How to Read a Peptide Analytical Report

An analytical report documents tests performed on one specific lot of material. Learning to read it helps you confirm that the material in your lab matches what you ordered, and it tells you which questions the report answers and which it leaves open.

Start with the Lot Number

The report applies only to the lot printed on it. Match the lot number on the report to the lot number on your vial label. A report for a different lot says nothing about the vial in front of you.

Also check the compound name, the stated amount per vial and the test date. A report with a missing lot number, a missing date or no named laboratory is incomplete, whatever its numbers say.

Identity: Mass Spectrometry

Identity is usually confirmed by mass spectrometry (MS), often coupled with liquid chromatography (LC-MS). The report lists the expected molecular weight and the observed mass. The two should agree within the stated tolerance of the method.

How the measurement works

Mass spectrometers measure ions, not neutral molecules. Two "soft" ionization methods made intact peptide and protein analysis routine in the late 1980s:

• Electrospray ionization (ESI), developed by John Fenn's group, sprays the sample from solution into the instrument. Larger molecules pick up several protons, so the spectrum shows a series of peaks for the same molecule at different charge states (Fenn et al. 1989).
• Matrix-assisted laser desorption/ionization (MALDI), developed by Michael Karas and Franz Hillenkamp, embeds the sample in a light-absorbing matrix and releases ions with a laser pulse, usually as singly charged ions (Karas & Hillenkamp 1988).

Fenn shared the 2002 Nobel Prize in Chemistry for this work on ionizing large biomolecules.

Reading the numbers

A report may quote a mass in two ways:

Because ESI adds protons, an observed peak is usually [M+H]⁺ or [M+2H]²⁺ rather than M itself. For BPC-157 the singly protonated ion falls near m/z 1419.7 and the doubly protonated ion near m/z 710.4. A report should say which species it is quoting, or show the deconvoluted mass.

What MS confirms and what it does not

A matching mass is strong evidence that the main molecule has the right composition. It does not, by itself, separate two peptides with the same composition in a different order, or distinguish L- from D-amino acids, which have identical masses. Tandem MS (MS/MS) fragments the molecule and can confirm the sequence. If a report includes MS/MS or a fragment table, that is a deeper identity check.

Purity: HPLC

Purity is commonly measured by high-performance liquid chromatography (HPLC) with UV detection. The result is the main peak's share of the total peak area, for example 98.7%.

How the separation works

Reversed-phase HPLC (RP-HPLC) is the standard method for peptides. The sample travels through a column packed with a water-repellent stationary phase, usually C18. A gradient of increasing organic solvent (typically acetonitrile in water) washes peptides off in order of hydrophobicity (Aguilar & Hearn 1996). An ion-pairing additive such as trifluoroacetic acid (TFA) or formic acid is often added to sharpen peaks.

Detection at about 214 nm is widely used because the peptide bond itself absorbs there, so almost every peptide species in the sample is seen. Published peptide methods routinely report 214 nm detection for both purification and purity checks (Sikora et al. 2018).

What the small peaks are

Peaks other than the main one are usually peptide-related impurities, molecules that are almost the target but not quite. A structured review of peptide impurities groups them into synthesis-related forms, such as deletion sequences (a missing residue), insertion sequences (an extra residue), diastereomers from racemization and incompletely deprotected adducts, and degradation forms, such as oxidation, deamidation, succinimide, pyroglutamate and diketopiperazine products (D'Hondt et al. 2014). Many of these differ from the target by one residue or one small chemical change, so they elute close to the main peak.

Two practical consequences follow:

1. Resolution matters. An impurity hiding under the main peak inflates the purity figure. Regulators reviewing synthetic peptide submissions ask for orthogonal methods, meaning separations based on different chemistry, partly for this reason (FDA SBIA presentation, 2022).
2. LC-MS can see what UV cannot separate. High-resolution LC-MS can identify and quantify impurities even when they co-elute with the main peak. An FDA laboratory showed impurities below 0.1% of the main compound could be detected this way (Zeng et al. 2015).

For context: how strict is "strict"?

For certain synthetic peptide drug products, FDA's 2021 guidance asks applicants to identify each peptide-related impurity present at 0.10% or more, and regards a new impurity above 0.5% as generally unacceptable without further data (FDA SBIA presentation, 2022). Those thresholds belong to regulated drug development, not to research materials, but they show how small the impurities are that analytical chemists care about.

Purity Is Not Content

HPLC purity is relative to what the detector can see. It does not measure water or counter-ion content, which is why net peptide content can differ from HPLC purity.

A lyophilized peptide powder contains three main things:

• the peptide itself;
• counter-ions, the negatively charged ions paired with the peptide's positive charges. Peptides made by Fmoc solid-phase synthesis and purified by RP-HPLC are usually isolated as trifluoroacetate (TFA) salts, and excess TFA can be present (Sikora et al. 2018);
• residual water, picked up during and after freeze-drying.

None of these show up as impurities in an HPLC purity figure. A sample can therefore be 99% pure by HPLC while the peptide itself makes up a noticeably smaller share of the weighed powder. If a report lists net peptide content (often by amino acid analysis or nitrogen analysis), water content (often by Karl Fischer titration) or counter-ion content (often by ion chromatography), those fields answer the "how much" question that purity does not.

Contaminant Tests

Some reports include tests for specific contaminants, each reported as a measured value against a limit:

• Endotoxin, by a Limulus amebocyte lysate (LAL) method. See What "Endotoxin" Means on a Lab Report.
• Residual solvents, usually by gas chromatography. Solvents used in peptide synthesis and purification include acetonitrile, dimethylformamide and dichloromethane; the ICH Q3C guideline classifies all three as Class 2 solvents, to be limited (ICH Q3C(R9)).
• Heavy metals or elemental impurities.

A test that was not run is not a pass. If a field is absent, the report simply does not address it.

Appearance and Other Fields

• Appearance: for lyophilized peptides, typically a white to off-white powder or cake.
• Solubility: sometimes noted for a stated solvent and concentration.
• Test date: when the analysis was performed.
• Laboratory: who performed the test.
• Signature or approval: who reviewed and released the result.

A Quick Checklist

1. Lot number on the report matches the vial.
2. Observed mass matches the expected mass, with the ion species stated.
3. HPLC purity, wavelength and method are given, with the chromatogram.
4. Any content, water or counter-ion figures are understood as separate from purity.
5. Contaminant tests list both the result and the limit.
6. The laboratory and date are named.

How We Apply This

Every Peptide Privé vial is batch-numbered, so each order can be traced to its lot. Every Peptide Privé batch is tested by an independent lab.

Sources

• Fenn JB et al. Electrospray ionization for mass spectrometry of large biomolecules. Science. 1989. PubMed 2675315: https://pubmed.ncbi.nlm.nih.gov/2675315/
• Karas M, Hillenkamp F. Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons. Anal Chem. 1988. PubMed 3239801: https://pubmed.ncbi.nlm.nih.gov/3239801/
• Aguilar MI, Hearn MT. High-resolution reversed-phase HPLC of peptides and proteins. Methods Enzymol. 1996. PubMed 8803962: https://pubmed.ncbi.nlm.nih.gov/8803962/
• D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014. PubMed 25044089: https://pubmed.ncbi.nlm.nih.gov/25044089/
• Zeng K et al. Liquid Chromatography-High Resolution Mass Spectrometry for Peptide Drug Quality Control. AAPS J. 2015. PubMed 25716148: https://pubmed.ncbi.nlm.nih.gov/25716148/
• Sikora K et al. Counter-ion effect on antistaphylococcal activity and cytotoxicity of selected antimicrobial peptides. Amino Acids. 2018. PubMed 29307075 (full text PMC5917001): https://pubmed.ncbi.nlm.nih.gov/29307075/
• U.S. FDA, SBIA 2022 presentation on peptide-related impurities in synthetic peptide ANDAs: https://www.fda.gov/media/166572/download
• Federal Register notice of FDA guidance, ANDAs for Certain Highly Purified Synthetic Peptide Drug Products (May 20, 2021): https://www.federalregister.gov/documents/2021/05/20/2021-10603/abbreviated-new-drug-applications-for-certain-highly-purified-synthetic-peptide-drug-products-that
• ICH Q3C(R9) Guideline for Residual Solvents (2024): https://database.ich.org/sites/default/files/ICH_Q3C%28R9%29_Guideline_MinorRevision_2024_2024_Approved.pdf
• Nobel Prize in Chemistry 2002, summary: https://www.nobelprize.org/prizes/chemistry/2002/summary/
• PubChem, BPC-157 (CID 9941957): https://pubchem.ncbi.nlm.nih.gov/compound/9941957

Further Reading

• The Label Is Not the Test: How a Lab Confirms What's in a Vial
• Storing Lyophilized Peptides in the Lab

For in-vitro laboratory research only. Not for human or animal use.