Identity, Purity, and Fill Weight Explained

Three critical metrics that together determine whether your peptide product matches its label and is suitable for research.

Quick Definitions

  • Identity: Confirms the peptide is what the label says it is. Measured by mass spectrometry (molecular weight matching).
  • Purity: Measures what percentage of the sample is the target peptide versus contaminants. Measured by HPLC (peak area ratio).
  • Fill weight: The actual mass of material in the vial. Measured by weighing the vial before and after removing contents.

All three are independent — a peptide can pass identity but fail purity, or pass purity but have low fill weight. Complete verification requires all three.

Peptide Identity Testing

What it tells you

Identity testing answers a single question: Is this peptide the compound the vendor claims it is? This is the most fundamental quality check. If you order tirzepatide and receive semaglutide instead (which has a similar appearance), only mass spectrometry can tell the difference.

How it works

Identity is confirmed by LC-MS (Liquid Chromatography-Mass Spectrometry). The mass spectrometer measures the molecular weight of the peptide with high precision. The measured mass is compared to the theoretical molecular weight calculated from the peptide's amino acid sequence. A match within ±0.1% (or better) confirms identity.

What can go wrong

  • Wrong peptide shipped: Vendor sends a different peptide (mix-up or mislabeling). Mass will not match.
  • Truncated sequence: Synthesis error produces a shorter peptide. Mass will be lower than expected.
  • Modified peptide: Oxidation, deamidation, or chemical modification changes the mass. LC-MS detects the mass shift.
  • Counterfeit: Inert substance with no peptide bond. Mass spectrum will show nothing.

Peptide Purity Testing

What it tells you

Purity testing answers: What percentage of the material in this vial is actually the target peptide? Even if the identity is correct, the peptide may contain impurities from the manufacturing process — truncated sequences, deletion peptides, residual protecting groups, salts, or solvents.

How it works

Purity is measured by HPLC (High-Performance Liquid Chromatography). The peptide sample is separated on a chromatographic column, and a UV detector records the absorbance at each point. The resulting chromatogram shows a peak for the target peptide and smaller peaks for impurities. The purity percentage is calculated as: (area of main peak) ÷ (total area of all peaks) × 100.

Acceptable purity levels

Purity Grade Typical Use
≥99% Premium research grade Clinical research, sensitive assays
≥98% Standard research grade Most research applications
95-97% Acceptable Non-critical research
<95% Low purity Not recommended — impurities may affect results

Common impurities detected

  • Truncated peptides: Incomplete synthesis produces shorter sequences. These elute at different times on HPLC.
  • Deletion peptides: Missing one or more amino acids. Mass spectrometry can identify the specific deletion.
  • Acetylated/deprotected intermediates: Residual protecting groups from solid-phase peptide synthesis.
  • Salts (TFA, acetate): Counter-ions from purification. TFA salts are common but can be exchanged for acetate or HCl.
  • Water: Lyophilized peptides retain 5-15% water by weight. This does not affect purity but affects fill weight.

Fill Weight Testing

What it tells you

Fill weight (also called net content) tells you how much material is actually in the vial. A vendor might sell you "5 mg" of peptide, but the vial may contain 4.2 mg, 5.5 mg, or exactly 5.0 mg. Without weighing, you cannot know.

How it works

Fill weight is measured by weighing the vial with the peptide, then removing the peptide and weighing the empty vial. The difference is the net fill weight. This is a simple gravimetric measurement but requires an analytical balance with 0.01 mg precision.

Why fill weight matters

Fill weight directly affects cost per milligram. If you pay $50 for "5 mg" of peptide but the vial contains only 3.8 mg, you are actually paying $13.16/mg instead of $10/mg — a 32% price premium you didn't know about. Conversely, some vendors overfill vials slightly, which is good for the buyer but indicates imprecise manufacturing.

Fill weight also affects dosing accuracy. If you reconstitute a "5 mg" vial with 2.5 mL of BAC water expecting a 2 mg/mL solution, but the vial actually contains 3.5 mg, your concentration is only 1.4 mg/mL — and your doses will be 30% lower than intended.

FAQ

What is the difference between identity and purity?

Identity confirms what the peptide is (correct molecular weight via mass spectrometry). Purity measures how much of the sample is the target peptide versus contaminants (via HPLC). A peptide can have correct identity but low purity, or high purity but wrong identity.

What fill weight should I expect?

Fill weight should match the label claim ±10%. A "5 mg" vial should contain 4.5-5.5 mg. Significant underfilling (more than 10% below label) is a quality control issue that should be reported to the vendor.

Does purity include water weight?

No. HPLC purity is measured on the dissolved peptide and reflects the chemical composition of the sample, not the water content. Water content is typically measured separately by Karl Fischer titration and reported on the COA as "residual moisture" or "water content."

Why do I need all three tests?

Each test answers a different question. Identity confirms the peptide is correct. Purity ensures it's free from contaminants. Fill weight verifies you received the amount you paid for. Without all three, you have an incomplete picture of quality.

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