What is a theoretical plate in chromatography?

A theoretical plate is a conceptual unit of separation efficiency. More plates mean narrower, better-separated peaks. A typical analytical HPLC column has 5,000–20,000 theoretical plates.

Which peak width method should I use?

Peak width at half-height (W½) is more accurate for tailing peaks and is the method recommended by USP. Width at base assumes a perfect Gaussian peak and is calculated as the intercept of tangent lines on the peak flanks.

What is a good HETP value?

Lower HETP means higher efficiency. For a 150 mm HPLC column with 10,000 plates, HETP = 0.015 mm (15 µm). Sub-2-µm particle columns can achieve HETP < 5 µm under optimized conditions.

What does the tailing factor indicate?

A tailing factor (T) of 1.0 is a perfect Gaussian peak. T > 1.2 indicates tailing (common with basic analytes on silica), while T < 0.8 indicates fronting. USP limits are typically 0.8–1.5.

How is resolution (Rs) calculated?

Resolution Rs = 2(tR2 − tR1) / (W1 + W2), where W is the peak width at base. Rs ≥ 1.5 is considered baseline resolution; Rs ≥ 2.0 is complete separation.

What is the difference between N and N_eff?

N (total plates) uses the retention time including dead volume. N_eff (effective plates) uses the adjusted retention time (tR − tM), which better reflects the true separating power for a retained analyte.

Theoretical Plates Calculator

Name: Theoretical Plates Calculator
Availability: InStock
Author: Nham Vu

Enter retention time and peak width to calculate column efficiency (N) and plate height (HETP) for any chromatography column.

Column Parameters

Column Length (mm)

Peak 1 (Analyte)

Retention Time t_R

Dead Time t_M

Peak Width Method

Peak Width W (same units as t_R)

Peak 2 (optional — for resolution)

Retention Time t_R2

Peak 2 Width W₂

Formulas

N (base) = 16 × (t_R / W)²

N (half-height) = 5.545 × (t_R / W½)²

HETP = L / N

N_eff = 16 × ((t_R − t_M) / W)²

T = (W_0.05 / 2) / f_0.05 (USP tailing)

Rs = 2(t_R2 − t_R1) / (W₁ + W₂)

Enter column length, retention time, and peak width, then click Calculate.

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