Internal Standard Method

The more accurate results in GC and LC are probably obtained using an internal standard. However, it can be difficult to find an appropriate substance that will elute in a position on the chromatogram that does not interfere or merge with any of the natural components of the mixture. Nevertheless, having identified a reference standard, the response factors for each component in the mixture must be determined (which may not necessarily include all the components). In many samples only certain components are required to be assayed. A synthetic mixture is then made up containing known concentrations of each of the components of interest together with the standard.

Now, if there are (n) components, and the (p) component is present at concentration (cp), the standard at a concentration (cst) and the response index of the detector can be considered to be unity.

Then, (17)

where (ap) is the area of the peak for component (p),

(ast) is the area of the peak for the standard,

and (yp) is the response factor for component (p).

Consequently, the response factor (yp) for the component (p) can be calculated as follows:

If peak heights are used instead of peak areas, then by simple replacement,

(19)

where (hp) is the height of the peak for component (p),

(hst) is the height of the peak for the standard,

and (yp) is the response factor for component (p).

Thus, rearranging, the response factor (yp) for the component (p) is given by

(20)

A weighed amount of standard is now added to the sample and chromatographed. Let the concentration of the standard be (Cst), the concentration of any component (p) be (Cp) and the peak areas of the component (p) and that of the standard (Ap), (Ast) respectively; then