Definition: Mass spectrometry (MS) is a lab method that weighs molecules. It turns molecules into charged particles and measures their mass very precisely. For peptides it is the usual identity test: if the measured mass matches the expected mass of the molecule, that supports the vial holding what it claims.
Worked example: matching a mass
Checking tirzepatide
- The Mounjaro label gives tirzepatide a molecular weight of 4813.53 daltons (Da). A dalton is about the mass of one hydrogen atom.
- A mass spectrometry result near 4813.5 Da supports tirzepatide being present.
- A main signal far from that, say 3,000 Da, would suggest a different or broken molecule, even if HPLC showed one clean peak.
Why it is paired with HPLC
HPLC separates a mixture and says how much of it is one substance. MS says what each substance is. Together (LC-MS) they answer both questions. Researchers used this method to find that a product sold as TB-500 contained a short, acetylated fragment of thymosin beta 4 (Esposito 2012), and to measure semaglutide in vials bought online (Ashraf 2024).
Licensed drug makers must run at least one identity test on each ingredient themselves, even when a supplier provides a report (21 CFR 211.84).
What it cannot tell you
- It does not measure sterility or endotoxin.
- A mass match alone does not tell you how many mg are in the vial. Amount is a separate line on the report. In the online semaglutide study, LC-MS found 28.56% to 38.69% more semaglutide than the labels stated (Ashraf 2024), so amounts can be off in either direction.
- It covers only the sample tested. Save the lot number with each vial you log.
Read more on Dose Track: TB-500 · Tirzepatide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.