Reasonable question which is likely the result of "to much' blogging on your behalf, mate.
The problem is how the term "purity" is often used on AAS forums. Suffice it to say essentially all drug must be mixed with other substances to balance PH, enhance absorption, facilitate uniform dispersion, prevent coalescence etc.
These compounds are collectively refered to as fillers and are NOT considered contaminants or impurities.
An
impurity is something that is either not removed or is indadvertedly added as a consequence of the production process itself. Some examples include; heavy metals, bacterial endotoxins, bacterial fragments, certain chemicals, dust, dirt or "protein" fragments etc.
For example whenever GH is being secreted by Ecoli MUCH of that "GH" is of poor quality some having 61AA, 112 AA, 181AA with the remainder, one hopes, having 191 AA aligned in the PROPER SEQUENCE and folded correctly bc of that sequence.
An HPLC looks for impurities OF CHEMICAL STRUCTURE which SHOULD have been detected AND removed as a consequence of the production process itself. Obviously this is no easy task and requires the purchase of very HIGH END devices capable of flirtation, ultrafiltration, ion exchange protein electrophoresis, en masse chromatography to name a few.
Once this is accomplished the "fillers" are added to "stabilize" the end product.
Now if a company has the capability to produce GH of high purity rest assured they KNOW exactly how much on a weight basis is being placed into each vial. So the amount of GH contained in each vial is/was NO MISTAKE, lol!
Hope that helps mate