Heavy isotope-labelled peptides help distinguish a standard from the matching unlabelled peptide in mass spectrometry. A signal ratio can be informative, but turning it into an absolute amount requires a known standard and a supported measurement model.
What the ratio compares
A light-to-heavy ratio compares two detected signals. It is not automatically a ratio of known amounts across every matrix and measurement condition. The assigned amount of the standard, assay range and response relationship matter.
A deliberately simple example
If a heavy standard contains a verified 10 pmol and the light-to-heavy signal ratio is 0.5, an ideal equal-response model would imply 5 pmol of light peptide. This is a teaching calculation. It does not validate the equal-response assumption, standard assignment or recovery in a real sample.
Why calibration adds information
Thermo Fisher’s SureQuant explanation distinguishes an absolute module using a calibration curve from a relative module. In that specific system, serially diluted light peptides are compared against a constant heavy-peptide background. The example illustrates a measurement approach, not an NHD service specification.
Do not confuse peptide and protein amounts
A labelled peptide added after digestion does not automatically correct for incomplete protein digestion before it was added. Specify the point of addition and what the assay is intended to quantify. For a custom-standard request, include sequence, label positions, amount assignment and the required documentation, then confirm feasibility.
Related materials and next steps
For a sequence or library project, begin with our custom peptide synthesis enquiry page. Describe the exact structure, quantities and analytical requirements; project feasibility and delivery scope need confirmation.
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: SureQuant absolute versus relative quantitation
- FDA: ICH Q2(R2), analytical procedure validation
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: SureQuant absolute versus relative quantitationChecked 2026-10-05; source scope is identified in the article.
- FDA: ICH Q2(R2), analytical procedure validationChecked 2026-10-05; source scope is identified in the article.
