I've seen confusion about what purity testing results mean when it comes to peptides. Purity does not mean the percentage of the bottle that's contaminants, excipient, etc. Purity refers to the fraction of the peptide molecule with the correct amino acid sequence. Impurity can arise due to deletion/truncation, and insertions, stereochemical errors, oxidation, deamidation, cleavage artifacts, residual protecting groups, and other process- or degradation-related variation. Reported peptide purity is the percentage of the area of the desired peptide peak in an analytical high-performance liquid chromatography trace relative to all UV-absorbing peptidic peaks. Water, salts, counterions, and other non-peptidic materials in the lyophilized cake affect weight but are not counted in the purity judgment. Those look like this: And to use a randomly-selected empirical example: Truncated peptides will tend to be less active. Mind you "tend". This is not always the case. For example, C-terminal truncation of GLP-1 reduces receptor affinity, but N-terminally truncated orexin peptides often retain their agonist activity. The activity impact is sequence- and receptor-specific. If you don't get what that means, that's fine because you probably don't need to worry about it. Manufacturers could remove truncated and otherwise impure peptides, but they frequently do not because of cost, yield loss, and time. In most cases people run into online, that means ending up with a product that's, say, 97% instead of 99% pure, and the physiological effect is this difference is usually (but not always) going to be too small to make a difference to the end consumer. All of this is to say, impurity probably isn't what you thought it was. In fact, because its real definition isn't all that similar to what people imagine it should be intuitively, even with a high purity level, you still have to think about issues from contamination. Have fun!