A retention-time ratio alone omits the hold-up subtraction required by the retention-factor definition.
Separate adjusted and total time
Chromatographic retention factor k can be written as adjusted retention time divided by hold-up time: k = (tR - tM)/tM. The total retention time includes the hold-up contribution. The time basis and a suitable hold-up observation must be defined for the chromatographic conditions. Primary reference: IUPAC: retention factor.
A simple total-time ratio tR/tM is consequently 1 + k, not k. Retention time by itself also does not establish retention factor when hold-up time is missing. Keep those quantities separate rather than relying on similar sounding labels.
Work through an invented time record
Take an original isocratic worksheet with tR = 6 minutes and tM = 2 minutes. Adjusted time is 4 minutes and k = 4/2 = 2. Dividing total time by hold-up gives 3. Entering 3 as k would therefore preserve the hold-up contribution that the definition requires subtracting.
For a second fictional condition, let tR = 3 minutes and tM = 1 minute. Its total retention time is shorter, but its k remains 2. The numbers show why comparing minutes and comparing normalized retention are different questions; they do not describe a change to a real chromatographic method.
Audit the definition on the worksheet
Record tR, tM, their common unit, adjusted tR, calculated k and the means used to establish hold-up. Link the observations to the same defined conditions. Reconstruct tR = tM(1 + k) to test arithmetic and distinguish a missing subtraction from a transcription error.
Then examine whether the method supports this calculation, especially when acquisition conditions vary during a run. A valid numerical k does not identify a compound or guarantee separation. This guide concentrates on the hold-up definition and avoids prescribing a marker, column or acceptable retention range.
Customer Questions
What is k for 6 and 2 minutes?
The adjusted-time calculation gives 2.
Is the total-time ratio also k?
It equals 1 + k here.
Does shorter total time always mean smaller k?
The hold-up time can change too.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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