# What is the RSD Limit for Replicate Standards in HPLC Analysis?

What is the Relative Standard Deviation (RSD)?
Relative standard deviation, which also may be referred to as RSD or the coefficient of variation, is used to determine if the standard deviation of a set of data is small or large when compared to the mean. In other words, the relative standard deviation can tell you how precise the average of your results is.

The relative standard deviation of a set of data can be depicted as either a percentage or as a number. The higher the relative standard deviation, the more spread out the results are from the mean of the data. On the other hand, a lower relative standard deviation means that the measurement of data is more precise.

How to Calculate Relative Standard Deviation?
The formula for calculating the relative standard deviation is as follows:

Relative Standard Deviation = (S x 100)/x

In this formula,
S stands for the standard deviation.
x stands for the mean of the data being used.

What is the use of Standard Deviation?
Standard deviation tells you how spread out the data is. It is a measure of how far each observed value is from the mean. In any distribution, about 95% of values will be within 2 standard deviations of the mean.

In HPLC, it is well known that system suitability parameters (including RSD of 5 or 6 replicate injections) apply to standard solution or system suitability solution.

How is RSD calculated in HPLC?
The relative standard deviation (RSD or %RSD) is the absolute value of the coefficient of variation. It is often expressed as a percentage. A similar term that is sometimes used is the relative variance which is the square of the coefficient of variation.

What is the limit of RSD?
The “usual” acceptable limit for repeatability is 2% (but depends on the concentration level of the analyte). Regarding the CV% (coefficient of variation) you are right, it’s the same as RSD (relative standard deviation).

What causes RSD failure in HPLC?
Column temperature fluctuations (especially evident in ion exchange systems). Column overloading. (Retention times usually decrease as mass of solute injected on column exceeds column capacity.) Sample solvent incompatible with mobile phase.

System Repeatability
The repeatability of response is computed using the following equation and represented as an estimated percentage relative standard deviation (%RSD) of a continuous series of measurements for at least three injections or applications of a reference solution.

xi = individual values expressed as peak area, peak height, or ratio of areas by the internal standardiza-tion method
X = mean of individual values
N = number of individual values

Assay of an Active Substance or an Excipient
When a relative standard deviation requirement is specified in an individual monograph, if the requirement is 2.0 or less the calculation is based on data from five replicate injections of the analyte, if the requirement is more than 2.0% data from six replicate injections are used.

In an assay of an active substance or an excipient, where the target value
is 100% for a pure substance, and a system repeatability requirement is not specified, the maximum permitted relative standard deviation(%RSD max) for the defined limits is calculated for a series (n = 3 to 6) of injections of the reference solution.

What is the RSD Limit for Replicate Standards in HPLC Analysis?
This question's solution is more complex than one may originally believe.
1. Regardless of any other choices or documentation, if you are using a previously developed and validated method, you must adhere to its instructions.
2. If you are developing a new method and are setting up system suitability criteria then you should look at several practices.
• Since internal SOPs represent your company's commitment to how all procedures will be handled, they will take precedence over all other documents.
• If you don't have a governing SOP, you should consult the ICH and USP for the right system appropriateness standards. The FDA will anticipate that you will at the very least comply with the USP's standards.

RSD Limit for Replicate Standards in HPLC Analysis
Unless otherwise specified in the individual monograph, data from five replicate injections of the analyte are used to calculate relative standard deviation (SR) if the requirement is 2.0% or less; data from six replicate injections are used if the relative standard deviation requirement is more than 2.0%.