Pharmaceutical Analysis
Chromatographic Resolution Calculator
Calculates the resolution (Rs) between two peaks in HPLC or GC, from peak data or column efficiency.
Resolution (Rs)
2.67
Excellent
Peak data
The earlier-eluting peak.
The later-eluting peak.
Width at the baseline, same unit as t.
Width at the baseline, same unit as t.
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Working
Simulated peaks
Two idealised peaks at your retention times, on a 0–10 min axis.
Resolution criteria
Bands used here: below 1.0 Poor · 1.0–1.5 Baseline not achieved · 1.5–2.0 Good · 2.0 and above Excellent.
Rs = 2 (t₂ − t₁) ÷ (w₁ + w₂)
t₁, t₂ = retention times of the two peaks; w₁, w₂ = their widths at the baseline, in the same unit. Resolution has no unit.
Rs = (√N ÷ 4) × ((α − 1) ÷ α) × (k ÷ (1 + k))
N = plate number (efficiency), α = selectivity (k₂/k₁), k = retention factor. This form shows which lever to pull: doubling N raises Rs only by √2, while a small increase in α often helps far more.
Sources: USP <621>; Snyder, Introduction to Modern Liquid Chromatography.
Keep learning
References
- Harris DC, Lucy CA. Quantitative Chemical Analysis. 10th ed. New York: W. H. Freeman (Macmillan Learning); 2019.
Common questions
Either, as long as all four values use the same unit. The units cancel, so Rs is the same number.
Retention time 2 was entered as the earlier peak. Resolution is defined for the later peak minus the earlier one; swap the two values.
The baseline width from tangents drawn to the sides of the peak (about 4σ). If your software reports width at half height, use Rs = 1.18 (t₂ − t₁) ÷ (w½,₁ + w½,₂) instead.
USP system suitability usually asks for Rs of at least 1.5 between the analyte and the closest eluting peak, though individual monographs can set their own limit.
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