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What is the difference between HPLC purity and mass balance?
The short answer
HPLC purity is relative: the target peak's share of total detected peak area. Mass balance is absolute: how the vial's total mass divides between peptide, water, counter-ion and other residues. HPLC answers how clean the peptide fraction is; mass balance answers how much of the powder is peptide. Serious assessments use both, and they can disagree.
In detail
A sample can be excellent by one measure and poor by the other. Highly purified peptide that absorbed water in storage keeps its HPLC purity and loses mass balance. Material cut with a UV-transparent bulking agent can also show clean chromatography.
The practical shortcut for buyers is quantification: measuring milligrams of peptide per vial against a standard captures most of what mass balance would tell you, at a fraction of the cost.
Full mass balance requires several techniques together and is a specification-development exercise rather than a routine buyer's check.
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