Analytical reports for research peptides almost always carry two headline results: an identity result and a purity percentage. They are easy to read as a single verdict on quality, but they are produced by different instruments and they answer different questions. Neither one can stand in for the other.

Identity asks which molecule is in the vial. Purity asks how much of what is in the vial is that molecule. A sample can pass one and fail the other, and understanding the difference is most of what it takes to read an analytical report accurately.

Identity: which molecule is this?

Identity testing compares a measured property of the sample against the value predicted from the molecular formula of the named compound. For peptides the usual method is mass spectrometry, which measures mass-to-charge ratio and lets an analyst calculate the molecular mass of the species present.

The report states an expected or theoretical mass, an observed or found mass, and a conclusion such as "conforms" or "consistent with". A match means the measured mass agrees with the mass the named compound should have, within the tolerance of the instrument. What mass spectrometry is used for covers the method in more detail.

What identity testing does not tell you is proportion. A mass measurement can confirm that the expected molecule is present without describing how much of the material in the vial it accounts for. Mass agreement is a statement about the species detected, not about the composition of the sample as a whole.

Purity: how much of the sample is that molecule?

Purity for peptides is normally reported from reversed-phase high-performance liquid chromatography. The sample is separated into its components as it passes through a column, a detector records each component as a peak, and the area under the main peak is divided by the total area of all peaks. The result is reported as a percentage. What HPLC testing measures goes through the method and the parameters that shape the number.

The figure is an area percentage at a stated detection wavelength, most often 214 nm or 220 nm. That detail matters, because the number describes relative detector response among the things the detector could see, not the mass fraction of the contents of the vial.

Why one does not substitute for the other

The two results fail independently, which is the practical reason both appear on a report.

  • A sample can contain the correct compound and still contain a substantial amount of other material. Identity confirms the molecule; it does not bound the rest.
  • A sample can produce a single sharp chromatographic peak that is not the named compound. Chromatography separates by how strongly components interact with the column, so a different molecule with similar behaviour can occupy the same position in the run. A high area percentage describes the peak, not its identity.
  • Related substances such as deletion or truncation sequences can be chemically similar enough to elute close to the main peak, and differ in mass. Whether they are resolved depends on the separation method used.

Read together, the two results say that the expected molecule was detected and that the main chromatographic peak accounted for a stated share of the detector response. That is the whole of the claim. Neither result, alone or combined, describes how a compound behaves or establishes that it is suitable for any particular use.

What a purity percentage does not describe

A common misreading is to read an HPLC purity figure as the percentage of the vial contents by weight. It is not. The percentage covers only material that the detector responded to under the conditions of that run.

  • Residual water, salts and counterions from synthesis and purification typically do not absorb at the detection wavelength, so they do not appear as peaks.
  • The share of vial mass that is peptide is a separate measurement, often called net peptide content, determined by methods such as amino acid analysis or nitrogen determination. It is a different number from chromatographic purity and is not always reported.
  • Residual moisture is sometimes reported separately from a Karl Fischer titration. It is a property of the material as received, not part of the purity calculation.

Reading the two results together

A useful habit is to read each result as a sentence with an explicit scope. The identity line says: the mass observed for the detected species agrees with the mass expected for this compound. The purity line says: under this method, at this wavelength, the main peak accounted for this share of total peak area.

Stated that way, the limits are visible. Both sentences are about a specific sample analysed under specific conditions on a specific date. Neither extends to a product line, to a different batch, or to any question the instruments were not measuring.

Where these results appear

Both results are reported on a certificate of analysis, alongside the batch they belong to and the methods used to produce them. How to read a certificate of analysis walks through the rest of the document, and published certificates for compounds in our catalogue are on the certificates of analysis page.

All compounds and research materials supplied by PepX Research are for laboratory and research use only. They are not for human consumption.