Why Two Certificates of Analysis Can Disagree on Purity
Two laboratories can analyse the same peptide and report different purity figures without either being wrong. Understanding why is essential to comparing suppliers on anything other than the size of the number printed on the page.
Purity is method-dependent by definition
A chromatographic purity figure is the area of the main peak as a percentage of total integrated peak area, under one specific set of conditions. Change the conditions and the number changes. It is a measurement, not an intrinsic property of the material.
Detection wavelength
Peptide analysis commonly uses 214 nm, which detects the amide backbone and therefore responds to essentially any peptide-related species, or 280 nm, which detects aromatic residues only. A sequence with no tryptophan or tyrosine is nearly invisible at 280 nm. Impurities lacking aromatics disappear at 280 nm while showing clearly at 214 nm, so the same sample commonly reports higher purity at the longer wavelength.
Gradient and column
A shallow gradient resolves closely eluting impurities that a steep gradient merges into the main peak. Deletion sequences — peptides missing a single residue — often elute very close to the target, and whether they appear as a separate peak or a shoulder is a function of the separation, not of the material.
Integration choices
Where the analyst sets the baseline and whether small peaks are excluded as noise both move the figure. These are judgement calls made by a person or a threshold setting in software.
What to compare instead
Rather than comparing bare percentages, compare the conditions. A certificate stating column chemistry, gradient, wavelength and run time is more informative than one reporting a higher number with no method. If the chromatogram itself is reproduced, better still — the shape of the baseline and the presence of shoulders tell you things the percentage does not.
Identity is a separate question from purity, and arguably the more important one. A mass spectrum confirming the expected molecular weight establishes that the main peak is the intended sequence. Purity without identity confirmation tells you the material is homogeneous, not that it is the right compound. Our note on HPLC and mass spectrometry verification covers how the two methods complement each other.
How our certificates work
Certificates for the compounds we supply are published on this site rather than shipped in the box. Vials are matched to their certificate by cap and crimp colour against the photographs shown on the certificate — the vials themselves do not carry batch numbers. Our certificates report third-party testing by HPLC and/or mass spectrometry where applicable, and state which methods were applied to a given batch.
Further background on interpreting these documents is in how to read a certificate of analysis.
ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use. Certificates of analysis for each compound are published on this site.
