What is Retention time?
Also called: tR, RT, Elution time, Relative retention time (RRT)
The time between injecting a sample and the apex of a component's peak on an HPLC trace; characteristic of a molecule only under one fixed method.
By the APL Research Team · Updated
Retention time (tR) is how long a component takes to travel from the injector to the detector, read at the apex of its peak on a chromatogram. In reversed-phase HPLC it reflects how strongly a peptide is held by the hydrophobic column, so it is reproducible for one molecule on one method and meaningless when lifted out of that method.
What sets a peptide's retention time
The sequence matters first. A model built from 346 tryptic peptides predicted retention by summing per-residue retention coefficients, with extra terms for the residues at the N-terminus and corrections for length; retention time was linear in the resulting hydrophobicity score with R² of about 0.94 [1]. The method matters almost as much:
| Variable | Effect on tR |
|---|---|
| Gradient slope | A shallower acetonitrile gradient spreads peaks out and moves them later |
| Column chemistry, length and age | Changes absolute times; worn columns drift |
| Temperature | Higher temperature usually shortens retention |
| Ion-pairing reagent | Retention rose with concentration for all four acids tested, and more with more hydrophobic anions (phosphate < TFA < PFPA < HFBA) [2] |
| Instrument dwell volume | Delays when the gradient reaches the column, shifting every peak |
Charge interacts with the ion-pairing reagent. In that study, peptides of different net charge moved relative to one another as reagent concentration changed, and above 10 mM heptafluorobutyric acid the elution order of a four-peptide set reversed [2]. Changing TFA concentration can therefore reorder peaks, not just shift them.
Worked example: retention factor and RRT
An illustrative run, not data from any batch: unretained material reaches the detector at t0 = 1.10 min and the main peak at 15.40 min.
- Retention factor k = (tR − t0) ÷ t0 = (15.40 − 1.10) ÷ 1.10 = 13.0.
- Relative retention time RRT = tR(impurity) ÷ tR(main peak).
| Peak | tR (min) | RRT | First hypothesis to test by mass spectrometry |
|---|---|---|---|
| A | 12.95 | 0.84 | A more polar variant of the target |
| B | 15.05 | 0.98 | Closely related impurity; check resolution from the main peak |
| Main | 15.40 | 1.00 | Target peptide |
| C | 16.20 | 1.05 | A more hydrophobic species, such as incompletely deprotected material |
Impurity specifications are usually written in RRT because it survives small run-to-run shifts better than absolute minutes. Some methods divide adjusted times (tR − t0) instead, so the convention should be stated.
Modifications have predictable directions but unpredictable sizes. Oxidising one methionine to the sulfoxide lowered reversed-phase retention by 2.37% acetonitrile on average, but the shift ranged from −9.1% to +0.4% depending on sequence, and in one structural context oxidation increased retention [3]. On a 1%-per-minute gradient, the average shift would put the oxidised form of a methionine-containing peptide such as Semax roughly 2.4 minutes earlier.
Common misunderstandings
- "Same retention time, same compound." Different molecules can co-elute. A match is supporting evidence; identity needs mass spectrometry or co-injection with a characterised reference standard.
- "A 12.8-minute peak on one COA should be 12.8 minutes on another." Different columns, gradients and TFA levels give different times [2]. Compare within one method, ideally by RRT.
- "Later peaks are bigger molecules." Reversed-phase order follows hydrophobicity; length enters retention models only as a correction term [1].
The HPLC testing guide covers method conditions and the COA guide shows where retention data appear in batch documentation.
References
- 1.Krokhin OV, Craig R, Spicer V, et al. An improved model for prediction of retention times of tryptic peptides in ion pair reversed-phase HPLC: its application to protein peptide mapping by off-line HPLC-MALDI MS. Mol Cell Proteomics. 2004. PubMed 15238601
- 2.Shibue M, Mant CT, Hodges RS. Effect of anionic ion-pairing reagent concentration (1-60 mM) on reversed-phase liquid chromatography elution behaviour of peptides. J Chromatogr A. 2005. PubMed 16013615
- 3.Lao YW, Gungormusler-Yilmaz M, Shuvo S, et al. Chromatographic behavior of peptides containing oxidized methionine residues in proteomic LC-MS experiments: Complex tale of a simple modification. J Proteomics. 2015. PubMed 26025879