USP <621> Chromatography

USP General Chapter <621> is an enforceable, harmonized standard that defines the principles, calculations, performance requirements, and allowable adjustments for liquid chromatography (LC) and gas chromatography (GC) procedures in USP monographs. It anchors system suitability testing (SST) and provides limits for adjusting chromatographic parameters while ensuring that monograph methods remain scientifically equivalent. The text is aligned with PDG-harmonized language and the corresponding Ph. Eur. 2.2.46 revisions, ensuring consistency across ICH regions.

Scope: Applies to LC and GC procedures in USP–NF monographs and any compendial testing referencing <621>.

Core principle: Methods may be adjusted within defined limits, provided all monograph SST criteria are met. Any changes outside these limits, or that alter critical selectivity, are considered method changes requiring revalidation/verification.

Harmonization: USP <621> is PDG-harmonized with Ph. Eur. 2.2.46 and JP equivalents; the latest revisions took effect with Ph. Eur. 11.0 (Jan 1, 2023).

System Suitability Testing (SST)

• SST verifies that the entire chromatographic system (instrument, column, mobile phase, and method parameters) is fit for use each day.

• Typical criteria set by monographs include:

  • Resolution between critical peaks

  • Precision (%RSD of replicate injections)

  • Column efficiency (plate count)

  • Tailing factor or symmetry

• Passing SST demonstrates that the adjusted conditions maintain equivalence to the validated method.

Allowable Adjustments (LC and GC)

• General allowances: Small changes in mobile phase composition, pH, temperature, and column dimensions are permitted within specified limits.

• LC-specific provisions:

  • Buffer pH: ±0.2 units

  • Buffer concentration: ±10%

  • Column temperature: ±10 °C

  • Mobile-phase composition: For isocratic methods, minor component changes within ±30% relative or ±10% absolute; gradient adjustments must follow the equations provided and account for dwell volume.

  • Column scaling: L/dp ratio may be adjusted −25% to +50% while maintaining efficiency/selectivity.

• GC provisions: Adjustments apply to parameters such as carrier-gas velocity, oven temperature programs, and injection volumes, provided SST is met.

Adjustments within <621> guardrails are permitted without revalidation, provided SST is demonstrated.

• Any adjustment outside these ranges, or that fails SST/selectivity criteria, must be validated as a method change.

• <621> also provides the official formulas and terminology (e.g., resolution, plate count, linear velocity, dwell volume) to standardize calculations across labs.