Relative Response Factor (RRF) Calculation

What is Relative Response Factor and its importance?

RRF (Relative Response Factor) is an analytical measure used in chromatographic courses to control impurities/degradants in drug substance and drug product (Ref. (ICH) Q3A R2 recommendations).
RRF is mainly used to correct the difference in detector response of impurities with sample (analyte) peak.
RRF is calculated by the slope method with a linear range (Linearity) of Sample solutions.
Different Pharmacopoeias refer the term RRF differently as described below,

The ratio of the responses of equal amounts of the Impurities and the drug substance is called as Relative response factor (RRF).
USP refers RRF as Correction factor (CF) or Response factor (RF) or Relative response factor (RRF).

The Relative detector response factor, usually referred as Response Factor (RF), represents a detector’s sensitivity for a given substance in comparison to a standard substance.
The correction factor is reciprocal of the response factor (RF).
Ph.Eur. denotes RRF as Correction factor or Response factor.

The Response Factor is a relative phrase that describes the response of equal weights of one substance to that of another under the test conditions.
BP refers RRF as Response factor (RF).

The establishment of RRF is required,
- For avoid the stability issues with standards or reference standard,
- To decrease the cost on the preparation of impurity standards,
- To lessen maintenance of impurity standards,
- Difficulty in synthesis and isolation of Impurity Standards, for convenience and time saving.

Relative Response factor (RRF) is used in different stages: Phase 1 to Phase 4 studies, in drug purity tests, Mass balance tests, in limit tests, In stability indicating methods etc.
Estimation of Relative Response factor (RRF)

A response factor (RF) is defined as the ratio between the concentration of a substance that are being analyzed and the response of the detector for that substance.

Response Factor (RF) = Peak Area / Concentration

Relative Response Factor (RRF) = Response Factor A / Response Factor B

RRF can be used for quantitative analysis of Unknown substances by means of known concentration of other substances as per the below equation,

Relative response factor of impurity = [Slope of impurity solution in curve/ Slope of standard solution in curve]

An internal standard (IS) is a compound that is similar in physical and chemical characteristics to the compound being analysed.
Internal Standard must be ideal, inert to the sample and must not react with the sample which is to be analysed or any of solvents used to dilute or being used in GC analysis.

Calculation:

RRF_A = Response factor A / Response factor IS

RRF_B = Response factor B / Response factor IS

Objects	RRF	RRT
Full Name	Relative Response Factor	Relative Retention Time
Use in HPLC	The RRF method is used to compensate for differences in detector response between contaminants and the analyte peak.	The relative retention time (RRT) is the comparison of the Retention Time (RT) of one compound to another
Calculation Formula	Relative Response Factor (RRF) = Response Factor A / Response Factor B	Relative Retention Time (RRT) = RT for Internal Standard/RT for Substance A

The RT means, in which time the main principle peak of substance is retained in minutes called as Retention time (RT)
Whereas the Relative Retention Time (RRT) stands for which time the impurity peaks are retained before and after the main principle peak and that retention time of impurity peaks is called RRT, i.e. relative retention time RRT to the base of main peak RT.
The relation between Retention time (RT) & Relative Retention Time (RRT) is always the main peak RRT is 1.0 and the impurity peak

RRT = IMPURITY PEAK RT / MAIN PEAK RT.

For example:

The main “XYZ”-peak RT is 15 min. and the “ABC”-Impurity RT is 5 min,
The RRT of “XYZ”-Peak =15/15 = 1
The RRT of “ABC”-Impurity peak = 5/15 = 0.333

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