I keep seeing "Identity confirmed by ESI-MS" on COAs and realized most people have no idea what that actually means or why it matters. So here's my attempt at explaining mass spectrometry for peptide identification in plain language. What is ESI-MS? ESI-MS = Electrospray Ionization Mass Spectrometry. It's a technique that measures the molecular weight of a compound with extreme precision. For peptide identification, it's the gold standard — HPLC tells you purity but not identity. Mass spec tells you identity. How it works (simplified): 1. Your sample is dissolved in a solvent 2. The solution is sprayed through a tiny needle with a high voltage applied (electrospray) 3. This creates a fine mist of charged droplets 4. The solvent evaporates, leaving behind charged molecules (ions) 5. These ions enter the mass analyzer, which separates them by their mass-to-charge ratio (m/z) 6. A detector records the signal at each m/z value 7. The resulting spectrum shows peaks at specific m/z values For semaglutide specifically: - Molecular weight: 4113.58 Da - In ESI-MS, you don't see a single peak at 4113.58. Instead, the molecule picks up multiple charges, producing a series of peaks: - [M+3H]³⁺ at m/z ≈ 1372.2 (molecular weight ÷ 3 charges + 1 for each proton) - [M+4H]⁴⁺ at m/z ≈ 1029.4 - [M+5H]⁵⁺ at m/z ≈ 823.7 - [M+6H]⁶⁺ at m/z ≈ 686.6 - Software "deconvolutes" these multiply charged peaks to calculate the actual molecular weight Why does this matter? If someone sells you "semaglutide" but it's actually a different peptide — say, tirzepatide (MW: 4813.45 Da) or a degraded fragment — the mass spec will immediately reveal it. The molecular weight is like a fingerprint. It's nearly impossible to fake. Limitations: - Mass spec confirms the molecular weight but doesn't tell you the sequence directly (for that, you need tandem MS or MS/MS) - Two different peptides could theoretically have similar molecular weights, but this is rare for commercially available peptides - It doesn't measure quantity (that's what HPLC is for) Any questions? This is one of those topics that sounds intimidating but is conceptually straightforward.

Thank you for this. I have a few follow-up questions: 1. On the COAs I've seen, the "observed mass" is sometimes reported as something like 4113.4 Da when the theoretical is 4113.58 Da. How close does the match need to be for it to count as confirmed? 2. What's the difference between ESI-MS and MALDI-TOF? I've seen both mentioned on different COAs. 3. Could a vendor substitute a cheaper peptide of similar molecular weight and fool the mass spec? Like — is there a peptide that weighs 4113 ± 2 Da that ISN'T semaglutide?

Great questions: 1. Mass accuracy: The acceptable tolerance depends on the instrument. For a standard quadrupole or ion trap mass spec (common in testing labs), ±2 Da is typical for a ~4000 Da peptide. That's about ±0.05% accuracy. So an observed mass of 4113.4 Da for theoretical 4113.58 Da is well within tolerance (difference of 0.18 Da). For a high-resolution instrument (like a Q-TOF or Orbitrap), accuracy is much tighter — ±0.01 Da or better. These instruments can distinguish between peptides that differ by less than 1 Da, but they're expensive and most third-party labs use standard-resolution instruments for routine testing. 2. ESI-MS vs MALDI-TOF: Both measure molecular weight, but the ionization method differs: - ESI (Electrospray): gentle ionization, works great for peptides and proteins. Produces multiply charged ions. Can be coupled directly to HPLC for LC-MS analysis (very powerful combo). - MALDI (Matrix-Assisted Laser Desorption/Ionization): the sample is mixed with a matrix compound and hit with a laser. Produces mostly singly charged ions. Very fast — you can analyze a sample in seconds. Good for screening. For semaglutide ID, both work. ESI is more commonly used in quality testing labs because it can be integrated with HPLC in a single workflow (LC-MS). MALDI is more common in research labs for quick molecular weight confirmation. 3. Could a cheaper peptide fool mass spec? Theoretically possible but practically very unlikely. You'd need to find a peptide with: - Molecular weight within ±2 Da of 4113.58 - Available commercially at a lower price - Visually similar (white lyophilized powder) The peptide sequence determines the molecular weight, and the probability of two random sequences producing the same MW decreases dramatically with peptide length. Semaglutide is a 31-amino acid peptide with a C18 fatty acid modification — finding a cheap substitute with the same MW would be like finding a needle in a molecular haystack. The more realistic fraud scenario is selling degraded or low-purity semaglutide, not substituting a different peptide. And that's what HPLC purity testing catches.

I want to add some practical context about what mass spec CAN'T tell you: 1. Stereochemistry: Mass spec measures molecular weight, which is identical for L-amino acid peptides and D-amino acid peptides. A peptide made with the wrong stereoisomer of an amino acid would have the correct mass but might not be biologically active. Detecting this requires chiral chromatography or enzymatic digestion analysis — tests that are almost never performed on community-submitted samples. 2. Post-translational modifications vs. synthesis artifacts: Semaglutide has a specific modification (the C18 fatty diacid on lysine-26 via a linker). Mass spec confirms the total mass is correct, which implies the modification is present. But it doesn't tell you WHERE the modification is attached. If it migrated to a different lysine during synthesis (there are multiple lysines in the sequence), the mass would be the same but the bioactivity might be affected. This requires peptide mapping (enzymatic digestion + LC-MS/MS) to verify — again, rarely done for community testing. 3. Aggregation state: Semaglutide can form oligomers (dimers, trimers) under certain conditions. These aggregates can be immunogenic. Mass spec under denaturing conditions (which ESI typically uses) will break up aggregates, so you'd see the monomer mass and miss the aggregation. Size exclusion chromatography (SEC) is needed to detect aggregates. Am I being paranoid? Maybe. But these are real quality attributes that matter for a pharmaceutical product, and they're completely invisible to the typical "HPLC purity + ESI-MS identity" testing panel that community COAs provide. It doesn't mean the testing is useless — it's extremely valuable for confirming identity and basic purity. But it's not the same as the full characterization that a pharmaceutical company performs.

Frank, you're not wrong, but let's contextualize this for the average person reading this thread: The tests that community members can reasonably afford and access: - HPLC purity: ~$50-80 - Content/potency by HPLC: ~$50-80 - ESI-MS identity: ~$40-60 - Full panel (purity + content + identity): $120-$150 The tests you're describing: - Chiral purity: $300-$500 - Peptide mapping by LC-MS/MS: $500-$1000+ - Size exclusion chromatography: $200-$400 - Full pharmaceutical characterization: $5,000-$10,000+ Nobody in this community is going to spend $5,000 to characterize a $100 vial of sema. And honestly, for the purpose of "is this actually semaglutide at the right purity and potency?" the standard panel (HPLC + MS) answers the question 95% of the time. The risk of stereochemical errors, modification migration, or aggregation exists, but it's low for reputable peptide manufacturers. These issues are more common with poorly optimized synthesis processes, and they'd likely manifest as reduced biological activity — which users would notice clinically. So yes, be aware of the limitations. But don't let perfect testing be the enemy of good testing. The standard COA panel is a massive improvement over no testing at all.