A customer sent us a vial of somatropin with a third-party COA already attached. The purity figure read 98.1%. We blinded it, ran it anyway, and found 11% of the protein had quietly dimerised.
High-Performance Liquid Chromatography separates a mixture and measures how much of each component is present. For peptides and proteins it is not enough, because HPLC measures what elutes, not what each peak actually is.
The peak that hides a second molecule
A growth-hormone dimer has nearly the same retention time as the monomer. To a UV detector it looks like the same clean peak. But a dimer is biologically inactive, and sometimes immunogenic.
Mass spectrometry resolved the shoulder into a distinct species at exactly twice the monomer mass.
“A purity number without a mass behind it is a measurement of confidence, not of content.”
This is why every protein and peptide sample that enters HELIXI is confirmed by LC-MS, not just quantified by HPLC.