Figure 53 : Add quantitatively an internal standard to the sample. This is normally possible on modern analytical balances by better than +- one %, if the standard substance has a purity of better than - 1 % and the main substance is stable by weight and light exposure. The chosen internal standard should have a chemical structure as much as possible similar than the to be quantitized substance of interest in the sample. The chromatogram must have an empty space for the position of the internal standard. The conditions to go “by internal standard” are quite critical. In addition you need a clean substance i - the product of analytical interest - to measure the correction factor necessary for the final calculation.