Peptide Testing Explained: HPLC, LC-MS and More

By Editorial team. Updated Oct 15, 2026. Awaiting editor sign-off

Lab reports for research peptides combine several different tests, and each answers a different question. A vendor can truthfully say “99% pure” while the vial holds the wrong amount, or the right molecule with a contamination problem. This guide explains what each common test measures, what it misses, and why a single test of a single vial is a data point rather than a verdict.

The tests at a glance

HPLC: purity

High-performance liquid chromatography separates the sample into components, and a detector records each one as a peak. Purity is the main peak’s share of the total, so “99%” means 99% of what the detector saw came from that peak. That leaves three blind spots:

• Quantity. A half-filled vial can still be 99% pure.
• Co-eluting impurities. Anything that leaves the column at the same time as the main peak is counted as part of it.
• Invisible contaminants. Material the detector doesn’t respond to never appears on the chromatogram.

Labs normally state the method alongside the result. One lab’s BPC-157 test, for example, lists HPLC as the method and reports finished vials as “mg per vial and peptide purity” [1].

Mass spectrometry: identity and contamination

Mass spectrometry weighs molecules. Coupled with chromatography (LC-MS), it confirms that the main peak has the expected mass. It can also reveal other peptides or degradation products. One lab sells an LC-MS screen that detects contamination with other peptides and degraded peptide. It notes that the screen only identifies peptides on its own list and doesn’t measure concentration unless that is ordered separately [1]. A purity number without an identity result hasn’t shown that the vial contains the peptide on the label.

Quantity and peptide content

Quantity is the measured amount of peptide in the vial, compared with the label claim. Peptide content is the share of the powder’s weight that is peptide at all. Freeze-dried peptides also carry residual moisture and counter-ions such as trifluoroacetate and acetate. Peptide content therefore typically falls around 70–90% [6]. Labs offer separate add-on tests for peptide content and for residual trifluoroacetic acid (TFA) [1].

These numbers can move independently, and real samples show it. In a 2024 study, researchers bought semaglutide from illegal online sellers and analyzed the three vials that arrived [7]. The measured content ran 28.56–38.69% above the label, while measured purity was only 7.7–14.37% against the 99% printed on the labels [7]. Neither number on its own would have described the product.

Endotoxin

Endotoxins are fragments of bacterial cell walls. They are a contamination concern for anything meant to be injected, and they survive even when no live bacteria remain. Labs usually measure them with an LAL assay and report EU per vial or EU per ml. Check the range the method covers: one lab states its standard test is accurate up to 10 EU per vial, estimates values above that, and recommends sending two vials [2]. In the 2024 semaglutide study, no live microorganisms were found, yet endotoxin was detected in every sample, at 2.1645 to 8.9511 EU/mg [7]. A clean microbial test is not a clean endotoxin test.

Microbial tests: counts versus sterility

Two different kinds of test get loosely called “sterility testing”:

• Microbial counts (TAMC/TYMC). These cultivation tests count total aerobic microbes and total yeasts and moulds. One lab sells this as “sterility testing (TAMC+TYMC)” with a pass/fail result, a 14-day average turnaround and a requirement for unopened vials [3].
• Pharmacopeial sterility testing. FDA guidance distinguishes microbial enumeration, which is for non-sterile products, from sterility testing. It says sterility testing should be conducted using USP General Chapter <71> [4].

Both have value. When a report says “sterile”, check which method was used.

Heavy metals

Heavy-metal screens usually cover arsenic, cadmium, lead and mercury [1]. The international drug-quality guideline on elemental impurities, ICH Q3D, puts these four in its highest-concern class [5]. Health Canada lists heavy metals among the contaminants that unauthorized injectable peptides may contain, alongside solvents, particles and microbes [8].

Impurities that are hard to see

Synthetic peptides can carry closely related by-products from manufacturing, known as peptide-related impurities, and peptide molecules can clump together (aggregate). In its notes on peptides it has reviewed for compounding, the FDA repeatedly flags immunogenicity risk from aggregation and peptide-related impurities, along with difficulty characterizing the active ingredient [9]. Routine purity tests can miss some of this. Specialized LC-MS work catches more, but it costs more.

Why one vial is not a verdict

• Batches vary. One vial describes one vial. Some labs sell variance testing across several vials from the same batch for exactly this reason [1].
• Samples change. Shipping, heat and storage can degrade a peptide after it leaves the vendor.
• Vendors choose what they submit. If a vendor knows a vial is going to a lab, it can pick which vial to send. That’s why blind purchases, made as an ordinary customer and only then sent for testing, carry more weight.
• Methods differ between labs. Results from different labs, or different methods, aren’t always comparable. A good report states its method.

How we use test results

Our own blind-purchase results, once published, will appear on our lab testing page with the lab, the method, the batch and a link to the full report. Meanwhile, third-party tested vendors lists which vendors publish batch COAs and which labs they name. Our methodology explains how we choose what to test. Vendor-supplied COAs are listed on vendor pages as the vendor’s claims, not as our findings. To read any single report critically, start with our guide to COAs.