> 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/assay.md).

# Assay

## Definition

Assay is the quantitative determination of the amount or concentration of a peptide in a sample, typically expressed as a percentage of the labeled content or as weight/weight (w/w) basis.

## Overview

While HPLC purity measures the relative proportion of peptide-related components, assay measures the actual amount of peptide present, accounting for water, residual solvents, counter-ions, and other non-peptide components.

## Technical Explanation

**Assay vs. Purity:**

* **Purity:** What fraction of the peptide-related material is the target peptide (area %)
* **Assay:** How much target peptide is actually present in the total sample mass (w/w %)

**Assay Methods:**

* UV spectrophotometry (using extinction coefficient)
* HPLC with reference standard calibration
* Amino acid analysis
* Nitrogen determination (Kjeldahl/Dumas)

**Factors Reducing Assay:**

* Water content (moisture)
* Residual solvents
* Counter-ion content (TFA, acetate)
* Inorganic salts

## Importance in Peptide Documentation

Assay data on the COA helps researchers accurately calculate the amount of active peptide for dosing. A product listed as "5 mg" may contain only 4 mg of actual peptide if the assay is 80% — the remaining 20% is water, salt, and counter-ion.

## Related Terms

* [Purity](/docs/glossary/analytical-terms/purity.md)
* [Identity](/docs/glossary/analytical-terms/identity.md)
* [Impurity](/docs/glossary/analytical-terms/impurity.md)
* [Counter-Ion](/docs/glossary/chemical-terms/counter-ion.md)
* [COA](/docs/glossary/quality-control/coa.md)

## Frequently Asked Questions

**Q: Why might assay be lower than HPLC purity?** A: This is normal. HPLC purity reflects only peptide-related components. Assay is lower because it accounts for water, residual solvents, and counter-ions that are invisible to HPLC area normalization.

**Q: What is a typical assay value for research peptides?** A: Typical assay values range from 70% to 95%, depending on counter-ion type, water content, and salt form. TFA salts generally have lower assay values than acetate salts at the same purity.


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