> For the complete documentation index, see [llms.txt](https://docs.rplpeptides.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rplpeptides.com/docs/glossary/analytical-terms/identity.md). # Identity ## Definition Identity is the confirmation that a peptide sample is the correct molecule as specified, verified through analytical methods that confirm its molecular weight, sequence, and structural characteristics. ## Overview Identity testing answers the question: "Is this the right peptide?" While purity measures how clean the sample is, identity confirms what the sample actually is. Both are essential quality attributes. ## Technical Explanation **Primary Identity Methods:** * **Mass Spectrometry (MS):** Confirms experimental molecular weight matches theoretical value (most common) * **Amino Acid Analysis (AAA):** Confirms amino acid composition * **Sequencing (MS/MS, Edman):** Confirms actual amino acid order * **Co-elution with Reference Standard:** Retention time match on HPLC **Identity Verification Flow:** 1. Calculate theoretical molecular weight from sequence 2. Acquire MS spectrum of the sample 3. Confirm observed molecular weight (m/z) matches theoretical 4. For modified peptides, confirm modification mass is included ## Importance in Peptide Documentation Identity data is a mandatory component of every Certificate of Analysis. Without identity confirmation, purity data alone is insufficient — a sample could be 99% pure but be the wrong peptide. ## Related Terms * [Purity](/docs/glossary/analytical-terms/purity.md) * [Mass Spectrometry](/docs/glossary/analytical-terms/mass-spectrometry.md) * [HPLC](/docs/glossary/analytical-terms/hplc.md) * [Assay](/docs/glossary/analytical-terms/assay.md) * [COA](/docs/glossary/quality-control/coa.md) ## Frequently Asked Questions **Q: What is the most common identity test for peptides?** A: Mass spectrometry (MS or LC-MS) is the most common and efficient method. It provides direct molecular weight confirmation. **Q: Can identity fail even if the purity is high?** A: Yes. A sample could be 99% pure by HPLC but be a different peptide entirely (or contain the wrong modification). This is why identity testing is mandatory alongside purity testing. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.rplpeptides.com/docs/glossary/analytical-terms/identity.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.