Technical data

Understanding Your Agilent ChemStation 251

Evaluating System Suitability

Definitions for Reproducibility

The sample standard deviation S differs in two points from the standard

deviation s for the whole population:

• instead of the real mean value only the sample mean value M is used and

• division by N-1 instead of N.

Relative Standard Deviation RSD[%] (USP)

The relative standard deviation is defined as

Standard Deviation of the Mean S

Let M be the sample mean and S the sample [or (N-1)] standard deviation. The

standard deviation S

of the sample mean M is determined by

This can be further illustrated by an example:

While the retention time of a certain compound may deviate slightly from the

calculated mean value during one sequence, the data from another sequence

may differ much more due to e.g. ambient temperature changes, degradation

of the column material over time etc. To determine this deviation the standard

deviation of the sample mean S

can be calculated according to the above

formula.