How Impurity Standards Are Used in HPLC and LC-MS: RRT, RRF and Method Validation

Impurity reference standards are the backbone of related-substances testing. This guide walks through how analysts actually use them in HPLC and LC-MS: from identifying peaks to calculating relative response factors and validating the method.

1. Identifying peaks: retention time and RRT

In a related-substances chromatogram, every small peak is a potential impurity. Injecting an impurity standard under the same conditions shows exactly where that impurity elutes.

Because absolute retention times drift between columns, instruments and days, methods use the relative retention time (RRT):

An RRT is a useful guide, but it is not proof of identity, because two different impurities can share the same RRT. Spiking the sample with the impurity standard (and, where needed, checking the mass spectrum by LC-MS) gives much stronger evidence.

2. Quantifying correctly: the relative response factor (RRF)

A UV detector does not respond equally to every compound. An impurity with a weaker chromophore than the API will give a smaller peak for the same amount, so its percentage calculated from area alone will be under-estimated.

The relative response factor corrects this:

The corrected impurity content is then calculated by dividing the impurity's area-based result by its RRF (or multiplying by the correction factor, 1/RRF). A common practice is to apply a correction when the RRF falls outside about 0.8-1.2.

Determining an RRF requires a pure, well-characterised impurity standard with a known potency, which is why the Certificate of Analysis matters so much.

3. Forced degradation studies

Forced (stress) degradation shows which degradation products can form and proves that the method can separate them from the API. Typical stress conditions are:

A degradation of roughly 5-20% is usually targeted. That is enough to generate the products without destroying the sample. Impurity standards of known degradation products confirm which peaks are which.

4. Method validation with impurity standards

Validation of an impurity method (under ICH Q2) relies on impurity standards at almost every step:

• Specificity: spike the API or product with all specified impurities and show they are resolved from each other and from the main peak (resolution is commonly expected to be at least 1.5).
• Limit of detection (LOD) and quantification (LOQ): typically at signal-to-noise ratios of about 3:1 and 10:1; the LOQ must be at or below the reporting threshold.
• Linearity: a calibration series of each impurity from the LOQ to above the specification limit.
• Accuracy (recovery): spike known amounts of each impurity into the sample at several levels and measure recovery.
• Precision: repeatability and intermediate precision at the specification level.
• Robustness: small changes in pH, column temperature, flow rate or mobile phase composition.
• Solution stability: how long standard and sample solutions remain stable.

5. System suitability

Every routine run should start with a system suitability check, often using a solution containing the API and one or more critical impurities. Typical criteria include resolution between a critical pair, tailing factor, theoretical plates and the %RSD of replicate injections. A "resolution solution" made from impurity standards is the simplest way to show that the column still separates what it needs to.

6. LC-MS and LC-MS/MS

LC-MS is used when UV is not selective or sensitive enough:

• Identifying unknown peaks found in stability or forced degradation samples.
• Trace impurities such as nitrosamines and other mutagenic impurities (see our nitrosamine guide).
• Bioanalysis of drugs and metabolites in plasma.

In quantitative LC-MS/MS, a stable isotope-labelled internal standard (a deuterated, ¹³C or ¹⁵N version of the analyte) is added to every sample. Because it behaves almost identically to the analyte but has a different mass, it corrects for ion suppression and recovery losses. Browse our isotope-labelled compounds and metabolites.

7. Good practice when handling impurity standards

• Store exactly as stated on the CoA, and let containers reach room temperature before opening.
• Weigh on a calibrated microbalance and use the potency from the CoA in your calculations.
• Prepare stock solutions in a solvent in which the impurity is stable, and record their preparation and expiry.
• Keep the CoA and lot number with your raw data for traceability.

Key takeaways

Building a new impurity method? Send us your list of specified impurities and we will supply the standards, including labelled internal standards and custom synthesis. Request a quote.