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Is this column packed well?

HETP, plate number and asymmetry factor calculator

From the peak of a tracer pulse run through the column, calculates the plate number, HETP and peak symmetry (asymmetry factor, As), so you can check in numbers whether the resin is packed evenly.

When to use itUse it right after packing a column, and for routine checks that performance has not dropped with use.

Prefilled with an example: a column with a 200 mm bed and a peak at 10 minutes. Change any value and the result updates.

min

Time from injection to the top of the peak. Use the same unit (minutes or seconds) as the half-height width below.

min

The full width of the peak at half its height. Use the same unit as the retention time.

mm

HETP comes out in this unit. Divide by 10 for cm.

At 10% of peak height, the width on the front (left) side of a vertical line dropped from the apex. Use the same unit as b.

At the same height, the width on the back (right) side of that line. Tailing makes b larger.

Height per plate (HETP)0.231 mm
Plate number (N)865.6 plates
Peak asymmetry factor (As)1.214

This column has about 866 plates, or about 4,328 plates per meter of bed. The peak is nearly symmetrical, so the packing reads as good. Record these values as the baseline for later checks.

How to read it

The row that matches your result is marked.

  • Small HETP (many plates)Well packedEach plate is short and the bed is working efficiently. What counts as good depends on bead size and bed height, so compare with the resin supplier's recommendation or your own criteria.
  • Fewer plates or worse As than right after packingSign of deteriorationSuspect resin damage from repeated use, bed compression, or a build-up of fines. Use it as input for repacking or replacing the resin. Keep a record, with the value right after packing as the baseline.
  • As between 0.8 and 1.8Your resultNearly symmetrical (sign of a good bed)Our related article cites roughly 0.8 to 1.8 as a common guide at many sites. It is not a single acceptance criterion; the actual range is set by your own validation for each resin, application and column diameter.
  • As above 1.8TailingSuspect uneven flow through the bed (channeling), a disturbed bed top, or gaps in the flow path such as dead volume in the piping. Judge plate number and As together, never one alone.
  • As below 0.8FrontingSuspect overpacking or bed compression. Even with enough plates, a poor As is a sign of disturbed flow.

How it is calculated

The plate number corresponds to how many separation steps happen in the column; the more plates, the sharper the column. HETP is the bed height divided by the plate number, the height of one plate, and smaller is better. The closer As is to 1, the more symmetrical the peak. Keep retention time and half-height width in the same unit, and a and b in the same unit.

N = 5.54 × (tR ÷ W½)² HETP = L ÷ N asymmetry factor As = b ÷ a

Source: Plate number N = 5.54 × (tR ÷ peak width at half height)² and HETP = bed length ÷ N. This is the column-efficiency definition in USP <621> Chromatography (half-height method).

With your numbers

  1. N = 5.54 × (tR ÷ W½)² = 5.54 × (10 ÷ 0.8)² = 865.6 plates
  2. HETP = bed length ÷ N = 200 ÷ 865.6 = 0.231 mm
  3. As = b ÷ a = 0.34 ÷ 0.28 = 1.214

Limits of this calculation

  • Plate number, HETP and As are not absolute pass/fail criteria. They are relative guides for establishing, by measurement, what a good pack looks like for that column and resin. Reasonable values depend on bead size, bed height and application.
  • There is no single acceptance range for As (the 0.8 to 1.8 in our related article is one example). It has to be set by your own validation for each resin, application and column diameter.
  • Results change with flow rate, tracer type and amount, injection volume and detection method. Values are only comparable when conditions are fixed and the same procedure is used.
  • N = 5.54 × (tR ÷ half-height width)² is the USP <621> half-height method, an approximation that assumes a nearly symmetrical peak. For strongly tailing peaks, it may not agree with other methods (such as the Ph. Eur. and USP six-sigma or integration methods).
  • Use the same unit for retention time and half-height width, and for a and b. HETP comes out in the unit of the bed length L you entered (mm in, mm out).
  • Neither plate number nor As alone can tell you whether a column is packed well, so look at both together.

Primary sources

Every calculator gives an estimate for learning, built on published information and primary sources. Real decisions follow the applicable guidance and expert judgment.