Peptide purity and laboratory testing: how to read a Certificate of Analysis (COA)

Peptide purity describes the composition of a sample determined using a specific analytical method. In itself, it does not confirm the full identity of the peptide, the actual amount of peptide in the sample, sterility, biological activity, stability, or safety. A certificate of analysis, most commonly referred to by the abbreviation COA, is most useful when every result provided in it is treated as an answer to a specific analytical question.

Therefore, information such as „99% purity” requires appropriate context.

Does 99% mean that the main chromatographic peak accounts for 99% of the total integrated detector response in the HPLC analysis? Does this value represent the mass fraction of the target peptide after accounting for water, counterions, and other components? Has the stereochemical purity been evaluated? What kinds of impurities can this method detect, and what kinds might fall outside its scope?

These distinctions are important because several different parameters are colloquially referred to by the same word „purity”, even though they actually describe different properties of the sample.

Peptide analysis research demonstrates why this matters. Structurally similar impurities can sometimes remain hidden within a seemingly simple chromatographic peak, whilst water, counter-ions, residues of other substances and related compounds can affect the total mass of the isolated peptide material. [1] [2]

These are distinct analytical problems and require different types of evidence.

Therefore, when evaluating a peptide's COA, more useful than the question itself:

„What is the percentage purity?”

There is a question:

„What exactly was measured, by what method, in which sample, and what conclusion actually follows from this measurement?”

What does peptide purity actually mean?

Cleanliness is a result based on a specific method

The word „purity” might suggest a single, easily measurable property. In practice, however, laboratories can express peptide purity in several different ways.

This may include, among other things:

• percentage of chromatographic area;
• mass fraction;
• chemical purity;
• stereochemical purity;
• sequence-related contamination profile;
• purity determined by a specific analytical method.

These values should not be treated as interchangeable.

Chromatographic purity typically describes the fraction of the integrated detector response attributed to the main component of the sample.

Mass fraction defines the mass of a specific substance in relation to the total mass of the tested material.

Stereochemical purity, on the other hand, relates to the proportion of different spatial configurations of the molecule or the amino acid residues that make it up.

Therefore, properly presented information on purity should, where possible, answer four questions:

what was measured + how it was measured + how the result was calculated + what sample was studied.

The statement alone:

„99% purity”

conveys significantly less information than:

„99% of the main peak area according to RP-HPLC-UV.”.

Even the second version still doesn't answer all the questions.

It is also necessary to know whether the chromatographic method used was capable of separating the relevant impurities and whether the detector used was able to detect them.

What does the percentage of peak area mean in HPLC?

Simply put, the calculation based on peak area normalisation can be represented as:

area of the main peak ÷ total area of included peaks × 100.

If the main peak has been assigned 990 area units and the sum of the accounted chromatographic signal is 1000 units, the area of the main peak will be:

990 ÷ 1000 × 100 = 99%.

This is information regarding the detector response obtained under specific chromatographic conditions.

It does not automatically mean that:

99% of the physical mass of the powder is the target peptide.

There are several reasons for this difference.

Different chemical compounds can generate different detector responses per unit of mass.

Some sample components may absorb radiation poorly at the applied wavelength.

Others may not be included in chromatographic calculations at all.

Some impurities may also elute together with the main component.

The word „included” is also important.

Chromatographic calculations depend on decisions concerning:

• integration of peaks;
• reporting thresholds;
• baseline selection;
• solvent-related signals;
• unresolved peaks;
• detector settings;
• the rule of excluding specific signals.

Therefore, the percentage of the HPLC area should be interpreted as the result of a specific chromatographic method, rather than as a universal description of the entire physical composition of the sample.

Related impurities can be difficult to separate

Impurities present in peptides can be structurally very similar to the intended molecule.

They may include, among others:

• deletion sequences;
• abbreviated sequences;
• altered side chains;
• oxidised forms;
• modified end groups;
• positional isomers;
• stereoisomers;
• products of incomplete decanning;
• other variants arising during peptide syntheses or demotion.

Some of these structures may differ significantly in chromatographic behaviour from the target peptide.

Others may behave very similarly.

One study used two-dimensional liquid chromatography coupled with mass spectrometry to assess the purity of the main peptide peak. The researchers applied additional separation conditions because peptide isomers with the same mass-to-charge ratio could not be distinguished based solely on routine mass measurement. [1]

In another study involving four synthetic cyclic peptides, reversed-phase chromatography was compared with hydrophilic interaction chromatography.

Both methods separated the contaminants in a different way. [3]

This does not mean that either of these techniques is always better.

It demonstrates the phenomenon of complementary selectivity: two analytical methods can resolve the same mixture differently because they exploit different physicochemical properties of the analysed molecules.

Therefore:

one clean chromatogram does not necessarily mean the absence of all structurally similar impurities.

Water and counterions also affect the composition of the material

The isolated peptide material may contain components that are not additional peptide sequences.

Important examples include:

• water;
• ion counter;
• solvent residues;
• inorganic substances;
• other components related to the production process.

A counterion is an ion of opposite electrical charge, bound to charged groups of the peptide.

In peptide chemistry, one can encounter, for example, acetate or trifluoroacetate.

Their presence does not have to mean a synthesis error or contamination. The peptide may intentionally be in a specific salt form.

However, these ingredients matter when we want to answer the question:

how much of the target peptide is actually in the weighed amount of material?

Interpretation therefore depends on whether the reported value relates to:

• just the peptide;
• peptide salts;
• dry material;
• anhydrous material;
• as-received material.

In the 2026 sublancin study, researchers quantified water, trifluoroacetate, organic compounds and inorganic impurities as part of a broader mass balance analysis. [2]

Ultimately, the assigned peptide content was clearly lower than 100%, even though structurally related organic impurities made up a relatively small portion of the material.

This is a practical example of an important difference:

A clean chromatographic peptide profile and a high mass fraction of the target peptide are not the same parameter.

Purity percentage is not a safety probability

The percentage value defining purity does not state what the probability is that a given material is safe.

For example, a purity result of 99% does not mean:

„99% of safety”.

It also does not inform about the biological significance of each substance contained in the remaining %.

A small amount of one compound may have greater analytical or biological significance than a larger amount of another compound, depending on:

• his identity;
• biological activity;
• concentrations;
• routes of exposure;
• intended use;
• other contextual factors.

Similarly, a correctly identified molecule of very high chemical purity may itself possess specific biological properties.

Chemical purity and biological safety therefore address completely different questions.

Purity testing serves to characterise a sample regarding the capabilities of the applied analytical method.

It does not convert the chemical result into a general human safety assessment.

HPLC and mass spectrometry

What can HPLC tell us?

High-performance liquid chromatography (HPLC) separates sample components based on differences in their interactions with a stationary phase and a flowing mobile phase.

Reverse-phase chromatography (RP-HPLC) is widely used in peptide analysis, often in combination with UV detection.

The chromatogram shows the detector response as a function of time.

Several important pieces of information can be obtained from it.

The retention time indicates the moment at which a given component leaves the chromatographic system under specified conditions.

The peak area represents the integrated detector response.

The peak shape provides information on the chromatographic behaviour of the substance.

Resolution describes how effectively adjacent components are separated from each other.

However, retention time is not a unique molecular „fingerprint”.

Different molecules can sometimes elute at a similar time.

The same peptide may also exhibit different retention times upon changing:

• columns;
• composition of the mobile phase;
• gradient;
• flow rates;
• temperatures;
• equipment configuration;
• other method parameters.

Studies comparing different peptide separation systems show how strongly chromatographic selectivity depends on the method used. [1]

Retention time alignment can therefore support identification in the appropriate context, but should not automatically be treated as full confirmation of the structure.

Why doesn't one peak have to mean one substance?

Coelution occurs when two or more components leave the chromatographic system at such a similar time that they are not properly resolved.

As a result, they may look like a single peak.

Even a visually symmetrical main peak does not therefore automatically prove that the entire signal originates from a single type of particle.

The composition of the peak can be studied in more detail by:

• change in chromatographic selectivity;
• use of a different stationary phase;
• multi-wavelength analysis;
• coupling of chromatography with mass spectrometry;
• two-dimensional chromatography;
• application of another complementary analytical technique.

However, none of these methods automatically detects every possible contaminant.

Closely related compounds may have:

• similar retention time;
• similar UV spectra;
• identical nominal mass;
• other common analytical properties.

A study of cyclic peptides, which compared reversed-phase chromatography with hydrophilic interaction chromatography, clearly showed different impurity elution profiles. [3]

This does not mean that every major peak hides additional impurities.

The application is more precise:

The deceptive simplicity of the chromatogram is a result obtained in a single separation system, and the application of a different selectivity may reveal components that the first method failed to resolve.

What does mass spectrometry measure?

Mass spectrometry (MS) detects ions based on their mass-to-charge ratio, written as m/z.

During analysis, a peptide may possess more than one electrical charge.

Therefore, a single peptide molecule can generate several signals corresponding to different charge states, instead of a single peak directly corresponding to the neutral molecular mass.

Proper data interpretation allows the fundamental molecular mass to be reconstructed or confirmed.

Mass spectrometry can provide valuable information regarding:

• molecular mass;
• of some chemical modifications;
• specified pollutants;
• isotope patterns;
• behaviours during fragmentation.

High-resolution mass spectrometry allows for a more precise distinction between molecules with very similar masses.

Tandem mass spectrometry (MS/MS) involves fragmenting selected ions and analysing the resulting products, thereby providing additional structural information.

However, an important rule still applies:

observing the expected molecular weight ≠ proving the complete molecular structure.

Different structures can have the same mass.

Stereochemical changes may not affect the elemental composition.

Positional isomers may also require additional evidence.

Research into impurities of the synthetic renin substrate peptide demonstrated this limitation. Additional information from fragmentation and derivatisation was required to distinguish between modifications occurring at different positions of the isobaric mixture. [4]

Sequence, end groups and stereochemistry

A peptide's identity involves much more than just its name.

Important structural features may include:

• amino acid sequence;
• configuration of individual residues;
• N-terminal structure;
• C-terminal structure;
• side chain modifications;
• disulphide bond arrangement;
• cyclisation;
• conjugation;
• other covalent characteristics.

For example, a peptide with a C-terminal carboxylic acid is a chemically different molecule to the corresponding peptide with a C-terminal amide.

Researchers have developed a derivatisation-assisted mass spectrometry method precisely in order to distinguish these final forms more effectively. [5]

This demonstrates an important principle:

The analytical method should be chosen to suit the specific structural question.

Stereochemistry poses an additional challenge.

A peptide containing a D-amino acid and the corresponding variant with an L-amino acid can have an identical molecular mass while differing in the spatial arrangement of their atoms.

In the study on the stereochemical purity of peptides, a method designed to account for racemisation occurring during sample hydrolysis was used. [6]

Without such control, sample preparation alone could change the property being measured.

Reliable peptide analysis therefore depends not only on instrument parameters, but also on proper sample preparation and method design.

LC-MS combines the advantages of both methods, but does not eliminate all limitations

Liquid chromatography–mass spectrometry (LC-MS) combines chromatographic separation with mass analysis.

This may make it possible to provide significantly more information than each of these methods used separately.

LC-MS can help, among other things, with:

• assign specific masses to impurity peaks;
• to chromatographically separate similar components with different masses;
• investigate degradation products;
• sequence-related impurities;
• support structural assignments.

In the study of synthetic oxytocin using an internally validated LC method coupled with high-resolution mass spectrometry, 18 structurally related impurities were identified and quantified. [7]

The researchers also explained why these impurities had to be taken into account when determining the purity of the reference material.

However, these results apply to a specific sample, method and analysed contamination profile.

They should not be treated as a universal list of impurities present in every material containing oxytocin.

LC-MS is therefore an extremely useful combination of analytical techniques.

A good LC-MS report should still explain:

• what was measured;
• how the individual peaks were assigned;
• how the proposed structures were identified;
• Have the assignments been confirmed?;
• what quantitative method was used;
• what analytical limitations remain.

Identity, purity and quantity are different parameters

Identity answers the question: what molecule is present?

Identity testing aims to answer the question of whether the analysed material corresponds to the intended substance.

Depending on the peptide and the issue under investigation, evidence of identity may include:

• the mass of the entire molecule;
• fragmentary data;
• sequence analysis;
• characterisation of end groups;
• amino acid profile;
• disulphide bond arrangement;
• spectroscopic data;
• chromatographic comparison with a reference material;
• combination of several methods.

One method may confirm a specific element of identity without resolving all other structural issues.

For example:

„the expected mass of the entire molecule was observed”

is a narrower conclusion than:

„the complete molecular structure has been confirmed.

Research into C-terminal amidation and isobaric peptide impurities shows why this distinction is important. [4] [5]

In both cases, information going beyond simply observing a mass peak near the expected value was required.

Purity answers the question: what else is in the sample?

Purity testing analyses the composition of a sample within a specified analytical range.

Depending on the method, the report may present:

• main ingredient;
• individual pollutants;
• the total amount of detected contamination;
• percentage of main peak area;
• stereochemical impurities;
• related substances;
• total mass fraction.

Different classes of contamination may require different methods.

The chromatographic method optimised for the detection of deletion sequences does not need to answer questions regarding stereochemistry.

The mass-based method may not distinguish between isomers.

However, the chiral method may provide little information regarding the water content.

Therefore, the aim of using complementary methods is not merely to gather a larger number of results.

It is about obtaining answers to various analytical questions that one method cannot resolve on its own.

The quantity answers the question: how many substances are present?

Quantity can refer to several different parameters.

It could mean:

• the concentration of the peptide in the solution;
• the total amount of peptide in the container;
• mass participation of the target peptide;
• mol concentration;
• quantity per unit of volume;
• another clearly defined quantitative parameter.

Each of these methods requires an appropriate unit and a calculation basis.

Quantitative chromatographic labeling usually requires proper calibration and should not rely solely on pixel surface normalization.

In appropriately developed methods, the following can also contribute to determining the quantity:

• amino acid analysis;
• quantitative magnetic resonance imaging;
• methods of isotopic dilution;
• other validated quantitative techniques.

Importantly, related peptide contaminants can affect some quantitative measurements.

Studies on the purity of oxytocin emphasized the need to correct results for such impurities instead of assuming that every measured amino acid or analytical signal comes exclusively from the target molecule.[7]

Example: percentage of HPLC area and actual amount of peptide

Imagine a hypothetical peptide powder for which the following was obtained:

RP-HPLC-UV, main peak area = 99.0%

and

The determined mass fraction of the target peptide = 78.0% in the material in the final state.

These results do not have to be contradictory.

They have different parameters and answer different analytical questions.

If 10.0 mg of such hypothetical powder were used, and the validated mass fraction value of the target peptide is 78.0%, the amount of peptide resulting from this determination would be:

10.0 mg × 0.780 = 7.80 mg.

It would not be appropriate to automatically accept:

10.0 mg × 0.990 = 9.90 mg

solely because the main peak accounted for 99.0% of the included UV area.

This is merely an illustrative illustration and does not describe any specific commercial product or experimental material.

The study of sublunines provides a real example of the principle underlying this distinction, because when assessing the composition of the material, water and anti-ions were taken into account separately, among other things.[2]

The numerical values from this study should not be transferred to other peptides.

Measurement uncertainty and measurement consistency

The analytical result should not be treated as an infinite-precision value.

This is an estimate obtained using a specific measurement procedure.

The measurement uncertainty describes the range or distribution of values that can be reasonably related to the measurement result according to the assessment method used.

This does not mean that the laboratory knows that its result is incorrect.

It, however, informs about the limits of numerical certainty.

Measurement consistency refers to the documented relationship between the result and the appropriate reference through a calibration chain.

It helps to ensure comparability of measurements, but only for a specific parameter that was actually measured, and for a defined reference chain.

The study on the development of the NT-proBNP reference material combined isotope dilution methods with contamination correction to assess the homogeneity and stability of the material [8].

This shows why reliable quantification requires much more than just reading the number generated by the device.

How to read the COA analysis certificate?

Start by identifying the sample and batch

Before interpreting any analytical result, it is necessary to determine which material was actually tested.

A useful COA should combine the results with identifying information, such as:

• name or description of the material;
• sample identifier;
• part or series number;
• appropriate dates;
• structural form;
• the form of the salt, if it matters;
• modifications of ends or other significant structural features.

This is important because the analytical results refer to the sample that was actually tested.

A certificate regarding one batch does not directly confirm the composition of another batch.

Similarly, analysis performed on a sample of aggregate material does not automatically prove the quantity or condition of the material in each individual container that is subsequently filled unless the sampling, filling, homogeneity assessment and process control system warrants such a conclusion.

These are issues related to the consistency and identity of the material.

They do not automatically mean improper conduct or low quality.

The reader of the report simply needs a connection that is defensible:

report → tested sample → batch or material whose result is to be determined.

Read the method, result, unit and specification together

The COA often presents both the criteria for acceptance and the actual result obtained.

For example:

Specification: not less than 98.0%

and

Result: 99.2%.

These are two different pieces of information.

The specification defines the criteria for acceptance.

The result represents the measured value for a specific sample.

Equally important are the unit and the basis of measurement.

Among others, you can meet:

• area%;
• mass%;
• mg/container;
• mg/mL;
• µg/mL;
• mol/L;
• result in terms of dry mass;
• the result in terms of anhydrous material;
• result for the material in the received state.

These values cannot be arbitrarily substituted.

For example, a result defined as „in the form received” takes into account the material in the form in which it was tested, whereas the anhydrous or corrected value can mathematically exclude water from the calculation basis.

The „pass” information means that the given result met the specified acceptance criteria for the specific test.

It does not extend the scope of this study.

Therefore:

passed identity test ≠ confirmed sterility

and

passed purity test HPLC ≠ determined peptide content,

unless these properties were measured independently.

Example interpretation of entries in the COA

The following entries are hypothetical and are intended solely to illustrate how analytical data should be interpreted. They do not describe the actual party nor do they propose acceptance criteria.

The distinctions presented are based on studies concerning the purity of the main peptide peak, anti-ions, and determining the mass contribution. [1] [2] [9]

The figures and table format provided are for educational purposes only.

Check the chromatogram and spectrum

Additional analytical data help to properly interpret the results summarized in the COA.

In the case of a chromatogram, it is worth paying attention to the following:

• sample identifier;
• method identifier;
• chromatogram axis;
• type of detector;
• wavelength;
• significant retention times;
• the markings of the spikes;
• table of pollutants;
• the results of integration;
• reporting thresholds.

The chromatogram, cut so that it shows only the large main peak, may omit information necessary for the correct interpretation of the purity result.

In the case of mass spectrometry, a useful context may include:

• the expected molecular form;
• the expected mass;
• observed ions;
• assigned charge states;
• deconvolution;
• mass error;
• fragmentary data, if applicable;
• the basis for assigning a structure.

The mass spectrum of the whole molecule, the deconvolution of the molecular mass, and structural evidence based on fragmentation are all related to each other, but they do not mean the same thing.

The COA does not necessarily have to include all the raw files from the equipment.

However, the analytical conclusions summarized therein should be possible to link to the source documentation, which can be verified if questions arise regarding the assignment of a structure or numerical result.

„Not detected” and „not studied” mean something completely different

„Not detected” means that the analytical method used did not detect the test component under the specified conditions and at its sensitivity.

This does not mean that the actual concentration is exactly zero.

„Below the limit of detectability” means that the component may be present at a level that a given method is unable to quantify reliably.

That's a different situation.

„Not studied” means that no measurement was performed for that property.

This should not be interpreted as:

„correct result”

Nor:

„substance absent”.

The correct interpretation of information about detection or quantification requires also knowledge of:

• the limit of detectability or detectability;
• units;
• the basics of reporting;
• the method used;
• the method of preparing the sample.

What does an independent laboratory study reveal?

An independent laboratory can provide an additional analytical measurement for the sample submitted.

This can increase the amount of available evidence regarding a specific material.

Just the term:

„reviewed by an independent laboratory”

However, it does not explain:

• what exactly was examined;
• how many samples were tested;
• who selected the sample;
• who took it;
• whether it was representative;
• from which party she came;
• what methods were used;
• whether the report can be verified;
• whether the sample corresponds to the material subsequently distributed;
• which properties the study did not cover at all.

The most useful questions therefore concern:

the identity of the sample + the traceability of the batch + the method of sampling + the analytical range + the authenticity of the laboratory report.

The independent study does not remove the limitations of the methods used.

HPLC performed by an external laboratory remains an HPLC study.

It does not become a test of sterility, a marker of peptide content, a stereochemical analysis, or a study of biological activity just because it was conducted by an independent entity.

Therefore, the COA is worth evaluating primarily based on its analytical scope and the ability to trace the sample, and not solely based on the laboratory logo or the high percentage value placed at the beginning of the document.

What do routine analytical tests not confirm?

HPLC does not confirm sterility

The routine HPLC peptide analysis examines the chemical components based on their chromatographic behavior.

It is not a method for detecting live microorganisms.

A pure chromatogram cannot therefore confirm that the sample is sterile.

Sterility is a microbiological issue that requires appropriate methods and control of the production process.

Even a separate sterility test has limitations related to sample collection, as only a portion of the material used in the test is directly tested.

For this reason, ensuring sterility in pharmaceutical production relies on a broader system of production and control, rather than solely on the final product testing.

The FDA guidelines for aseptic production treat microbiological control and sterility testing as separate elements of the process. [10]

Therefore:

High purity HPLC ≠ sterility

and

absence of visible chromatographic impurities ≠ absence of microorganisms.

The endotoxin study answers another question

Endotoxins are components of bacteria, in particular material associated with lipopolysaccharides of Gram-negative bacteria.

They are not the same as live bacteria.

The study of bacterial endotoxins therefore answers a different question than the sterility test.

Theoretically, the sample may not contain detectable live bacteria, yet it may contain certain components of bacterial origin.

Similarly, a sterility test cannot automatically replace endotoxin testing.

The sample matrix can also influence the endotoxin test.

Therefore, it is important to properly adjust the method and control interference.

The FDA guidelines on pyrogens and endotoxins discuss analytical limitations and the difference between bacterial endotoxins and other pyrogenic substances.[11]

Routine HPLC purity analysis cannot replace either sterility testing or endotoxin testing.

Chemical purity does not confirm biological activity

The sample can have:

• the expected molecular mass;
• a high surface area of the main chromatographic peak;
• a relatively small amount of detected chemical pollutants,

and at the same time not have a confirmed specific biological function.

Functional behavior may also depend on:

• folding;
• disulphide bridge distributions;
• aggregation;
• structural conformity;
• state of modifications;
• test conditions;
• the applied biological system.

The study on the synthesis of BPTI clearly shows the value of dividing these questions into separate parts.

The researchers combined analytical and biophysical characterization with a trypsin inhibition test. [12]

Overall, these results provided stronger evidence regarding the material under study.

No single measurement replaced the others.

Therefore:

chemical purity ≠ biological activity

and

biological activity in one test ≠ complete structural or production characteristics.

The COA is not a clinical safety assessment

The COA presents the results of analytical tests performed on the described sample.

It does not determine:

• clinical tolerance;
• safety of human exposure;
• the appropriate dose;
• interaction;
• effectiveness;
• long-term effects;
• suitability for a specific person.

These questions require other types of evidence.

This principle remains true even when the analytical results are accurate and the identity of the peptide has been correctly confirmed.

The cleanliness test can characterize certain aspects of the chemical composition.

It cannot convert the analytical report into a clinical study or an assessment of the safety of use in humans.

One time point does not confirm the shelf life period

The COA usually describes the material at a specific point in the study.

Peptides can then undergo changes related to:

• degradation;
• oxidation;
• hydrolysis;
• deamidation;
• aggregation;
• absorption of moisture;
• structural reconfigurations;
• other processes dependent on the properties of the molecule and the environmental conditions.

The result obtained at one moment therefore does not confirm unlimited stability.

The shelf life requires data linking defined storage conditions with the analytical material over time.

In the study of exenatide, a chromatographic method was developed that indicated stability and characterized the degradation products formed under several stress conditions.[13]

This study shows how degradation products can be separated from the parent peptide.

However, it does not specify a universal time for the storage of all peptides, and artificial stress conditions should not be automatically equated with normal storage conditions.

The strongest argument should correspond to the strength of the evidence

A scientifically cautious interpretation should not go beyond the scope of the measurements performed.

For example, the COA can justify the statement:

„In the tested sample, the mass yield of the entire molecule was obtained consistent with the assigned peptide; the determined share of the main peak area in the HPLC method used, and the separately determined peptide content.”.

This is a valuable set of analytical evidence.

However, this does not automatically mean:

„The peptide has been fully characterized; it is sterile, biologically active, safe, stable indefinitely, and suitable for human use.

It is not about questioning the value of laboratory tests.

One should also not expect that one analytical method will answer every possible question.

The most important thing is the connection:

any claim → with a measure that can actually justify it,

with a clear indication of the questions that did not receive an answer.

Frequently asked questions

1. Does 99% purity mean that the product is safe?

No. The purity percentage describes the composition of the sample according to a specific analytical method, not the probability of safety. Moreover, this value can indicate the contribution of the chromatographic signal rather than the physical mass contribution of the target peptide. Studies of the mass balance of peptides show why water, antisolvents, and other components can cause differences between these values.[2] Regardless of the analytical purity, the target molecule itself can also have its own biological properties. A high purity score can therefore support the chemical characteristics of the sample under investigation, but it does not replace toxicological, pharmacological, microbiological, or clinical evidence regarding safety.

2. Does HPLC confirm sterility?

No. Routine HPLC peptide analysis separates and detects chemical components based on their chromatographic behavior. It does not determine whether there are live microorganisms in the sample. Sterility is a separate property requiring appropriate microbiological testing and control of the production process. The FDA guidelines for aseptic manufacturing discuss microbiological control independently of chemical analysis. [10] Therefore, a clean chromatogram, a high percentage of the main peak area, or the expected peptide retention time cannot substitute for evidence of sterility.

3. What exactly does the percentage of HPLC surface area mean?

The HPLC surface area percentage usually indicates the proportion of the integrated signal assigned to the main peak in the total chromatographic signal taken into account. The result depends on the quality of the separation, the detector response, the wavelength, the integration rules, the reporting thresholds, and other methodological conditions. It therefore does not automatically mean the physical contribution of the mass peptide to the entire sample. Studies of the purity of the main peptide peak show why unseparated isomers can complicate interpretation.[1] The HPLC surface area percentage should therefore be considered a result of a specific analytical method, the measurement basis of which should be clearly defined.

4. Can one HPLC peak contain several substances?

Yes. Two or more components can elute so close together that they will not be properly separated in a given chromatographic system and will appear as a single peak. Therefore, even a visually symmetrical peak is not an absolute proof of molecular homogeneity. The study of cyclic peptides showed that a different chromatography mode provided a different way of separating impurities than RP-HPLC.[3] This does not mean that every major peak contains hidden substances. It does, however, indicate that the separation ability depends on the method's selectivity and, in the case of closely related structures, additional analytical techniques may be needed.

5. Does the correct molecular mass confirm the correct peptide sequence?

No. The mass of an entire molecule consistent with the expected value constitutes valuable evidence regarding identity, but it does not necessarily confirm the sequence of amino acids, stereochemistry, type of terminal groups, position of modifications, or all other structural features. Studies of synthetic peptide impurities have shown components of the same mass but with modifications located at different positions, which required additional analytical information. [4] Therefore, a full assessment of identity requires methods tailored to the specific structural ambiguities of a given peptide.

6. How is the content of the peptide different from purity?

The content of the peptide determines how much of the defined target peptide is present in the material based on a specific quantitative basis. However, purity may refer to the proportion of the chromatographic signal attributed to the main component or another defined purity parameter. Water, anti-ions, process residues, and structurally related compounds can cause these values to vary. In the sublactin study, several such components were measured separately during the mass balance assessment. [2] The result of the peptide content and the HPLC surface area percentage should therefore be interpreted separately, unless a validated analytical system unequivocally establishes the relationship between them.

7. Can water and anti-caking agents affect the amount of peptide in the powder?

Yes. Water and counterions increase the total mass of the sample and therefore may reduce the mass fraction attributable to the target peptide itself if the result is reported for the material in the form obtained. Studies of sublactin and peptide counterions show why these components may require separate labeling.[2][9] Their presence does not necessarily indicate synthesis error or contamination, because the counterion may be part of the intended salt form. Therefore, key questions concern the type of molecular form being analyzed, the quantities of individual components, and the basis of the calculation used.

8. Is anti-jon the same as peptide contamination?

Not always. The anti-jon can be an expected component of peptide salt, whereas the sequence-related contamination is another peptide molecule. These are therefore different analytical categories, although both can affect the total composition of the sample. In studies comparing peptide anti-jons, differences in membrane permeation experiments have been observed depending on the sequence and salt form.[9] If the anti-jon is relevant to the material’s characteristics, its type and quantity should be reported separately, rather than inferred from the chromatographic purity of the main peptide peak.

9. Can conventional mass spectrometry distinguish between D- and L-amino acid variants?

The measurement of the mass of the entire molecule itself usually does not allow distinguishing structures with identical elemental composition, which differ only in stereochemistry. Therefore, variants D and its corresponding variant L may have identical molecular mass. If the question concerns spatial configuration, a corresponding chiral or stereochemical analysis is required. In the study of the spatial configuration of peptide purity, a specially designed process was used that takes into account the stereochemical changes occurring during sample preparation. [6] The problem is of a structural nature, therefore, simply increasing the accuracy of the mass measurement alone does not resolve this ambiguity.

10. Why is the party number on the COA important?

The batch or series number helps to link analytical results to the specific material that was tested. Results obtained for one batch cannot be automatically transferred to another batch or to each container prepared at a different time. Studies involving certified reference materials assess such properties as uniformity and stability, rather than assuming them based on a single measurement.[8] Therefore, a consistent batch number is an important element of traceability, although in itself it does not prove the correctness of sampling, storage, handling of the material, or all other elements of the control system.

11. Does „not detected” mean zero?

No. „Not detected” means that the method used did not detect the analyte under the specified conditions and within its sensitivity limits. This does not imply that the actual amount is exactly zero. A result below the detection limit or limit of quantification should be interpreted together with the corresponding limit value, unit, sample size, and analytical conditions. Endotoxin testing guidelines also discuss the significance of method sensitivity and possible interference.[11] Therefore, a scientifically justified conclusion should refer to what the method could have detected, rather than an absolute claim of complete absence of the substance.

12. What does the measurement uncertainty in the report mean?

The measurement uncertainty describes the dispersion of values that can be reasonably associated with the obtained result according to the assessment applied. It does not mean that the laboratory considers its result to be erroneous. It rather indicates the limits of the numerical certainty of a given estimate. In studies of reference materials for assigning values, uncertainties related to the measurement itself, homogeneity and stability can be taken into account, among others. [8] Therefore, uncertainty should be interpreted together with its definition, units, the applied level of coverage, and the precisely defined measured quantity.

13. Does the test by an independent laboratory confirm that all the containers contain the same material?

No. An independent study provides evidence regarding the samples or samples actually submitted for analysis. The ability to transfer these results to each container depends, among other things, on the method of sampling, the traceability of the batch, the homogeneity of the material, the filling process, storage, and other control elements. In developing certified reference materials, homogeneity is evaluated rather than simply assumed based on a single measurement. [8] The laboratory's independence may increase the independence of the measurement itself, but it does not automatically prove the homogeneity of the entire batch.

14. Can the sterility result and the endotoxin result be used interchangeably?

No. The sterility test concerns live microorganisms, whereas the bacterial endotoxin test concerns specific bacterial components that may remain in the material even in the absence of live bacteria. These are therefore two separate analytical questions. Interference from the sample matrix may additionally affect the determination of endotoxins, therefore the appropriate adaptation of the method is important. [11] Neither sterility nor the endotoxin level can be inferred from the routine purity of the HPLC. If these tests have been performed, they should be reported separately.

15. Does chemical purity confirm biological activity?

No. Chemical analysis and functional testing provide other types of information. A peptide may have an expected molecular mass and high chromatographic purity without proven activity in a specific biological test. In the BPTI study, structural and biophysical characterization was combined with a trypsin inhibition assay, rather than assuming that one analytical result is sufficient for all conclusions. [12] Functional evidence is also specific to the molecule under study, the experimental system, and the specific test. Therefore, high-purity samples should not be considered functionally confirmed if the relevant activity has not actually been investigated.

16. Does COA confirm that the peptide will remain stable indefinitely?

No. The COA describes the sample at the time or at the stage of the study indicated in the report. During subsequent storage, the peptide may undergo degradation, oxidation, hydrolysis, aggregation, absorption of moisture, and other changes depending on its properties and the environment. Studies of exenatide degradation have shown the importance of methods that indicate stability, which are able to separate degradation products from the parent molecule. [13] Stress studies do not, however, automatically determine the storage time under normal conditions. Any claim regarding shelf life requires appropriate stability data obtained under specific conditions and time.

17. What should be checked first when comparing two COA peptides?

First, it is worth comparing the identity of the samples, batch numbers, analytical methods used, units, and the basis of measurement, rather than immediately aggregating the highest percentage values visible in the documents. Two reports can present a value defined as „purity,” while measuring other properties or having a different ability to separate individual classes of impurities. Studies using orthogonal chromatographic methods and mass balance determination show the importance of these differences. [2] [3] Without establishing whether these two numbers refer to the same size and with sufficiently comparable methods, a direct comparison of percentages can be misleading.

18. Does 99% purity HPLC mean 99% peptide content?

No. A result of 99% HPLC most often means that the main peak has been assigned 99% responses from the chromatographic detector. A result of 99% peptide content would require quantitative measurement showing that the target peptide represents 99% of the appropriate amount of sample based on a clearly defined basis. Water, counterions, solvent residues, and other components can increase the physical mass of the sample without proportionally affecting the RP-HPLC-UV surface area percentage.[2] Therefore, both values can vary significantly, although neither measurement needs to be erroneous.

19. Can a peptide pass the HPLC purity test and still have an incorrect stereochemistry?

Potentially, yes. Conventional RP-HPLC does not necessarily separate all stereochemical variants, especially when the molecules have very similar physicochemical properties. Similarly, a conventional mass measurement of the entire molecule does not allow the distinction of stereoisomers with identical elemental composition. Studies concerning the stereochemical purity of peptides show why specialized methods and precise control of sample preparation may be necessary.[6] Whether a specific HPLC method separates a given stereoisomer depends on the demonstrated selectivity of that method, and not merely on the fact that an HPLC analysis was performed.

20. Does the COA confirm that the entire batch is identical to the tested sample?

Not independently. The COA presents the results related to the sample and batch indicated in the analytical documentation. Extending these results to the entire batch requires an appropriate sampling system, consistency of the process, uniformity, traceability, and other production controls. Developing reference materials shows why uniformity is assessed as a separate property rather than simply assumed. [8] The COA can therefore provide important analytical evidence regarding the batch, but the strength of the conclusions regarding the entire material depends on a broader quality system linking the sample under investigation with the batch represented by the report.

Restrictions on the interpretation of peptide purity and COA

When interpreting peptide analytical reports, several limitations should be kept in mind.

First, purity depends on the method. The RP-HPLC-UV result describes what a specific chromatographic system and detector measured. Another separation mechanism may reveal components that the first method did not separate.[1][3]

Second, the chromatographic surface area and the physical mass have different measurement bases. Water, counterions, residues of other substances, and differences in the detector response can cause the percentage of HPLC surface area to not correspond to the mass fraction of the target peptide.[2]

Thirdly, the expected molecular mass provides strong but incomplete evidence of identity. Isomers, positional variants, and stereochemical differences may require the use of additional structural methods. [4] [5] [6]

Fourthly, analytical methods alone can cause artifacts. Sample preparation, hydrolysis, derivatization, storage, and the way the material is handled can alter the analyte or the characteristic being studied.[6]

Fifthly, a tested sample is not automatically equivalent to every container or the same material at a later time. Homogeneity, batch identity, and stability are separate issues. [8]

Sixthly, chemical tests do not answer microbiological questions. HPLC and mass spectrometry do not confirm sterility or endotoxin levels. [10] [11]

Seventhly, the chemical characterization does not confirm biological activity or clinical safety. These are separate issues that require other types of evidence. [12]

Finally, the result obtained at one point does not confirm long-term stability. Evaluation of stability requires relevant data obtained over time and under specific conditions. [13]

These limitations do not mean that COA is of little value.

They explain, however, how it should be properly interpreted.

Summary

The peptide analysis certificate is most useful when we treat it as a collection of separate analytical measurements, rather than as a single quality assessment.

The purity value given at the beginning is only the beginning of the interpretation.

A result such as 99% RP-HPLC surfaces describes the distribution of the detector response in a specific chromatographic method. It does not automatically mean that the 99% physical mass of powder constitutes the target peptide.[1][2]

Mass spectrometry can provide strong evidence regarding the molecular mass and some structural characteristics, but the consistency of the entire molecule’s mass with the expected value does not independently confirm all aspects of the sequence, the terminal structure, stereochemistry, or molecular connectivity.[4][5][6]

The content of the peptide requires its own quantitative basis.

Water, antisolvent, residues of other substances and related peptide impurities can affect the relationship between the mass of the charged material and the actual amount of the target peptide.[2][7][9]

Therefore, when reading the COA, it is worth asking several separate questions:

Which sample was tested?

Which party is it about?

What analytical method was used?

What exactly does the reported percentage value mean?

What units and what calculation basis were used?

Was the molecular identity tested independently of the chromatographic purity?

Has the actual content of the peptide been determined?

Were water or antidotes measured in the appropriate cases?

Is the stereochemistry evaluated if it is relevant to the given molecule?

Which properties have not been studied?

It is particularly important to distinguish between „not detected” and „not studied”.

The absence of a result should not be interpreted as proof of a positive test result.

Similarly, a study conducted by an independent laboratory can be a valuable additional source of information, but the mere designation „third-party tested” does not describe the method of sampling, its relationship to the batch, the scope of the analysis, uniformity, or whether the result can be transferred to any container.

Routine chemical methods also have clearly defined limits.

The HPLC does not confirm sterility.

The purity of HPLC does not confirm the level of endotoxins.

Chemical purity does not confirm biological activity.

The COA does not constitute an assessment of clinical safety.

One analytical result obtained at a given time does not confirm unlimited stability. [10] [11] [12] [13]

The most scientifically justified interpretation is therefore always as broad as the available evidence allows – and not broader.

A well-documented analytical report can provide strong information about the identity, composition, and quantity of the material contained in the sample being examined.

Its value increases when the methods used, the measurement basis, the material's identity, and the limitations of individual tests are clearly described.

The most important principle is simple:

Identity, purity, quantity, stereochemistry, microbiological status, biological activity, stability and safety are different questions. A reliable analysis certificate helps answer questions that have actually been investigated – it should not be used to suggest answers to issues that have not been investigated.

Disclaimer

The article is purely educational and scientifically informative in nature. It explains the general principles of analytical chemistry of peptides, the interpretation of analysis certificates (COA), HPLC chromatography, mass spectrometry, peptide content determination, contamination assessment, material identity, and related research methods. It does not constitute medical advice, product recommendations, safety assessment, production authorization, or confirmation that any peptide or material containing a peptide is suitable for human use. Analytical results should always be interpreted in relation to the specific sample, batch, method used, measurement basis, specifications, the analytical problem being investigated, and the limitations of the test performed.

References

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2. Chen, W., Yan, J., Yang, M., Wang, M., Zhou, J., Li, F., & Li, L. (2026). SI-traceable purity assignment for peptide sublancin using mass balance approach and isotope dilution mass spectrometry based on amino acid analysis. Journal of Chromatography B, 1277, Article 125038. https://doi.org/10.1016/j.jchromb.2026.125038
3. Yoshida, K., Kato, S., Nagai, K., Shimamoto, S., Onishi, T., & Ohnishi, A. (2025). Impurity profiling of synthetic cyclic peptides based on orthogonality between hydrophilic-interaction and reversed-phase liquid chromatography. Journal of Chromatography A, 1745, Article 465748. https://doi.org/10.1016/j.chroma.2025.465748
4. Mathews, W. R., Runge, T. A., Haroldsen, P. E., & Gaskell, S. J. (1989). Characterization of impurities in a synthetic renin substrate peptide by fast-atom bombardment mass spectrometry and hybrid tandem mass spectrometry. Rapid Communications in Mass Spectrometry, 3(9), 314–319. https://doi.org/10.1002/rcm.1290030912
5. Kuyama, H., Nakajima, C., Nakazawa, T., Nishimura, O., & Tsunasawa, S. (2009). A new approach for detecting C-terminal amidation of proteins and peptides by mass spectrometry in conjunction with chemical derivatization. Proteomics, 9(16), 4063–4070. https://doi.org/10.1002/pmic.200900267
6. Goodlett, D. R., Abuaf, P. A., Savage, P. A., Kowalski, K. A., Mukherjee, T. K., Tolan, J. W., Corkum, N., Goldstein, G., & Crowther, J. B. (1995). Determination of peptide chiral purity: Hydrolysis in deuterated acid, derivatization with Marfey’s reagent and analysis using high-performance liquid chromatography-electrospray ionization-mass spectrometry. Journal of Chromatography A, 707(2), 233–244. https://doi.org/10.1016/0021-9673(95)00352-n
7. Li, M., Josephs, R. D., Daireaux, A., Choteau, T., Westwood, S., Martos, G., Wielgosz, R. I., & Li, H. (2021). Structural-related peptide impurity identification and accurate quantification for synthetic oxytocin by liquid chromatography-high-resolution mass spectrometry. Analytical and Bioanalytical Chemistry, 413(7), 1861–1870. https://doi.org/10.1007/s00216-021-03154-5
8. Xiao, P., Chen, J., Wu, P., Zhang, W., Sun, Z., Ma, J., & Li, H. (2024). Development of an SI-traceable N-terminal pro-B-type natriuretic peptide certified reference material using structure-based impurity-corrected isotope dilution mass spectrometry approaches. Analytical and Bioanalytical Chemistry, 416(14), 3447–3458. https://doi.org/10.1007/s00216-024-05295-9
9. Erckes, V., Streuli, A., Chamera Rendueles, L., Krämer, S. D., & Steuer, C. (2025). Towards a consensus for the analysis and exchange of TFA as a counterion in synthetic peptides and its influence on membrane permeation. Pharmaceuticals, 18(8), Article 1163. https://doi.org/10.3390/ph18081163
10. U.S. Food and Drug Administration. (2004). Sterile drug products produced by aseptic processing—Current good manufacturing practice: Guidance for industry. Official guidance
11. U.S. Food and Drug Administration. (2026). Pyrogen and endotoxin testing: Questions and answers—Guidance for industry. Official guidance
12. Ferrer, M., Woodward, C., & Barany, G. (1992). Solid-phase synthesis of bovine pancreatic trypsin inhibitor (BPTI) and two analogues. A chemical approach for evaluating the role of disulfide bridges in protein folding and stability. International Journal of Peptide and Protein Research, 40(3–4), 194–207. PubMed record
13. Badgujar, D., Maskar, T., Paritala, S. T., & Sharma, N. (2024). Development and validation of a stability-indicating assay method and identification of force degradation products of glucagon-like peptide-1 synthetic analog Exenatide using liquid chromatography coupled with Orbitrap mass spectrometer. European Journal of Mass Spectrometry, 30(3–4), 171–186. https://doi.org/10.1177/14690667241262935