DOCUMENTED BY BATCHTwo analytical methods sit at the centre of peptide quality control, and they answer two different questions. High-performance liquid chromatography (HPLC) answers “how pure is this material?” Mass spectrometry, run as LC-MS or high-resolution mass spectrometry (HRMS), answers “is this the correct molecule?” A rigorous Certificate of Analysis reports both, because a single number from either method alone leaves the other question open. This article explains what each technique measures, why parts-per-million mass error matters, and how the two combine on a batch record.
What HPLC measures: purity by separation
HPLC pushes a dissolved sample through a packed column under high pressure. Components travel at different rates according to how strongly they interact with the stationary phase, so the target peptide and any related impurities, truncated sequences, or process residues elute at different retention times. A detector, usually UV at 214 nm where the peptide bond absorbs, records each species as a peak.
Purity is calculated as the area of the main peak divided by the total area of all peaks, expressed as a percentage. A result such as 99.2% area purity means the target species accounts for 99.2% of the detected material. What HPLC does not do is tell you the mass or the sequence of that main peak. Two different peptides can share a retention time, so a clean chromatogram establishes homogeneity, not correctness. That limitation is precisely why a second, orthogonal method is required for compounds like retatrutide and tirzepatide.
What LC-MS and HRMS measure: identity and mass accuracy
Mass spectrometry ionises the molecule and measures its mass-to-charge ratio. For a peptide of known composition, the theoretical monoisotopic mass is fixed by its molecular formula, so comparing the observed mass to the calculated mass is a direct identity check. Coupled to a chromatographic front end (LC-MS), the instrument separates the sample first and then weighs each eluting component, linking retention time to molecular weight in one run.
High-resolution instruments such as Orbitrap and time-of-flight analysers resolve isotope patterns and report mass to several decimal places. Because peptides carry multiple charges under electrospray ionisation, the raw spectrum shows a charge-state envelope that is deconvoluted back to the neutral monoisotopic mass. When the measured mass matches the theoretical formula within a tight tolerance, identity is confirmed for that specific sequence.
Why parts-per-million mass error matters
Mass accuracy is the difference between the observed and theoretical mass, expressed in parts per million (ppm). The formula is (observed − theoretical) / theoretical × 10⁶. A sub-5 ppm result is typical of high-resolution instruments and represents a strong identity assignment; older unit-resolution instruments might report tens of ppm and cannot distinguish near-isobaric species. On a molecule the size of semaglutide (formula C₁₈₇H₂₉₁N₄₅O₅₉), a few ppm of error corresponds to a fraction of a dalton, which is why ppm, not absolute mass, is the meaningful figure of merit. A low ppm error narrows the set of formulas that could produce the observed peak, which is what makes HRMS a confirmatory identity method rather than an approximate one.
Why both methods are required
The two techniques are orthogonal: they fail in different ways, so together they close the gaps each leaves open. HPLC can flag an impurity that co-ionises poorly and would be invisible to MS; MS can catch a mislabelled or mis-synthesised sequence that co-elutes cleanly on HPLC and would pass a purity check unnoticed. This is why the accepted standard for peptide characterisation pairs an HPLC purity value with an LC-MS or HRMS identity confirmation, and why smaller research peptides such as bpc-157 (sequence GEPPPGKPADDAGLV, formula C₆₂H₉₈N₁₆O₂₂) are held to the same two-method expectation as the larger lipidated compounds.
How this appears on a Certificate of Analysis
A complete COA section reports, for identity: the theoretical mass, the observed mass, the charge states detected, the ppm accuracy, and the instrument type, usually with the annotated spectrum attached. For purity, it reports the HPLC area percentage, the column and gradient conditions, the detection wavelength, and the chromatogram. Reading a COA is a matter of checking that the observed mass matches the formula within the stated ppm window and that the main HPLC peak dominates the total area. livvmore documents both figures per batch on its quality and testing page, alongside the identity work done for the GLP research peptides collection.
Testing at the USC Alfred E. Mann core
livvmore’s differentiator is that identity and purity are verified at the USC Alfred E. Mann Multi-Omics Mass Spectrometry Core, a university laboratory, using LC-MS and HRMS on a per-batch basis, with the resulting COAs published. Routing characterisation through an independent academic mass-spectrometry facility means the reported mass, ppm accuracy, and purity are generated on instruments and by analysts outside the supply chain, and the underlying certificate is available for inspection rather than summarised as a single unsupported number.
For laboratory research use only. Not for human or veterinary use.
Frequently asked questions
What is the difference between HPLC and mass spectrometry in peptide testing?
HPLC (high-performance liquid chromatography) measures purity by separating a peptide from its impurities and reporting the main peak as a percentage of total peak area. Mass spectrometry, run as LC-MS or high-resolution mass spectrometry (HRMS), measures identity by comparing the observed molecular mass to the theoretical mass of the expected sequence. HPLC confirms homogeneity but not identity, so both methods are used together for a complete characterisation.
What does ppm mass error mean on a peptide Certificate of Analysis?
Parts-per-million (ppm) mass error is the difference between the observed mass and the theoretical mass of a peptide, calculated as (observed minus theoretical) divided by theoretical, times one million. It expresses how closely a mass-spectrometry measurement matches the compound's known molecular formula. A sub-5 ppm result is typical of high-resolution instruments such as Orbitrap and time-of-flight analysers and represents a confident identity assignment.
Can HPLC alone confirm a peptide's identity?
No. HPLC establishes purity by measuring peak area but cannot determine the mass or sequence of the main peak, and two different peptides can share a retention time. Identity confirmation requires mass spectrometry, which weighs the molecule and matches its observed mass to the theoretical mass of the expected formula. This is why a complete Certificate of Analysis pairs an HPLC purity value with an LC-MS or HRMS identity result.
What are the CAS number and molecular formula of retatrutide?
Retatrutide (house code GLP3-RT) has CAS number 2381089-83-2 and the molecular formula C221H342N46O68, with an average molecular weight of about 4731 g/mol. It is a synthetic 39-amino-acid peptide with the backbone conjugated to a C20 fatty diacid moiety at position 17. These identity parameters are what a high-resolution mass-spectrometry measurement is checked against on a batch certificate.
Where are livvmore peptides tested for identity and purity?
livvmore peptides are tested at the USC Alfred E. Mann Multi-Omics Mass Spectrometry Core, a university laboratory, using LC-MS and high-resolution mass spectrometry on a per-batch basis. Each batch's Certificate of Analysis reports the observed mass, ppm mass accuracy, charge states, and HPLC purity, and the certificates are published for inspection.