Calculate column efficiency (N), HETP, and reduced plate height from chromatographic peak data.
Choose the half-height or tangent method depending on which peak width measurement you have. Half-height is more commonly used and more reliable for asymmetric peaks. Enter the retention time and peak width in the same time units (both in minutes or both in seconds). Optionally enter column length and particle size to get HETP and reduced plate height.
Half-height method: N = 5.545 × (tR / W½)². This uses the peak width measured at 50% of peak height, which is easier to measure accurately than base width.
Tangent method: N = 16 × (tR / Wb)². This uses the peak width at the base, determined by drawing tangent lines on the peak inflection points. Both methods give identical results for perfectly Gaussian peaks.
HETP (Height Equivalent to a Theoretical Plate): H = L / N, where L is column length. Lower H means better efficiency. For a 150 mm column with 15,000 plates: H = 150/15000 = 0.010 mm = 10 µm.
Reduced plate height: h = H / dp, where dp is particle diameter. This normalizes for particle size, allowing comparison across different columns. A reduced plate height of 2–3 indicates well-packed columns operating near optimum.
Troubleshooting: Decreasing plate count over time suggests column degradation (voiding, contamination, or frit blockage). Sudden drops usually indicate a void forming at the column head.