Retention Factor k' Calculator

Calculate the retention factor (k prime) in chromatography, a key metric for compound separation efficiency. Designed for students, researchers, and lab technicians. All calculations run offline. Supports decimals and scientific notation (e.g., 1.2e-3).

Educational only. Follow your lab SOPs and SDS.

Parameters

Retention Time of Analyte (t_R)

Example: 2.5, 0.05, or 1.5e1

Retention Time of Unretained Compound (t₀ or t_M)

Often the solvent front or a non‑interacting marker.

Result Precision (decimal places)

Result

3.00

Retention Factor (k′)

Unitless (ratio)

Intermediate Values

Adjusted t_R: 5.2 min
Adjusted t₀: 1.3 min
t_R − t₀: 3.9 min

Understanding Retention Factor (k′)

The retention factor (k′, also called capacity factor) is a fundamental metric in chromatography that describes how long a compound is retained on the column relative to an unretained compound. It is independent of column dimensions and flow rate, making it ideal for method development and transfer.

How to Use This Tool

Enter the retention time of your target analyte (t_R).
Enter the retention time of an unretained compound (t₀), often measured as the solvent front or using a non‑interacting marker like uracil in reversed‑phase HPLC.
Ensure both times are in the same unit (use the unit toggles if needed).
Click "Calculate k′". The tool will compute k′ = (t_R − t₀) / t₀.

Realistic Examples

Scenario	t_R	t₀	Calculated k′	Interpretation
Early eluting peak	2.1 min	1.0 min	1.10	Low retention, may co‑elute with solvent front.
Ideal separation	8.5 min	1.2 min	6.08	Good retention; optimal for most methods.
Strongly retained	22.0 min	1.5 min	13.67	High retention, long run time; consider gradient.

Common Mistakes Checklist

Using inconsistent units (e.g., minutes for t_R, seconds for t₀).
Misidentifying t₀ – it must be a compound with no interaction with the stationary phase.
Using t₀ = 0 (causes division by zero).
Ignoring extra‑column volume when measuring t₀ on a different instrument.

k′ Scenario Explorer

Adjust t₀ (hold‑up time) to see how it affects k′ for typical analyte retention times.

Set t₀ (hold‑up time)

1.5 min

Typical range: 0.5–3 min for analytical columns.

t_R (min)	k′	Retention Strength

Frequently Asked Questions

What is a good k′ value?

For isocratic separations, k′ between 1 and 10 is generally desirable. Values below 1 may lead to poor resolution from the solvent front; above 10, run times become long and peaks broaden.

How do I measure t₀ accurately?

Use a non‑interacting marker (e.g., uracil for reversed‑phase HPLC, methane for GC). The marker should elute at the same time as the solvent front. Avoid using the injection peak.

Can k′ be negative?

No. If t_R < t₀, the compound elutes before the unretained marker, which is physically impossible in normal chromatography. Check your t₀ assignment.

Does k′ depend on flow rate?

No, k′ is a thermodynamic parameter (ratio of time spent in stationary vs. mobile phase). However, measured t_R and t₀ scale with flow rate, so their ratio remains constant.

Safety considerations?

Always follow SDS for solvents and analytes. Ensure proper ventilation when measuring t₀ with markers like methane (flammable). Waste disposal per local regulations.

Formulas & Assumptions

Formula: k′ = (t_R − t₀) / t₀

Assumptions: Linear chromatography (no overloading), constant flow rate, t₀ accurately represents the mobile‑phase hold‑up time.

Constants used: None (pure ratio).

Rounding: Results rounded to the selected decimal places. Intermediate calculations use full precision.

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Retention Factor k Prime Calculator

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