Dear Sir. Concerning your issue about how to quantify the term" % Purity" in fabricated known materials inside lab. The practical aspects of purity determination can be done by differential scanning calorimetry (DSC) and explains the way that the purity calculation works. This will give thermal analysts sufficient information to make successful purity determinations using their calorimeters and understand how to avoid some of the common errors that may be encountered. Also instrument like Avomeen’s purity testing laboratory which specializes in material analysis of contaminants, testing impurities, determine residuals that may be left behind, and test for degradation over time. The designation of a substance as experimental material (“research grade”) makes it clear that it differs from material intended for human use (“pharmaceutical grade”). Following the guidance of global pharmacopoeial and regulatory frameworks, materials for clinical use require a detailed characterization and need to fulfill certain criteria. For example, the purity of pharmaceutical grade materials is rigorously defined. Generally, purity assessment of pharmaceutical grade materials involves both the structural characterization and the quantification of the impurities, frequently down to the 0.1% w/w level. Analogous criteria for research grade materials are generally much less rigorous, partially incomplete, and/or poorly followed. For purity assay based on peak area in HPLC analysis three approaches are recommended: Normalization procedure - just calculate percentage of peak area in relation to total area of peaks under interests or in relation to main peak (in case of small impurities to be determined). On the background there is an assumption that (A) extinction coefficient at given wavelength is the same for main peak and all impurities and (B) detector signal is linear in a conc. range between main compound level and impurity level (eg. 0,1%) that is not always the case. Purity assay in relation to main compound content - run your test solution to integrate all impurity peaks; next run test solution diluted eg. 100x which corresponds to 1% of test solution (1% reference) ; calculate the content of impurities referring to the reference; the dilution of your test solution should be adjusted to expected level of impurities. On the background there is an assumption that extinction coefficient at given wavelength is the same for main peak and all impurities. The advantage of the approach is that U can treat the diluted test solution as X% reference regardless of its initial concentration and/or purity and U don't need any other standard. For final result it should be stated that the value is expressed in relation to main compound content. The approach is very often applied in Pharmacopoeias. Purity assay in relation to external standard of the impurity - well known procedure with use of qualified material of the impurity requiring weighting, diluting to the level of the impurity, and calculating against the signal obtained. The method is the most accurate in terms of chemistry if individuals determined, but can be limited with impurity standard availability and/or price and uncertainty of its purity evaluation.. I think the following below links may help you in your analysis:

https://www.perkinelmer.com/lab-solutions/resources/docs/APP_Practical_Purity_Determination_by_DSC_011667_01.pdf

http://www.avomeen.com/services/materials-testing/purity-testing-analysis/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4255677/

Thanks