Response factor

Where the true amount of sample and standard are equal: where A is the signal (e.g. peak area) and the subscript i indicates the sample and the subscript st indicates the standard.

Injection volumes for GCs can be 1 microliter (μL) or less and are difficult to reproduce.

To compensate for this error, a known amount of an internal standard (a second compound that does not interfere with the analysis of the primary analyte) is added to all solutions (standards and unknowns).

This comparison of runs also applies to solutions with different concentrations of the analyte.

The number of moles of compound is equal to the concentration (molarity, M) times the volume, n = MV.

From these equations, the following derivation is made: Since both compounds are in the same solution and are injected together, the volume terms are equal and cancel out.

In practice, a solution containing known amounts of both octane and nonane is injected into a GC and a response factor, F, is calculated.

The response factor is applied to the data from the second solution and the unknown concentration of the octane is found.

This example deals with the analysis of octane and nonane, but can be applied to any two compounds.