As an example is GC, an internal standard is put to control the differences in sample volume for one. Propanol in the analysis of ethanol content in wine is one example where I have used it like that anyway. It takes care of losing some of the sample and is used as a ratio of the standard to the compound - so it would be the ratios of the peak heights or areas of EtOH and PropOH in the case above.
An external standard is such for titrations and the like using 0.1M Na2SO3 or similar standard is it not. (?) :animatedfear: