Resolution (Rs)
Definition
Rs = 2(tR2 − tR1) / (W1 + W2). Above 1.5 means baseline separation; below 1.0 means peaks overlap and quantitation is unreliable. Improving Rs is the main reason a chemist changes mobile-phase composition, column length or particle size.
- Baseline-separation threshold
- Rs ≥ 1.5 (conventional standard in forensics)
- Overlap risk
- Rs < 1.0 means peaks overlap and quantitation is unreliable
- Formula
- Rs = 2(tR2 − tR1) / (W1 + W2)
Common questions
What is Rs and why does it matter in chromatography?+
Rs is a numerical measure of how well two adjacent peaks are separated in a chromatogram. It's calculated from the distance between peak positions and their widths. In forensic work, an Rs of 1.5 or higher means the peaks are fully baseline-separated, which is essential for reliable identification and quantitation of compounds.
What happens if Rs is below 1.5?+
When Rs falls below 1.0, peaks overlap significantly and quantitation becomes unreliable. Even between 1.0 and 1.5, separation is incomplete and forensic identification can be questionable. This is why chemists adjust mobile-phase composition, column length, or particle size to improve Rs for critical analyses.
How do you calculate Rs?+
The formula is Rs = 2(tR2 − tR1) / (W1 + W2), where tR is the retention time and W is the peak width. This gives you a single number that tells you whether your separation is good enough for forensic work.
Related terms
- Mobile Phase
- The solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching...
- Stationary Phase
- The immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it...
- Limit of Detection (LOD)
- The lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviation of...
- Retention Time (tR)
- Time from sample injection to peak maximum at the detector in HPLC or GC. Characteristic of an analyte under a defined column...
- Rf (Retention Factor)
- Distance travelled by the analyte divided by the distance travelled by the solvent front, on a TLC or HPTLC plate. A unit-less...
- Rf Value
- The retention factor, defined as the distance the spot travels divided by the distance the solvent front travels on the same plate....
- Theoretical Plate Count (N)
- A measure of column efficiency, N = 16(tR/W)² for a column peak, where tR is retention time and W is peak width...
- Visualising Reagent
- A spray or dip that converts a colourless TLC spot into a coloured or fluorescent one so the chemist can locate it....
Explained in these topics
- Chromatographic Techniques: TLC, HPLC, HPTLC and GLCA numerical measure of how completely two adjacent peaks are separated, calculated from peak spacing and peak width. An Rs of 1.5 is the conventional baseline-...
- Chromatography Fundamentals: Paper, Column and TLCRs = 2(tR2 − tR1) / (W1 + W2). Above 1.5 means baseline separation; below 1.0 means peaks overlap and quantitation is unreliable. Improving Rs is the main reas...