A purity figure on an analytical report is the output of a specific test method run on a specific instrument. Whether that number can be trusted depends on more than the instrument. It depends on whether the method has been shown to measure what it claims, across the range it is used for, with known accuracy and precision. Showing that is called analytical method validation.
The main international reference is the International Council for Harmonisation guideline ICH Q2(R2), Validation of Analytical Procedures (adopted 2023). It states that the objective of validation is to demonstrate that an analytical procedure is fit for its intended purpose. This guide walks through the main performance characteristics Q2(R2) describes, using peptide HPLC purity testing as the running example.
ICH guidelines are written for pharmaceutical development and registration. Research-grade materials are not registered products, but the same concepts are the clearest way to judge whether a lab result means what it appears to mean.
Method, Procedure, Validation: The Vocabulary
- Analytical procedure (method): the full description of how a test is done: sample preparation, instrument, column, mobile phases, gradient, detection, calculations.
- Validation: a planned study showing the procedure performs acceptably for its purpose.
- Intended purpose: what the result is used for. An identity test, an assay of content and an impurity test have different requirements.
- Reportable range: the span of results the method can deliver with suitable accuracy and precision.
A method validated for one peptide is not automatically validated for another. Sequence, hydrophobicity and the impurities that need to be separated all change how a method behaves.
Specificity and Selectivity
Specificity (or selectivity) is the ability to measure the analyte without interference from other things that may be present, such as impurities, degradation products, counter-ions or matrix components.
Q2(R2) says specificity can be demonstrated by showing absence of interference or by comparison with an orthogonal procedure, meaning a method based on a different separation or detection principle.
For a peptide HPLC purity method, the key question is: are closely related impurities, such as deletion sequences or oxidised forms, separated from the main peak? If an impurity co-elutes with the main peak, the chromatogram looks clean and the purity number is too high. Our guide on Peptide-Related Impurities explains which impurities are most likely. Mass spectrometry coupled to the separation is a common way to check whether a peak is truly one species.
Stability-Indicating Methods
Q2(R2) calls a validated quantitative procedure stability-indicating when it can detect changes in relevant quality attributes during storage. To demonstrate that, the validation should include samples containing relevant degradation products, for example samples deliberately exposed to stress conditions such as heat, light, oxidation or pH extremes.
This connects directly to stability work. A stability study is only as good as the method used to measure it. See How Stability Is Studied for the study designs.
Range and Linearity (Response)
A method must give results that are proportional to the amount of analyte, or follow a known relationship, across the range it is used for.
- Linearity. Q2(R2) recommends a minimum of five concentrations appropriately distributed across the range to assess a linear response.
- Range. Q2(R2) gives typical reportable ranges for common uses. For a purity result expressed as area percent, it lists 80% of the lower specification limit up to the upper specification limit or 100%. For an impurity, the range typically runs from the reporting threshold to 120% of the specification limit.
A practical consequence: an impurity method needs to be able to see small peaks reliably. That is where detection and quantitation limits come in.
Detection Limit and Quantitation Limit
- Detection limit (DL): the lowest amount that can be reliably detected.
- Quantitation limit (QL): the lowest amount that can be measured with acceptable accuracy and precision.
Q2(R2) describes several ways to estimate them:
- Signal-to-noise. A ratio of 3:1 is generally acceptable for the DL. For the QL, a ratio of at least 10:1.
- From a calibration line. DL = 3.3σ / S and QL = 10σ / S, where σ is the standard deviation of the response and S is the slope of the calibration curve.
Why it matters: if a report lists "no impurities above 0.1%," that statement only means something if the method's QL is at or below 0.1% for those impurities.
Accuracy
Accuracy is closeness to the true value. Q2(R2) says it should be assessed across the reportable range, for example 3 concentrations with 3 replicates each of the full procedure. Results are reported as the mean percent recovery of a known added amount, or as the difference from an accepted reference value, together with a confidence interval.
For peptides, accuracy depends heavily on the reference standard. A method can be very precise and still be inaccurate if the reference material's own content is wrong. That is one reason net peptide content and HPLC purity are different numbers. See Net Peptide Content vs HPLC Purity.
Precision
Precision is how close repeated measurements are to each other. Q2(R2) describes three levels.
| Level | What varies | Q2(R2) minimum |
|---|---|---|
| Repeatability | Nothing: same analyst, instrument, short time | At least 9 determinations across the range (e.g., 3 × 3), or at least 6 at 100% of the test concentration |
| Intermediate precision | Within one lab: different days, analysts, equipment, conditions | Extent depends on how the method will be used |
| Reproducibility | Between laboratories | Inter-laboratory trial; usually needed when a method is standardised |
Precision is usually reported as a standard deviation or relative standard deviation. A purity figure with one decimal place implies a precision that the method should actually have.
Relative Response Factors
HPLC area percent assumes every component gives the same detector response per unit of mass. That is not always true. An impurity with a different UV absorbance can be over- or under-counted.
Q2(R2) states that where an analyte responds differently from the reference material, relative response factors should be determined, and if a relative response factor is outside 0.8-1.2, a correction factor should be applied. This is a quiet but real source of error in "area %" purity figures.
Robustness
Robustness is how sensitive a method is to small, deliberate changes, such as mobile-phase composition, pH, column temperature or flow rate. Q2(R2) notes that robustness is normally evaluated during method development, as described in the companion guideline ICH Q14. A robust method gives the same answer on a slightly different day, column lot or instrument.
System Suitability
Validation shows a method works in principle. System suitability testing (SST) checks that it is working today. Q2(R2), referring to ICH Q14, calls SST an integral part of the analytical procedure and a regular check of performance.
Typical SST criteria for an HPLC method include resolution between the main peak and a critical impurity, peak tailing, and repeatability of replicate runs of a standard. If SST fails, the run should not be used to report results.
Questions to Ask About a Purity Result
When you read an analytical report, these questions help judge the method behind it:
- What method was used? Column, gradient and detection wavelength should be stated or available.
- Is it specific for this peptide? Were related impurities shown to be separated, or confirmed by mass spectrometry?
- What are the DL and QL? Do they support the impurity statements made?
- Was the method validated or verified for this compound, and by whom?
- Did system suitability pass for the run that produced the result?
- Is the lab competent and independent? Accreditation to ISO/IEC 17025 is one signal. See GMP, GLP and ISO/IEC 17025.
Our guide How to Read a Peptide Analytical Report covers how to read the chromatogram and mass spectrum themselves.
FAQ
Is a "validated method" the same as a correct result? No. Validation shows the method can perform. Each run still depends on correct sample handling, a good reference standard and passing system suitability.
Why are 99.5% and 99.0% sometimes not meaningfully different? If the method's intermediate precision is several tenths of a percent, results that close may be within normal variation.
Does HPLC purity tell me how much peptide is in the vial? No. It describes the proportion of the UV signal from the main peak. Content requires a separate measurement.
Sources
- International Council for Harmonisation. ICH Q2(R2) Validation of Analytical Procedures, final version adopted 1 November 2023: https://database.ich.org/sites/default/files/ICH_Q2%28R2%29_Guideline_2023_1130.pdf
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