verification
Reading a Peptide Certificate of Analysis: Field-by-Field Reference
A certificate of analysis records the results of tests performed on one batch. This is a field-by-field reference to what each entry states, how it is determined, and where it fails.
A peptide certificate of analysis reports the results of a defined set of tests performed on one specific batch, and nothing more. It is a record, not a warranty. Its value is a function of three variables: which tests were run, on which batch, and by whom. Change one and the document establishes something different.
Two certificates can carry the same headline number and support entirely different conclusions. A purity figure without its method is not comparable to one with it. A mass result on an unnamed batch is not evidence about the vial in hand.
Function of the Document
A certificate is issued against a batch, or lot: material produced in one synthesis and purification campaign and filled together. The results apply to that batch alone. A repeat synthesis of the same sequence needs its own certificate.
It is generated at release and states the condition of the material on the date of analysis. Temperature excursions, light and moisture after that date leave no mark on the document.
Published analyses of unregulated product show what testing exists to detect. A 2015 study in Talanta on illegal peptide biopharmaceuticals encountered by controlling agencies reported samples in which the declared peptide could not be detected, and samples whose measured amount differed from the label 1. A 2018 Talanta study profiling impurities in falsified polypeptide drugs on the Belgian market reported synthesis-related impurities, including truncated and deletion sequences, in the material examined 2. Neither result extrapolates to other supply.
Field Reference
The table is the summary. The sections after it expand the fields whose failure mode is not obvious from one line.
| Field | What it states | How determined | Common failure mode |
|---|---|---|---|
| Batch or lot number | The run the results apply to | Assigned at manufacture | Absent, or not matching the vial |
| Name and sequence | The molecule tested, with terminal modifications | Synthesis record, confirmed by analysis | Trade name only, no sequence to check |
| Formula and theoretical mass | Mass expected from the sequence | Calculated from the formula | Formula omits a modification or counterion |
| Observed mass | Mass measured on this batch | Mass spectrometry, commonly electrospray | Basis and tolerance not stated |
| Purity | Main peak area as a share of total | Reversed-phase HPLC with UV detection | No method or chromatogram given |
| Net peptide content | Share of gross mass that is peptide | Amino acid analysis or nitrogen determination | Omitted; gross mass assumed peptide |
| Counterion | Salt form, usually acetate or trifluoroacetate | Ion chromatography or titration | Not stated; mass contribution unquantified |
| Residual solvents | Volatiles left from synthesis | Headspace gas chromatography | Conformance claimed, no per-solvent figures |
| Water content | Water held in the lyophilised solid | Karl Fischer titration | Omitted; adds to gross mass |
| Appearance | Physical description of the solid | Visual inspection | Wording no observation could contradict |
| Solubility | Solvent and concentration of dissolution | Bench test at a stated concentration | Solvent named, concentration omitted |
| Storage condition | Conditions under which results hold | Manufacturer specification | No temperature or moisture qualifier |
| Date of analysis | When the tests were performed | Laboratory record | Missing, or inconsistent with release |
| Analyst or laboratory | Who performed and signed the work | Signature, laboratory name, report number | Unattributed, no report number |
Batch, Sequence and Mass
The batch number is the only field connecting the document to the material. Check it against the container label first. With no batch identifier, or one that does not match, every remaining field is an untestable claim about an unknown quantity.
The sequence is given in one- or three-letter code, N-terminus to C-terminus, with terminal modifications noted: Ac- for acetylation, -NH2 for amidation. Disulfide bridges are stated separately. Check it against the sequence ordered, not the trade name. Trade names are uncontrolled; a sequence can be checked, a name cannot.
Two masses appear. Theoretical mass is calculated from the molecular formula, and a formula omitting the terminal modifications or the counterion cannot match the measurement. Observed mass is what was measured on the batch. Monoisotopic and average masses differ by of the order of one unit at a few thousand g/mol, so the certificate must state which is quoted. The two should agree within the stated instrument tolerance; where they do not, that is the finding, whatever the purity line says.
Purity by HPLC
Reversed-phase HPLC with UV detection is the standard purity measure for synthetic peptides 3. The figure is not an absolute quantity of peptide: it is the main peak area as a percentage of total integrated peak area, under one set of separation conditions.
Purity is therefore method-dependent. A shallow gradient resolves species a steep gradient merges, and a merged pair integrates as one peak and reports as higher purity. Column, particle size, temperature, modifier and run length all move the number. A percentage without its method is not comparable with another: two certificates reading 98% may describe different materials.
Detection wavelength decides what is visible. Detection at 214 nm responds to the peptide bond and sees peptide-related species generally. Detection at 280 nm responds to aromatic side chains — tryptophan, tyrosine, weakly phenylalanine — and reports only on species carrying them. The two answer different questions; a sequence without aromatic residues gives almost no signal at 280 nm.
| Parameter | Typical entry | Effect on the reported number |
|---|---|---|
| Column | C18, 4.6 × 250 mm, 5 µm | Sets resolution; unresolved impurities integrate into the main peak |
| Gradient | 5–65% B over 30 min | Shallower gradients separate close-eluting species and lower reported purity |
| Mobile phase | Water and acetonitrile, 0.1% trifluoroacetic acid | The modifier changes retention and which species co-elute |
| Flow and temperature | 1.0 mL/min, 25 °C | Both shift retention and resolution |
| Detection wavelength | 214 nm | 214 nm sees the peptide bond; 280 nm only aromatic residues |
| Integration | Area percentage, main peak | Threshold and manual integration change the number, not the sample |
| Run time | 40 min | Late-eluting impurities are invisible if the run ends first |
A purity figure is interpretable only alongside the chromatogram. Check baseline return between peaks, that the main peak is not flat-topped at the detector ceiling, and that the run continues past it.
Identity by Mass Spectrometry
Mass spectrometry measures the mass-to-charge ratio of ionised species 4. On an intact peptide this yields the molecular mass. A match with the theoretical mass establishes that the material has the expected mass. It does not establish that the residues are in the expected order.
Sequence isomers are isobaric: any permutation of the same residues has the same formula and mass, so a transposition passes an intact-mass check silently. Leucine and isoleucine are identical in mass. Lysine and glutamine differ by about 0.036 units, resolvable only at high mass accuracy. Deamidation adds roughly one unit and reads as noise at low resolution.
Sequence order is established by tandem mass spectrometry, in which the selected ion is fragmented and the fragment masses read as a ladder 4. Where a certificate states that sequence is confirmed, check whether the data behind it is an intact mass or a fragmentation spectrum.
Content, Counterion, Solvent and Water
Net peptide content states the proportion of gross weighed mass that is peptide; the remainder is counterion, water and residual solvent. It is determined by amino acid analysis or nitrogen determination, and it converts a balance reading into an amount of peptide. A separate reference on this site covers it in full, with the calculation. Its absence is itself information: gross mass is then the only figure available, and it overstates peptide.
The counterion is the salt form, usually acetate or trifluoroacetate, determined by ion chromatography or titration. Its mass is part of the gross mass, and trifluoroacetate carried through from purification matters to experiments sensitive to it. Unstated, part of the weighed mass is unaccounted for.
Residual solvents are volatiles left from synthesis and purification, determined by headspace gas chromatography and reported per solvent against a limit. A line stating only that the material conforms records a decision, not a measurement.
Water content in a lyophilised solid is determined by Karl Fischer titration and reported as a mass percentage. Lyophilised peptides are hygroscopic: a high figure means less peptide per weighed milligram and faster degradation in storage.
Physical Description and Provenance
Appearance is a visual description, typically a white to off-white lyophilised powder or cake. It is the only field verifiable without instrumentation. Discolouration, an oily or collapsed cake, or visible particulates is a discrepancy and is recorded as one.
Solubility states a solvent and a concentration at which dissolution was observed — water to 1 mg/mL, for example. Without a concentration the statement is not testable. Some sequences need dilute acetic acid or another co-solvent before dilution into the working buffer.
Storage condition specifies the conditions under which the reported results are expected to hold: a temperature, with protection from light and moisture. It is a specification, not a measurement.
Date of analysis places the results in time; they describe the batch as it was on that date. Undated, a certificate cannot be aged against a retest interval. The signature, laboratory name and report number identify who is answerable. Unattributed, there is nothing to query.
Outside the Scope of a Chemistry Certificate
A certificate of analysis is a chemistry document. It reports composition. It does not report whether the material is sterile, free of bacterial endotoxin, or active in any biological system. Those are separate tests, implied by no purity or identity result.
| Test | What it establishes | Method | Relation to the certificate |
|---|---|---|---|
| Sterility | Absence of viable microorganisms | Membrane filtration or direct inoculation | Separate report; purity says nothing about it |
| Bacterial endotoxin | Endotoxin per milligram of material | Limulus amoebocyte lysate or recombinant factor C | Not visible in a UV chromatogram |
| Biological activity | Response in a defined assay system | Cell-based or receptor-binding assay | Chemical identity does not establish activity |
| Elemental impurities | Residual metals from reagents or hardware | Inductively coupled plasma mass spectrometry | Reported separately where required |
| Stability | Behaviour over time under stated storage | Stability study sampled at intervals | The certificate is one time point, not a trend |
Independence of the Testing Party
A certificate has two properties, assessed separately: what it reports, and who reports it. The first is judged from the fields above. The second is a question about the document.
A certificate produced by the party selling the material is self-reported data. It may be entirely accurate, and in-house quality control is standard practice across chemical manufacturing. It carries less independent weight than a result from a laboratory with no interest in the outcome, because the reader cannot separate the measurement from the incentive.
Third-party analysis is a service that can be commissioned. Contract laboratories run HPLC purity, mass confirmation and amino acid analysis on submitted samples and report against the sample as received. That converts a claim on a document into a measurement on your own sample.
Three features make independence checkable: the testing laboratory named distinctly from the seller, a report number it can retrieve, and raw data or chromatograms rather than a summary alone. Their absence removes the means of checking.
Acceptance Checks
- Confirm the batch number on the certificate matches the container label.
- Compare the stated sequence in full against the sequence ordered.
- Check the formula accounts for terminal modifications and yields the theoretical mass.
- Check observed against theoretical mass, both quoted on the same basis.
- Establish whether identity rests on intact mass or on fragmentation.
- Read purity only with its column, gradient, mobile phase, wavelength and run time.
- Confirm the detection wavelength suits the sequence; 280 nm needs aromatic residues.
- Locate the chromatogram; check baseline return and run length past the main peak.
- Record whether net peptide content is stated, and note it where absent.
- Record counterion, water content and residual solvents as figures, not conformance.
- Compare the appearance description against the container and record discrepancies.
- Check solubility carries both a solvent and a concentration.
- Note the storage condition and confirm it has been held since receipt.
- Confirm date of analysis, laboratory identity, and a retrievable report number.
- Identify who performed the analysis and whether that party supplies the material.
Recording a Certificate Against a Batch
A certificate is useful later only if it is bound to the physical material in the laboratory record.
- Assign an internal reference on receipt and mark it on the container.
- File the certificate unaltered under that reference; annotate a copy, never the original.
- Transcribe into the batch record: batch number, sequence, theoretical and observed mass, purity with its wavelength, net peptide content, counterion, water content, date of analysis.
- Record absent fields explicitly as absent; a blank cell reads as unchecked.
- Record date of receipt, condition on arrival, storage location and temperature.
- Record the appearance check against the certificate, with date and initials.
- Where a calculation uses gross mass, record which content figure was applied and whether it was stated or assumed.
- Cross-reference every experiment to the internal reference so results trace to the batch.
- File any commissioned independent report under the same reference, with the submission date.
- Retain the record for as long as any dataset from that batch is in use.