> 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/formulation-terms/concentration.md).

# Concentration

## Definition

Concentration is the amount of peptide dissolved in a given volume of solvent, typically expressed as mg/mL (mass concentration) or mM/mol (molar concentration).

## Overview

Accurate concentration calculation is critical for reproducible research. After reconstitution, the peptide concentration depends on the amount of peptide in the vial and the volume of solvent added — but must account for peptide content (assay), not just fill weight.

## Technical Explanation

**Concentration Calculations:**

Mass concentration: Concentration (mg/mL) = Peptide content (mg) / Volume (mL)

Molar concentration: Concentration (mM) = Mass (mg) / (MW × Volume)

**Important Correction:** The labeled vial weight (e.g., 5 mg) includes peptide + counter-ion + water. The actual peptide content is: Corrected mass = Fill weight × (Assay / 100)

**Example:**

* Fill weight: 5 mg
* Assay: 85%
* Actual peptide content: 5 × 0.85 = 4.25 mg
* Reconstitute with 1 mL → Concentration = 4.25 mg/mL (not 5 mg/mL)

## Importance in Peptide Documentation

Using the uncorrected fill weight overestimates concentration and leads to dosing errors. The COA provides the assay value needed for accurate concentration calculations.

## Related Terms

* [Assay](/docs/glossary/analytical-terms/assay.md)
* [Reconstitution](/docs/glossary/formulation-terms/reconstitution.md)
* [Molecular Weight](/docs/glossary/chemical-terms/molecular-weight.md)
* [Purity](/docs/glossary/analytical-terms/purity.md)

## Frequently Asked Questions

**Q: Why can't I just use the labeled vial weight?** A: The labeled weight includes peptide, counter-ion, and residual water. Using it directly overestimates the actual peptide content. Always correct using the assay value from the COA.

**Q: Should I use mg/mL or Molar when reporting peptide doses?** A: Both are valid, but molar concentrations (mM/µM) are preferred in research because they account for molecular weight differences between peptides and enable direct comparison of molar ratios.


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