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System suitability

USP <621> plates, resolution, and separate 5% and 10% peak-shape factors.

Based on:USP <621>Ph. Eur. 2.2.46IUPAC 2017

Rules version 1.80 · Last checked 2026-10-31

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INPUT

Plates and resolution use this convention. The result names it. Peak shape below does not follow this menu.

Theoretical plates N

Same units as the widths. The ratio cancels the unit.

Used by N = 5.54 (tR / W0.5)^2.

Used by N = 16 (tR / W)^2. Extrapolate the relatively straight sides to the baseline.

Resolution Rs

Peak 2 is the later peak.

Half-height Rs = 1.18 (tR2 − tR1) / (W0.5,1 + W0.5,2).

Tangent Rs = 2 (tR2 − tR1) / (W1 + W2).

USP tailing factor at 5% height

USP <621> symmetry factor, also called the tailing factor, and the Ph. Eur. 2.2.46 symmetry factor. Both are measured at 5% of peak height (one-twentieth of the peak height): T = W0.05 / (2f).

Distance from the perpendicular at the peak maximum to the leading edge, at 5% of peak height.

10% asymmetry b/a

10% peak-height asymmetry As = b/a. IUPAC Recommendations 2017 names b/f at 10% of peak height the tailing factor. This ratio is not the Ph. Eur. 2.2.46 symmetry factor and not the USP <621> tailing factor.

As = b / a, measured at 10% of peak height.

USP <621> and Ph. Eur. 2.2.46 measure the symmetry factor at 5% of peak height. The 10% b/a ratio is a different number. Reporting the 10% value as the European Pharmacopoeia symmetry factor, or as the USP tailing factor, is a common audit mistake.

Optional acceptance checks

Limits are optional and start blank. Enter a minimum or maximum only when your monograph or validated method states one. This page does not fill in official defaults. The harmonized chapter says that, unless otherwise stated, the symmetry factor of the peak used for quantification is 0.8–1.8. That sentence is not entered here for you. Replicate-injection %RSD is on the RSD calculator.

Compared with the 5% tailing factor, not with the 10% asymmetry.

Not a substitute for CDS-calculated, validated results. The monograph or the validated method sets the acceptance criteria. This page does not invent official limits. Confirm the current USP <621> and Ph. Eur. 2.2.46 text.

N

Enter a retention time and peak width for plates, two peaks for resolution, or the 5% and 10% widths for peak shape.

N = 5.54 × (tR / W0.5)²

Rs = 1.18 × (tR2 − tR1) / (W0.5,1 + W0.5,2)

Half-height: USP <621> and Ph. Eur. 2.2.46

T = W0.05 / (2f) at 5% height

As = b / a at 10% height

Calculation values stay on this device.

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CH 22 · notes

How it works

USP <621> Chromatography, in the harmonized text official 1 December 2022, defines column efficiency and resolution from the peak width at half-height. The Definitions section gives the plate number as N = 5.54 (tR / Wh)^2 and the resolution as Rs = 1.18 (tR2 − tR1) / (Wh1 + Wh2). Ph. Eur. 2.2.46 uses that same PDG text. The width convention on this page is labeled on every plate and resolution result.

Earlier USP <621> also gave the tangent equations N = 16 (tR / W)^2 and Rs = 2 (tR2 − tR1) / (W1 + W2), with W taken by extrapolating the relatively straight sides of the peak to the baseline. The PDG revision removed the baseline resolution equation. This page still calculates those tangent values when you enter baseline widths, and it names them as the earlier convention. They are not the current harmonized definition.

Peak shape is kept apart from that menu. Ph. Eur. 2.2.46 incorporates the PDG harmonised text (signed off 28 September 2021; implementation 1 January 2023). Its symmetry factor is As = w0.05 / (2d) at one-twentieth of the peak height, which is 5% height, the same measurement as USP tailing factor T = W0.05 / (2f). EDQM states that the default symmetry-factor range moved from 0.8–1.5 to 0.8–1.8 for tests and assays, unless the monograph states otherwise. The 10% ratio As = b/a is a different measurement. IUPAC Recommendations 2017 names b/f at 10% of peak height the tailing factor, and names (f + b) / (2f) at 10% of peak height the asymmetry factor. It notes that some pharmacopoeia definitions measure peak asymmetry at 5% of peak height. This page does not call the 10% b/a ratio a Ph. Eur. symmetry factor.

System suitability in the harmonized chapter also covers repeatability. Percent relative standard deviation of replicate injections is on the RSD calculator. Detection and quantitation limits are on the LOD/LOQ calculator. Relative response factors are on the relative response factor calculator. Last checked 2026-10-31.

Not a substitute for CDS-calculated, validated results. The monograph or the validated method sets the acceptance criteria. This page does not invent official limits. Confirm the current USP <621> and Ph. Eur. 2.2.46 text.

Worked example

Load example uses an invented chromatogram, not a monograph limit. Retention time 12 and half-height width 0.40 give half-height N 4986. The same retention time and baseline width 0.80 give tangent N 3600. Peaks at 10 and 12 with half-height widths 0.50 and 0.50 give half-height Rs 2.36. Baseline widths 1 and 1 give tangent Rs 2.00. W0.05 = 1.10 and f = 0.50 give tailing factor T 1.10 at 5% height. Front 0.40 and back 0.50 at 10% height give asymmetry As 1.25. The acceptance boxes stay empty.

Sources

  • USP <621> Chromatography — Definitions: plate number N = 5.54 (tR/Wh)^2; resolution Rs = 1.18 (tR2−tR1)/(Wh1+Wh2); symmetry factor As = W0.05/(2d) at 5% height. System suitability peak symmetry 0.8–1.8 unless the monograph states otherwise. Assay repeatability %RSDmax. PDG: baseline resolution equation removed.
  • Ph. Eur. 2.2.46 Chromatographic separation techniques — harmonised with USP <621>: half-height plate number and resolution; symmetry factor at 5% height (w0.05/2d); default symmetry range 0.8–1.8; system repeatability (assay), Table 2.2.46.-1
  • IUPAC tailing factor (10% peak height) — tailing factor b/f at 10% height (Gold Book 09905); asymmetry factor (f+b)/(2f) at 10% height (09902); some pharmacopoeias measure peak asymmetry at 5%

FAQ

Which plate-count equation does USP <621> use?

The harmonized USP <621> (official 1 Dec 2022), Definitions, plate number, uses N = 5.54 (tR / Wh)^2 with the peak width at half-height. Ph. Eur. 2.2.46 uses that equation. Earlier USP <621> also gave N = 16 (tR / W)^2 from the baseline width obtained by extrapolating the relatively straight sides of the peak. This page calculates the convention you select and names it on the result.

How is chromatographic resolution calculated?

The harmonized USP <621> and Ph. Eur. 2.2.46 use Rs = 1.18 (tR2 − tR1) / (Wh1 + Wh2), with widths at half-height. The PDG revision removed the baseline resolution equation. Earlier USP <621> used Rs = 2 (tR2 − tR1) / (W1 + W2) with baseline widths. Peak 2 is the later peak. The result names which equation was used.

Is the Ph. Eur. 2.2.46 symmetry factor the same as the USP tailing factor?

Ph. Eur. 2.2.46 incorporates the PDG harmonised text (signed off 28 September 2021; implementation 1 January 2023). Its symmetry factor is As = w0.05 / (2d) at one-twentieth of the peak height, which is 5% height, the same measurement as USP tailing factor T = W0.05 / (2f). EDQM states that the default symmetry-factor range moved from 0.8–1.5 to 0.8–1.8 for tests and assays, unless the monograph states otherwise. Both are T or As = width at 5% height divided by twice the front half-width.

What is the 10% asymmetry factor?

As = b/a, the back half-width divided by the front half-width, both at 10% of peak height. IUPAC Recommendations 2017 names b/f at 10% height the tailing factor, and names (f+b)/(2f) at 10% height the asymmetry factor. It is not the Ph. Eur. 2.2.46 symmetry factor and not the USP <621> tailing factor. Reporting the 10% value under either of those names is a common audit mistake.

What does the worked example calculate?

An invented chromatogram with retention time 12 and half-height width 0.40 gives N 4986 by N = 5.54 (tR / W0.5)^2. Baseline width 0.80 gives N 3600 by N = 16 (tR / W)^2. Peaks at 10 and 12 with half-height widths 0.50 and 0.50 give Rs 2.36. Baseline widths 1 and 1 give Rs 2.00. W0.05 = 1.10 and f = 0.50 give T 1.10. Front 0.40 and back 0.50 at 10% height give As 1.25. These are not monograph limits.

Does this replace a chromatography data system?

No. Not a substitute for CDS-calculated, validated results. The monograph or the validated method sets the acceptance criteria. This page does not invent official limits. Confirm the current USP <621> and Ph. Eur. 2.2.46 text. Optional N, Rs, and T limits stay blank until you enter the criterion from the monograph or the validated method. Replicate-injection %RSD is on the RSD calculator. Values were last checked 2026-10-31.