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what is the difference between chromatograhic purity and related substances?
You should find suitable definitions in the relevant pharmacopoeia monograph for the compounds of interest. In the USP, that is General section <1086> Impurities in Official Articles.
Simply, chromatographic purity is a test ( usually TLC or HPLC ) where the sample is compared to some standards, and the spots/peaks of any impurities detected must not exceed the prescribed limits for size and number. Those impurities can be from any source, including process contamination, and thus can appear almost anywhere in the chromatography.
Related substances can be determined using similar procedures, but the peaks usually have defined retention behaviour, as they are structurally related to the drug substance. The peaks don't have to be identified, but they are known to be structurally related, usually because they are degradtion products or process impurities. The monographs may define the expected retention properties, and the limits for these impurities.
Note that another category "process impurities" specifically excludes related substances and water.
Bruce Hamilton
