There may not be a noticeable difference between ≥99% and ≥95% pure products. However, the problem is that you don’t know what’s in the impurity. It is usually residual reagents, byproducts or side products. In some cases these impurities can be toxic. And to a degree, impurities cannot be fully eliminated. It is difficult to characterize every byproduct and side product of synthesis and some impurities are just too small to measure. But impurities can be minimized. For that reason the standard chromatographic purity in the pharmaceutical and nutraceutical industries is ≥99%. Whenever possible we carry ≥99% products. If this is not possible at production scale we carry the next best option (e.g. ≥98%, ≥95%) and make this publicly known on our COAs. Where appropriate we test for specific toxic residuals, byproducts and heavy metals.
Peptide products are an exception in that the only residual will be amino acids that did not form the correct chain to form the peptide.