usp calculation

[olivia]

Hi, I am doing a test for the calculation of Lidocaine according to the USP. Part of the calculation states Ru/Rs where Ru and Rs is the ratio of the peak area of the standard and sample. There is internal standard in both the standard and sample. Should I take the ratio of the active to internal standard in both the standard and sample and then take the ratio of the areas, or is it just the ratio of the active peaks it means in both the standard and sample.
Thanks Liv

[bartjoosen]

You should use the ratio of the area of the active substance to the internal standard to do the calculations.
Otherwise the internal standard is of no use.

Bart

[AdrianF]

Here is a useful link explaining the use of internal standards.

http://www.lcresources.com/wiki/index.p ... alStandard