Peptide Purity: What HPLC and Mass Spectrometry Show

A peptide certificate of analysis can look authoritative. There may be a purity percentage, a chromatogram, a mass spectrum and a batch number. The problem is that those pieces of information answer different questions. A single number such as 99% does not automatically mean that 99% of everything in the vial is the intended peptide, and a mass spectrum that shows the expected molecular mass does not automatically prove that no important impurity is present.

Pharmaceutical peptide testing treats identity, purity, assay and impurities as separate parts of a larger quality picture. The European Medicines Agency now recommends at least two orthogonal methods for identification of synthetic peptide drug substances and specifically warns about co eluting impurities that can be difficult to separate ¹.

That is a useful starting point for understanding what the common tests on a peptide COA really mean.

What does peptide purity actually mean?

Purity sounds like one simple property, but laboratories can measure different kinds of purity.

A chromatographic purity result asks how much of the detected signal belongs to the main peptide peak compared with other detected peaks under a defined method. An assay asks how much target peptide is actually present compared with a reference standard. Other tests may measure water, residual solvents, counterions, inorganic material or specific process related impurities.

Those measurements are related, but they are not interchangeable.

A 2023 paper describing how USP establishes synthetic peptide reference standards illustrates the difference clearly. HPLC impurities were measured as a percentage of total detected chromatographic area, while counterions, residual solvents and other components were measured separately on a weight basis before the final peptide content was assigned ².

That means a high HPLC area percentage can coexist with water, salts, counterions or other material that does not appear as a peptide related peak in that chromatogram.

What HPLC tells you

HPLC stands for high performance liquid chromatography. In peptide analysis, a common version is reverse phase HPLC.

The sample travels through a chromatography column while the composition of the mobile phase changes. Different molecules interact differently with the column and leave it at different times. A detector records the material coming out, producing the familiar chromatogram with peaks across the page.

For a reasonably well designed purity method, the intended peptide should appear as the dominant peak and related impurities should separate into other peaks.

The EMA synthetic peptide guideline says methods used to control peptide related impurities should be able to detect and quantify relevant impurities, and it specifically highlights the risk of impurities co eluting with the intended peptide ¹.

So HPLC is not simply a machine that reveals a universal purity percentage. The answer depends on the method.

Why an HPLC percentage is method dependent

Change the column chemistry, mobile phase, gradient, temperature, detector wavelength or integration rules and the separation can change. An impurity that separates well under one method may overlap another peak under a different method.

That is why regulators focus heavily on validation.

ICH Q2(R2), the international guideline for analytical procedure validation, says an analytical procedure should be demonstrated to be fit for its intended purpose. Depending on the test, validation considers properties such as specificity, accuracy, precision and reportable range ³.

For peptide related impurities, specificity matters because the method needs to distinguish the intended peptide from structurally similar material.

A screenshot of a chromatogram without the method, detector conditions and integration information is therefore much less informative than it appears.

Does 99% HPLC purity mean the vial is 99% peptide?

Not necessarily.

HPLC area purity is commonly calculated from the proportion of detected chromatographic area assigned to the main peak. It does not automatically measure every component by weight.

The USP reference standard work gives a practical example. Its peptide purity calculation separately accounted for chromatographic impurities, counterions, residual solvents and other contributions rather than treating the HPLC percentage as the final peptide content ².

This is the distinction between chromatographic purity and peptide content.

A material can produce a very clean peptide chromatogram while still containing water or a counterion associated with the peptide salt. Those components do not necessarily create ordinary peptide peaks in the chromatogram.

That does not make the HPLC result false. It means the number has to be interpreted as the result of that analytical test, not as a complete chemical inventory of the vial.

What mass spectrometry tells you

Mass spectrometry answers a different question.

The technique measures ions according to their mass to charge ratio. For a synthetic peptide with a known expected molecular mass, a mass spectrum can show whether the material contains a species whose measured mass agrees with the intended peptide.

FDA chemistry guidance lists spectroscopy and other identity techniques separately from HPLC chromatograms used to support purity and impurity control ⁴.

Mass spectrometry is particularly valuable because peptide synthesis can produce deletion sequences, additions, oxidation products and other related species whose masses differ from the intended product.

Modern liquid chromatography coupled to mass spectrometry can go further by separating components before they enter the mass spectrometer and helping identify structurally related peptide impurities. This is why LC MS is widely used in pharmaceutical peptide characterisation ⁵.

Does the correct molecular mass prove the peptide is correct?

It is strong evidence for identity, but it is not the whole identity package.

Different structures can sometimes have the same or very similar overall mass. Isomers, stereochemical differences and some sequence questions can require additional analytical information.

The EMA therefore recommends at least two orthogonal methods for peptide identification. Its list includes mass, relative retention time, LC MS, peptide mapping, bioactivity, amino acid analysis and NMR, with the combination chosen to confirm the sequence unambiguously ¹.

A mass spectrum showing the expected mass is useful. Treating that one spectrum as complete proof of identity, purity and structural correctness goes further than the test supports.

Why HPLC and mass spectrometry work better together

The two techniques solve complementary problems.

HPLC separates components. Mass spectrometry helps identify them.

Imagine a chromatogram with one main peak and several smaller peaks. HPLC can tell you that those components behave differently during separation, but the chromatogram alone may not identify each impurity. Adding mass information can help determine whether a smaller peak represents a truncated peptide, an oxidised species or another related molecule.

The reverse can also be true. A mass spectrum may show a strong signal at the expected mass, while chromatography reveals that other components are also present.

This is the basic reason regulators favour orthogonal testing rather than one impressive looking result.

What should a certificate of analysis contain?

For pharmaceutical active ingredients, ICH Q7 gives a useful description of what a certificate of analysis should contain. It says the certificate should list each test performed, the relevant acceptance limits and the numerical results when the test produces a number. It should also identify the batch and be authorised by the quality unit ⁶.

That does not mean every document called a COA on the internet was produced under pharmaceutical GMP. It gives a useful benchmark for understanding what the document is supposed to communicate.

A meaningful COA is linked to a specific batch, identifies what tests were actually run, gives the method or method reference where appropriate, reports acceptance criteria and provides the actual result.

A purity percentage with no clear batch link, method context or underlying analytical data is weaker evidence.

A COA only answers the tests it contains

This point is easy to miss.

If a certificate reports HPLC purity and mass spectrometry, those tests address chromatographic purity and chemical identity. They do not automatically answer unrelated questions that were never tested.

For example, neither an ordinary HPLC purity chromatogram nor a mass spectrum is itself a sterility test. The same applies to bacterial endotoxin, residual solvents, water content and particulate contamination. Those require their own suitable analytical procedures when they are relevant.

ICH Q7 makes the same principle clear in another way: a certificate should list each test performed and the corresponding result ⁶.

If a test is absent, the certificate does not prove that attribute.

Why synthetic peptides create difficult impurities

Peptide synthesis builds a chain through repeated chemical steps. Each cycle creates opportunities for incomplete coupling, deletion, side reactions and stereochemical changes. Later purification and storage can introduce additional degradation products.

Some peptide related impurities are chemically very similar to the intended sequence. That similarity is exactly what makes analytical separation difficult.

The EMA notes that synthetic peptide impurity methods have to consider structural complexity and the risk of co eluting impurities. It also says additional independent methods may be required when one analytical method cannot adequately separate all relevant impurities ¹.

This is why a clean looking chromatogram should not be read as proof that every possible impurity has been excluded.

For a deeper look at how those impurities arise, see How Peptides Are Actually Made.

What does pharmaceutical quality testing look like?

It looks less like one purity number and more like a collection of measurements.

FDA guidance for investigational drug substances expects information supporting identity, strength, quality and purity, including analytical methods and acceptance limits ⁴.

The EMA synthetic peptide guideline separates identification, assay and impurity control, and it recommends orthogonal methods for identity. ICH Q2(R2) then provides the framework for showing that each analytical procedure is suitable for what it claims to measure.

The lesson is simple: quality comes from a testing system, not from one attractive chromatogram.

That same evidence hierarchy applies elsewhere on BioRx. The guide to what peptides are explains the molecules themselves, while the BPC-157 evidence review shows why the type and quality of evidence matters as much as the headline claim.

Frequently asked questions

Is 99% HPLC purity good?

A high HPLC area percentage can indicate that the main detected chromatographic peak dominates under that method. It does not by itself establish total peptide content, identity, sterility or the absence of every relevant impurity. The method and the other tests matter.

Can mass spectrometry tell if a peptide is real?

Mass spectrometry can provide strong evidence that a sample contains a molecule with the expected mass, and more detailed MS methods can provide additional structural information. Regulators still recommend complementary identification methods for synthetic peptides rather than relying on one mass result alone.

What is the difference between purity and assay?

Purity generally describes the relationship between the intended material and impurities under defined tests. Assay measures the amount or strength of the intended substance against an appropriate reference. A high chromatographic purity result and the actual mass content of peptide in a material are not necessarily the same number.

Does a COA prove a peptide is sterile?

Only if appropriate sterility testing was performed and reported. HPLC purity and mass spectrometry are chemical analytical tests, not substitutes for microbiological tests.

Why can two laboratories report different purity results?

Analytical results depend on the method. Different columns, gradients, detectors, integration rules and validation approaches can change how well related impurities separate and how the chromatogram is interpreted. A valid method should be demonstrated to be fit for its intended purpose under recognised validation principles.

References

Studies marked "View study in BioRx" sit in the clinical studies index with their design and evidence grade.

Peptide Purity Tests Explained: HPLC, Mass Spectrometry and COAs