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  • Determination of Response factors of Impurities in Drugs by HPLC

    Importance of Relative Response Factor in Impurities Quantification

    Impurities developed in active pharmaceutical ingredients (APIs) and in formulation products during the manufacturing process stages or during degradation need to be controlled. The isolation, identification, and quantification of these impurities are important for drug development and also as per the regulatory requirement and defined guidelines. Impurities present in drug products and drug substances require impurity profiling to have control over quality.

    The international conference on harmonization (ICH) mentions the guidelines for reporting the identification, quantification, and threshold values for impurities present in drug products and new drug substances. These impurities need to be analyzed and for this impurity, standards are required to quantify the impurity. The most accurate method to quantify these impurities is the use of external standards of the impurity.

    Importance of Relative Response Factor in Impurities Quantification

    Relative Response Factor (RRF) is an important parameter in impurities quantification in analytical chemistry. Impurities can be present in pharmaceuticals, chemicals, and other materials, and they can affect the quality and safety of the final product. The determination of impurities levels is necessary to ensure that the product meets regulatory requirements and is safe for use.

    RRF is used in impurities quantification to correct for differences in detector response between the impurity and the reference compound. The reference compound is typically a related compound that is structurally similar to the impurity but not present in the sample. The RRF is calculated by dividing the detector response of the impurity by that of the reference compound, both of which are determined under identical conditions.

    Once the RRF is established, it is used to quantify the impurity in the sample. The concentration of the impurity is determined by multiplying the peak area or height of the impurity by the RRF.

    The RRF is important in impurities quantification as it corrects for any variations in detector response due to differences in chemical structure, retention time, or detector sensitivity between the impurity and the reference compound. It enables accurate and precise quantification of the impurity levels in the sample. Therefore, it is essential to establish the RRF for each impurity and reference compound pair under the specific analytical conditions used.

    Relative Response Factor (RRF) in related substances

    Relative Response Factor (RRF) is a term used in analytical chemistry to describe the ratio of the response of a detector to the amount of analyte present in a sample compared to a standard compound. It is typically used in chromatography, such as gas chromatography (GC) or liquid chromatography (LC), where a sample is separated into its individual components and each component is detected using a detector.

    Different pharmacopeia mentions the term relative response factor (RRF) differently. As per U.S. Pharmacopoeia (USP), the Relative response factor is the ratio of the responses of equal amounts of the Impurities to the drug substance or its reference standard. USP refers to the correction factor or response factor or relative response factor.

    According to European Pharmacopoeia (Ph. Eur), the relative detector response factor often referred to as the response factor reveals the sensitivity of a detector for the given substance relative to a standard substance. The correction factor is reciprocal of the response factor. Ph. Eu refers to the correction factor or response factor or relative response factor.

    British Pharmacopeia (BP) refers to the response factor as a relative phrase that expresses the response of equal weights of one substance to that of another in the test conditions as described in the test. British pharmacopeia refers to RRF as a response factor.     

    How do we calculate Relative Response Factor (RRF)?

    As per the International Conference on harmonization (ICH) guidelines, the foremost main aim is to estimate the impurities accurately and consistently as per the defined reporting threshold, identification threshold, and quantification threshold.

    Quantification of impurity by using external standards is an impeccable method but it was impossible in any respect at the time because of a lack of sufficient impurities. Impurity isolation and synthesis is a difficult task and even should confirm its purity, quality, and integrity by doing periodic assessments.

    A precise response factor is resulting from the assessment of every compound by using the appropriate detector. RRF is determined by using the following equation:

    The relative response factor is convenient for the estimation of impurities when it’s difficult to synthesize, very unstable, or in absence of its standard. Probable robustness study of RRF to be performed during method development or validation. Accurately established RRFs are utilized within the strategy of study with acceptable conditions.


    Why do we use relative response factor (RRF)?

    Relative response factor (RRF) is used to obtain the exact quantification of impurities in drug substances and drug products by using High-performance Liquid Chromatography (HPLC).

    What is RF (response factor)?

    The relative response factor (RF) is the ratio between the concentration of a composite and its equivalent analytical response.

    What is RRF in HPLC analysis?

    In High-Performance Liquid Chromatography (HPLC) analysis, RRF stands for Relative Response Factor. It is a ratio of the detector response for a target compound to that of a reference compound, typically an internal standard, under identical analytical conditions.

    The RRF is used to calculate the concentration of the target compound in a sample by comparing its detector response to that of the reference compound. The RRF corrects for differences in the detector response between the target compound and the reference compound due to factors such as differences in their chemical structures, retention times, and detector sensitivity.


    1. United States Pharmacopeia, <1225> Validation of Compendial Procedures,
    2. ICH Q2(R1), Harmonised Tripartite Guideline Validation of Analytical Procedures: Text and Methodology
    3. ICH, Q3A (R2) Impurities in New Drug Substances: Text and Methodology,
    4. ICH, Q3B (R2) Impurities in New Drug Substances: Text and Methodology.
    5. European Pharmacopoeia 7.0, Section 2.2.46 Chromatographic Separation Techniques, (2010)
    6. United States Pharmacopeia USP 34-NF 29, Chapter, 621, (2011)