HPLC vs Mass Spectrometry: How Peptide Purity Is Measured

When a seller states "99% purity" for a product, the actual truth behind that percentage rests with the method used. Two methods dominate the analytical world of peptides: reversed-phase high-performance liquid chromatography (RP-HPLC), and liquid chromatographic mass spectrometry (LC-MS). Each method checks for different parameters, and realizing this difference between the two methods allows you to read the Certificate of Analysis better than most buyers.

Two Main Methods Labs Use

Peptide purity testing relies on two complementary analytical techniques. HPLC measures how much of the sample is the target compound, while LC-MS confirms what that compound actually is. Most reputable labs use both, but many certificates of analysis only report one. Understanding each method helps you evaluate what a COA is really telling you.

HPLC - High Performance Liquid Chromatography

In RP-HPLC, separation happens based on the interaction between specific compounds in the nonpolar stationary phase (usually C18 silica) and the polar mobile phase (made up of water, acetonitrile, and a small amount of trifluoroacetic acid). The separation process results in a chromatogram where peaks appear in accordance with the speed of each component. The target peptide emerges as the most prominent peak, while impurities appear as smaller peaks in the chromatogram.

Purity can be determined using area normalization whereby the area of the primary peak is divided with the total areas of the peaks to give the percent value. If purity is measured as 98.5%, then it can be said that the sample peptide accounts for 98.5% of the ultraviolet active components as analyzed by the detector. As mentioned by 2023 article on USP peptide reference standards, optimized RP-HPLC is the standard analytical method for calculating peptide purity as well as impurity profile data for the material suitable for usage in biomedicine.

HPLC separates by hydrophobicity. If two compounds happen to have similar retention times, they will co-elute and appear as a single peak. The detector does not know whether that peak is one compound or two stacked on top of each other. HPLC also cannot identify what an impurity actually is. It tells you something is there, and roughly how much, but not its molecular structure. For a 15-amino-acid peptide like BPC-157, deletion sequences (missing one residue) may have similar enough hydrophobicity to overlap with the parent peak. This is the fundamental blind spot of HPLC-only testing.

LC-MS - Liquid Chromatography Mass Spectrometry

LC-MS performs the same chromatographic separation as HPLC while applying the output to a mass spectrometer. The mass spectrometer ionizes each component and records its mass to charge ratio (m/z). As a result, you are able to obtain two types of information from the process, namely, retention time derived from the chromatographic step, and molecular weight gained from the mass spectrometry process.

With LC-MS, impurities that HPLC was unable to detect can now be detected. Since impurities have a different molecular weight, their detection becomes possible with LC-MS. This technique confirms that a target peptide is indeed the correct peptide as it compares the mass observed with the mass of the peptide calculated based on its amino acid sequence. Using MS/MS fragmentation it is then possible to verify the peptide sequence.

What 99% Purity Actually Means

A certificate of analysis showing 99% purity means that when HPLC analysis was performed, the main peak accounted for 99% of the total peak area in the chromatogram. The remaining 1% consists of other UV-absorbing components that eluted at different retention times. This number does not tell you what those impurities are, whether the main peak is truly a single compound, or whether non-UV-absorbing contaminants are present.

For a peptide intended to be reconstituted and injected, knowing the molecular identity of what is in the vial matters as much as the percentage. LC-MS analysis confirms the presence of the correct peptide and verifies that deletion impurities and modification impurities are below detection limits. By using both methods together, a lab obtains substantially more information than by using either of them alone. A purity of 95% or higher is generally considered acceptable for peptides intended for research use, whereas reputable manufacturers are expected to provide products with a purity of at least 98%.

How to Read a Purity Result on a COA

Not all certificates of analysis are the same. Whenever you check a COA, ensure you look for specific fields. The lot number must correspond to the particular product that you received. The testing method should be stated. The purity value should indicate how it was calculated. The name of the laboratory and the date of testing should be visible. Third-party testing (a lab other than the manufacturer) carries more weight than self-testing because it eliminates conflict of interest.

Which Vendors Use Which Method

How vendors handle purity testing varies considerably. Some publish full dual-method COAs on every product page, while others provide no documentation at all. The table below summarizes what is publicly available from the vendors tracked on this site. This dual-method approach is consistent with how pharmaceutical reference standards are characterized, as described in the 2023 USP peptide standards article.

Real Peptides publishes COAs from Kovera Labs on each product page. These reports include both HPLC purity data and LC-MS mass confirmation. The HPLC result gives the percentage; the LC-MS result confirms the peptide identity and checks for mass-shifted impurities. PeptideGod does not provide a Certificate of Analysis on their product pages. There is no way to know what type of testing is involved, which laboratory did it, or the purity of the final product. This does not imply that the product has not been tested. It simply means that the documentation is not available for the customer to examine.