Guide
What HPLC-MS actually measures, and what it doesn't
Almost every peptide certificate you will ever read reports HPLC-MS. It is two techniques doing two different jobs, and the number it produces is narrower than it looks. Understanding what the method can and cannot see is the difference between reading a certificate and trusting one.
Two instruments, two questions
HPLC-MS is a separation technique coupled to a detector that weighs molecules. They answer different questions and it is worth keeping them apart.
High-performance liquid chromatography pushes a dissolved sample through a packed column under pressure. Different molecules travel at different speeds depending on how strongly they interact with the packing, so a mixture that entered as one plug leaves as a series of separated bands. A detector at the far end records each band as a peak against time. HPLC answers: how many different things are in here, and in what proportion.
Mass spectrometry ionises what comes off the column and measures mass-to-charge ratio. For a peptide this gives an observed molecular mass that can be compared against the mass calculated from the expected sequence. MS answers: is this the molecule it is supposed to be.
Where the purity percentage comes from
The figure on a certificate is almost always area percent: the area under the main peak divided by the total area of all integrated peaks, times one hundred.
That definition has two consequences people rarely think about. First, it is a ratio, not a quantity - it says nothing about how much material is in the vial. Second, it only counts what the detector can see.
- It is relative. A 99% result means the main peak is 99% of the detected peak area. If the method misses something entirely, that something is not in the denominator.
- Integration is a judgement. Where an analyst sets the baseline and where they cut the edges of a peak changes the number. Two competent labs can report slightly different figures from the same trace.
- Co-elution hides impurities. If a related substance travels at nearly the same speed as the target, it arrives inside the main peak and is counted as the target.
- Detector response is not uniform. A UV detector responds to chromophores, not to mass. Two impurities present in identical amounts can produce very different peak areas.
What the method cannot see at all
This is the part that matters most, and the part a purity figure quietly implies is handled.
Peptide UV detection is typically at around 214 nm, which responds to the peptide bond, or 280 nm, which responds to aromatic residues. Anything without the relevant chromophore is close to invisible.
- Water. Lyophilised peptide holds residual moisture. It contributes mass in the vial and nothing to the chromatogram.
- Counterions. Synthetic peptides are usually isolated as salts - trifluoroacetate or acetate. That salt is a real fraction of the powder's weight and does not appear as a peak.
- Residual solvent. Traces from synthesis and purification are generally not detected by this method.
- Endotoxin. A separate assay entirely. HPLC-MS says nothing about it.
- Sterility. Also a separate test. A purity certificate is not a sterility certificate.
- Biological activity. Nothing in the method addresses whether a molecule does anything. It confirms mass and proportion, not function.
Why identity usually reads as 'conforms'
Identity is generally reported as a pass or fail rather than a number. The laboratory compares the observed mass against the calculated mass for the expected molecular formula, and if they agree within the instrument's tolerance the result conforms.
It is worth knowing what this does and does not rule out. Mass confirms the molecular formula is consistent. It does not by itself distinguish molecules of identical mass - a sequence with two residues transposed has exactly the same mass as the correct one. Distinguishing those requires fragmentation, usually written as MS/MS or tandem MS, which breaks the peptide into pieces and reads the sequence from the fragment masses.
Reading a chromatogram rather than a summary
A summary table is somebody's reading of the evidence. The chromatogram is the evidence.
On a trace worth trusting you can see the full run rather than a window around the main peak, a flat and quiet baseline, a main peak that is symmetrical rather than tailing badly, and any minor peaks visible with their retention times. A trace cropped tightly around the main peak has removed exactly the region where problems appear.
What a good result actually tells you
A clean HPLC-MS certificate is meaningful. It says that on the day of the test, on the sample submitted, the material was overwhelmingly one substance and that substance had the expected molecular mass. That is a real and useful thing to know, and a supplier who cannot produce it is asking for trust they have not earned.
It is simply narrower than 'this material is good'. It is a snapshot of one sample, by one method, on one date - and the methods that would answer the other questions are different tests with different names.
Common questions
What is the difference between HPLC and mass spectrometry?
HPLC separates a mixture into its components and measures their relative proportions, which is where a purity percentage comes from. Mass spectrometry measures molecular mass and is used to confirm identity - that the material is the compound named. They are usually run together and answer different questions.
Does HPLC purity tell me how much peptide is in the vial?
No. Purity is area percent - the proportion of detected material that is the target compound. It is a ratio, not a quantity. How much peptide is in the vial is a separate measurement, usually called net peptide content, and it is not the same number.
Can HPLC-MS detect endotoxin or confirm sterility?
No. Both are separate assays. An HPLC-MS certificate reports identity, purity and mass, and says nothing about endotoxin, sterility, or biological activity.
Why do two laboratories report slightly different purity numbers?
Peak integration involves judgement about where the baseline sits and where a peak begins and ends, and column, gradient and detector settings differ between methods. Small differences between competent laboratories on the same sample are normal.
What does 'conforms' mean on a certificate?
It means the measured value matched the expected value within tolerance - most often that the observed molecular mass agreed with the mass calculated for the expected formula. It is a pass or fail result rather than a measurement.
Check the evidence for a Causa lot
Our lot pages publish the available results for assayed batches. Request the original laboratory Certificate of Analysis using the lot number, and compare it with the material you are considering. Materials without a qualifying result are marked assay pending. A summary is not a substitute for the original laboratory report.
Related research documentation guides
Research use only. Not for human or veterinary use. This guide explains how to read an analytical document. It is not medical advice and not guidance on use.