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Plate Number, HETP & Resolution

Measure column efficiency from a chromatogram: theoretical plate count (N), height equivalent to a theoretical plate (HETP), resolution between two peaks, and the USP tailing factor.

How to use this tool

Score how well a column is performing straight from a chromatogram. Read a peak's retention time and width off the trace, type them in, and get the plate count, plate height, peak resolution and tailing, the standard measures of column health.

What to enter

Retention time t_R: when the peak apex elutes, in minutes.
Width at half-height w½: the peak's width measured halfway up, in minutes. Narrower means a more efficient column.
Baseline width: optional; width where tangents to the peak sides meet the baseline. Used only if half-height is left blank.
Column length: optional, in mm; needed to convert plate count into HETP.
Peak 2: optional second peak (retention time + baseline width) to get the resolution between the two.
Tailing factor: total peak width at 5% height and the leading half-width, to score peak symmetry.

Reading the result

The headline is N, the theoretical plate count, bigger is better, tens of thousands for a good modern column. HETP is the height per plate (smaller is better). Resolution R_s ≥ 1.5 means two peaks are baseline-separated; tailing factor T_f ≈ 1.0 is a symmetric peak, above 2.0 is significant tailing.

Worked example

A peak at t_R 5.0 min, w½ 0.20 min on a 150 mm column gives N ≈ 3460 plates and HETP ≈ 43 µm; with a second peak at 5.8 min the resolution is R_s ≈ 2.3 (baseline) and the tailing factor T_f ≈ 1.15 (symmetric).

Peak 1

Retention time t_R

min

Width at half-height w½

min

Baseline width w (tangent), optional

min

Half-height is used if given; otherwise baseline width. Column length for HETP, optional

Peak 2 optional, for resolution & tailing

Retention time t_R

min

Baseline width w

min

Tailing factor peak 1, at 5% height

Total width W_0.05

min

Leading half-width f

min

Result

N (plate count) and HETP gauge column efficiency, more plates and a smaller HETP are better. Resolution R_s ≥ 1.5 means two peaks are baseline-separated; tailing factor T_f ≈ 1.0 is a symmetric peak, above 2.0 is real tailing. Each metric appears only once you've supplied the inputs it needs.

Methodology

Plate count from half-height width: N = 5.54 (t_R/w½)²; from baseline-tangent width: N = 16 (t_R/w)². HETP = L / N. Resolution R_s = 2|t_R2 − t_R1| / (w₁ + w₂). USP tailing factor T_f = W_0.05 / (2f), where W_0.05 is the full peak width at 5% height and f is the leading edge to peak apex.

Reading the numbers

R_s ≥ 1.5 is baseline resolution; ≥ 2.0 is a robust separation.
T_f ≈ 1.0 is symmetric; > 2.0 indicates significant tailing.