Mass spectrometry measures mass-related signals. It is powerful, but some different structures share the same mass. Leucine and isoleucine are a useful example for anyone reading a peptide identity report.
Same formula, different arrangement
Leucine and isoleucine have the same elemental formula. Exchanging one for the other within an otherwise identical peptide gives a calculated intact-mass difference of 0 Da. Measuring the intact mass more precisely does not create a mass difference that is not there.
What the example does and does not mean
Consider the hypothetical sequences AGLK and AGIK. Their compositions have the same mass, while the third residue is different. This is a teaching pair, not a claim that an NHD sample contains a substitution. The example explains why “mass matched” and “every residue position verified” are different statements.
Can other methods help?
A suitable chromatographic separation, reference comparison or specialized structural analysis may address a particular isomer question. Routine fragment spectra also have limits, so do not assume that a generic MS/MS label automatically resolves every leucine/isoleucine assignment. Ask the analyst which specific ambiguity the evidence can distinguish.
Make the requested proof explicit
For a sequence-sensitive project, provide the exact sequence and identify critical isomer or stereochemical concerns. Confirm which tests are feasible and which remain unproven. The purpose is to choose evidence suited to the question, rather than to dismiss a useful intact-mass measurement.
Related materials and next steps
Explore the NHD catalogue records for LL-37. Confirm the required molecular form, quantity and lot-document availability before relying on a listing for procurement. A catalogue record does not establish equivalence to a published study material.
Send the product or sequence, quantity and document requirements. For more background, browse the analytical methods guides and research glossary.
Sources and reading scope
- Thermo Fisher: chromatographic separation of isobaric leucine and isoleucine
- Bachem: Quality Control of Amino Acids & Peptides
Source links checked 5 October 2026. Research summaries use the accessible original abstract unless a full-text source is explicitly identified. Guidance documents and external supplier explanations are technical references, not NHD batch evidence or a claim of regulatory compliance. Any example introduced as hypothetical or teaching is a calculation, not a measured product result.
NHD catalogue materials are for qualified laboratory research, development and manufacturing enquiries. This article does not provide instructions for human or veterinary use.
Sources & editorial method
AI-assisted writing and source checks; conceptual illustrated cover.
2 linked records are listed in the references below. Read the editorial and AI-assistance policy.
How to interpret this article
Study results and labeled examples are separated. Original-source, arithmetic, readability and internal-link checks; no independent human peer review recorded.
Research-use boundary: Catalog materials discussed on this website are for laboratory research, development and manufacturing use only, not for human or veterinary use. This content is not medical advice and does not provide administration instructions.
Primary records and authoritative sources
- Thermo Fisher: chromatographic separation of isobaric leucine and isoleucineChecked 2026-10-05; source scope is identified in the article.
- Bachem: Quality Control of Amino Acids & PeptidesChecked 2026-10-05; source scope is identified in the article.
Continue with structured records
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