Can anyone please tell me the difference between a "related substances method" and a "chromatographic purity method" regarding drug substance/drug product? Do they have different goals or are they just two names for the same thing? How should a "chromatographic purity method" be validated?
I would deeply appreciate any help, mainly references of good bibliographic support on this subject.

I my place of work we use the term "chromatographic purity method" when the results are reported as %Area of the various peaks.
Where as the term "related substances method" is used when one uses a diluted standard to quantify the known and unknown impurities in the sample and report the results in %w/w terms.

To validate a "chromatographic purity method" one has to carryout study of all the parameters use for "related substances method" viz., specificity, forced degradation, linearity, system precision, method precision and intermediate precision, stability in analytical solution etc. Here the importance of LOD is more than LOQ as is the case in "related substances method". Also there is no need to perform the recovery/accuracy part of the validation study.

I am not sure if I will be able to give you any bibiliographic support info.