Peptide Purity Calculator¶

Calculate the active (peptide-only) mass and impurity content based on stated purity. Essential for accurate dosing — a 100 mg peptide at 95% purity contains only 95 mg of active peptide.

Calculator¶

Stated Purity (%) Total Amount

Molecular Weight (Da) — optional (for molar corrected values)

Purity-Adjusted Results

Active Peptide Mass	—
Impurity Content	—
Active Peptide Percentage	—
Effective Concentration Adjustment	—

How Purity Affects Dosing¶

Active peptide mass is always less than the total mass you weigh out. Impurities (truncated sequences, salts, water, residual solvents) contribute to the total mass but not to biological activity.

Concept	Formula
Active peptide mass	Active = Total × (Purity ÷ 100)
Impurity content	Impurity = Total − Active
Correction factor	CF = 100 ÷ Purity (multiply your desired dose by this)

Why HPLC purity ≠ biological activity: HPLC (high-performance liquid chromatography) measures chemical purity — the proportion of your target peptide sequence relative to other UV-absorbing species. It does not measure: - Bioactivity — whether the peptide folds correctly or binds its target - Salt/water content — counterions (TFA, acetate) and residual water that add mass - Peptide content — the actual peptide fraction after accounting for counterions

A peptide at 98% HPLC purity could still have only 75–85% peptide content due to salts and water. This is why some labs report both "purity" (HPLC) and "peptide content" (by AAA or gravimetric analysis).

Core Formula¶

The mathematics behind purity correction is straightforward:

\[ m_{\text{active}} = m_{\text{total}} \times \frac{P}{100} \]

Where: - m_active = Active peptide mass (what you can actually use) - m_total = Total mass weighed (what the vial says) - P% = Stated purity percentage

Correction Factor (CF):

\[ \text{CF} = \frac{100}{P} \]

Use the correction factor to adjust any dose calculation:

\[ \text{Adjusted Dose} = \text{Target Dose} \times \text{CF} \]

Example: For a peptide at 95% purity, CF = 100 ÷ 95 ≈ 1.0526. To get 5 mg of active peptide, weigh 5 × 1.0526 = 5.26 mg total.

Real-World Scenario¶

Situation: You need 2 mg of active peptide for your experiment. Your vial is labeled "10 mg, ≥ 95% purity."

Step 1 — Calculate actual active: 10 mg × 95% = 9.5 mg active peptide (not 10 mg)

Step 2 — Calculate adjusted weight for your dose: 2 mg (target) × (100 ÷ 95) = 2.11 mg total mass needed

Why dosing by total mass causes systematic under-dosing:

If you ignore purity and dose based on the total mass labeled on the vial, you consistently deliver less active peptide than intended. The error scales linearly with impurity.

	Truth (95% purity)	Assumption (100% purity)
Total vial mass	10.00 mg	10.00 mg
Active peptide in vial	9.50 mg	10.00 mg
Dose target	2.00 mg active	2.00 mg active
Mass actually needed	2.11 mg total	2.00 mg total
Active delivered	2.00 mg (correct)	1.90 mg (5% low)

Over many doses, this 5% error can shift dose-response curves, reduce statistical power, and lead to incorrect conclusions. For premium-grade peptides (≥ 98%), the error is smaller (~2%); for standard-grade (85–95%), it can be 5–15%.

Frequently Asked Questions¶

**How is peptide purity determined?**

Peptide purity is most commonly measured by RP-HPLC (Reverse-Phase High-Performance Liquid Chromatography). The sample is run through a column, and the area under the target peak is compared to the total area of all peaks. This gives an area-percentage purity. Supplementary methods include Mass Spectrometry (MS) for identity confirmation and Amino Acid Analysis (AAA) for peptide content.

**Does 98% purity mean 2% is something harmful?**

Not necessarily. The remaining 2% typically consists of: - Truncated sequences — shorter peptide fragments from incomplete synthesis - Deleted sequences — missing amino acids in the chain - Oxidized variants — methionine or cysteine oxidation - Residual solvents — TFA, acetonitrile, or water These are generally not toxic at the small quantities present, but they can interfere with sensitive assays or cause off-target effects in biological systems.

**Should I always dose based on active mass?**

Yes — for any quantitative experiment where dose accuracy matters (dose-response curves, in vivo studies, cell-based assays). The only exceptions are: - Crude peptides used for preliminary screening where exact dosing is not critical - When the supplier explicitly states the mass is already "peptide content" adjusted For best practice, always use a purity correction when preparing stock solutions.

**What's the difference between peptide content and peptide purity?**

	HPLC Purity	Peptide Content
What it measures	Chemical purity of the peptide sequence	Actual peptide mass fraction of the powder
Influenced by	Truncations, deletions, byproducts	Salts, water, counterions (TFA, acetate)
Typical value	95–99% for premium grade	70–90% after accounting for counterions
How measured	RP-HPLC (UV absorbance)	Amino Acid Analysis, gravimetric
A peptide can be 98% pure by HPLC but contain only 80% peptide content if it carries heavy counterions.

**Can I adjust my molarity calculations for purity?**

Absolutely. When preparing a solution of known molarity, multiply the molecular weight by the correction factor (100 ÷ P%) or divide the calculated molarity by (P% ÷ 100). For example, to make a 1 mM solution from a 95% pure peptide, your effective molecular weight becomes MW × (100 ÷ 95) for weighing purposes. The Molarity Calculator includes a purity field for this exact purpose.

Purity Reference for Common Peptide Grades¶

Grade	Purity	Typical Applications
Crude	70–80%	Initial screening, exploratory research
Standard	85–95%	General research, in vitro assays
Premium	≥ 98%	Cell-based assays, in vivo studies
Ultra-Pure	≥ 99%	Clinical research, structural studies

Why Purity Matters¶

Peptide purity directly affects experimental results. A common scenario:

Example: You need 5 mg of active peptide for an in vivo study. Your peptide is labeled "100 mg, ≥ 95% purity."

Active content: 95 mg (not 100 mg)

If you dose based on the total mass (100 mg), you under-dose by 5%

For dose-response studies, this can lead to systematically shifted curves

Always dose based on active peptide mass, not total vial mass.

Dilution Calculator — Reconstitution using corrected mass
Molarity Calculator — Concentration conversions
Molecular Weight Calculator — Calculate MW from sequence