304. With a 150 mm column the average plate count is 1952, which is a plate height of about 0.0768 mm.
Improving a separation
Resolution depends on three things: how selective the system is (alpha), how long the peaks are retained (k) and how narrow the peaks are (N). Changing the mobile phase or the stationary phase changes alpha and is the most powerful lever. A longer column increases N, but resolution rises only with the square root of the length, and the run takes longer.
Quantities in the sample are often given in different units. See the concentration unit converter and the solution dilution calculator.
Measuring the widths
The base width is the width of the peak at the baseline between tangents drawn to the sides of the peak. Some software reports the width at half height instead, and then the plate count formula uses 5.54 in place of 16 and the resolution has a different constant. Use the base width on this page to get the results shown.
To compare a measured retention time with a known value, use the percent error calculator.
Assumptions and limits
- Peaks are roughly symmetrical and Gaussian, and widths are measured at the baseline.
- All times are in the same unit, and retention times are above the dead time.
- The second peak elutes after the first.
- Requirements for resolution come from your method or regulator, not this page.
Frequently asked questions
What does a resolution of 1.5 mean?
It means the two peaks are separated almost to the baseline, so their areas can be measured separately. Lower values mean the peaks overlap.
What is the retention factor?
It is the time a compound spends in the stationary phase compared with the time in the mobile phase. It equals the retention time minus the dead time, divided by the dead time.
What does selectivity tell me?
It is the ratio of the retention factors of the two peaks. A value of 1 means they co-elute, and larger values mean an easier separation.
How is the plate count found?
From the retention time and the base width of a peak: N = 16 x (t / w)^2. Higher counts mean narrower peaks and a more efficient column.