Mass Spec Identity vs Chromatographic Purity on a Peptide COA

Byline: Momentum Editorial

Summary

On a peptide certificate of analysis, identity and chromatographic purity answer different questions. When a report names a mass-spectrometric identity check (or a combined UHPLC-MS method), that field supports whether the measured species matches the expected mass or identity result for that sample. When a report prints a purity percentage under a named chromatographic method, that figure usually reflects relative peak area of the main analyte versus other peaks in that chromatogram. Do not treat one printed field as proof of the other — copy each only as the COA states it.

Why identity and purity are separate documentation lines

Laboratory documentation often separates what was detected from how clean the main peak looked under a stated method. Analytical and pharmacopeial practice treats chromatographic purity assessments as method-bound peak evaluations. Mass spectrometric identity checks, when named, support compound identity for the tested sample. A high purity percentage does not, by itself, document identity; an identity “Conforms” line does not, by itself, document a purity percentage.

If the COA leaves either field blank or unnamed, leave it blank in your inventory notes (see How to Read a Peptide Certificate of Analysis).

What “identity” usually means when MS is named

When a certificate prints an identity result and names mass spectrometry (MS), LC-MS, or a combined chromatographic–MS method such as UHPLC-MS, record:

1. The exact identity result printed (for example, Conforms — only if printed).
2. The method name exactly as printed.
3. The lot, product name, and strength already matched to the vial.

Do not upgrade a generic “screening” label into “mass spectrometry” unless those words appear on the report. Do not invent expected m/z values that the COA does not print.

What chromatographic purity usually means when HPLC / UHPLC is named

When purity % appears and a chromatographic method is named (HPLC, UHPLC, HPLC-UV, or similar), the figure typically reflects that method’s purity assessment for the tested sample. It does not automatically equal:

• net peptide content (a different content question when stated — see HPLC Purity vs. Net Peptide Content);
• identity, unless a separate identity field is also printed;
• the same result for every strength of the same catalog name.

Quantity-only reports without a purity percentage are not purity claims.

Combined methods (example: UHPLC-MS)

Some third-party reports name a combined technique such as UHPLC-MS. That label can support both chromatographic assessment and mass-spectrometric identity when the report prints both kinds of results. Still file each printed line separately: purity % (if present), identity result (if present), method string, lab, and test date. Never infer a purity number from an identity line, or an identity result from a purity % alone.

How to log fields without inventing method names

1. Match vial lot to the library row.
2. Confirm product name and strength.
3. Copy identity only if printed.
4. Copy purity % only if printed.
5. Copy method only if named — do not add HPLC or MS by assumption.
6. Note lab and test date exactly.

Storage of the dry solid is a separate topic: Storing Lyophilized Research Peptides.