The journal
HPLC and mass spectrometry: what each one actually measures
Every certificate we issue carries two measurements. They are not redundant, and neither one substitutes for the other. Here is what each instrument is actually doing, and why a certificate carrying only one of them has verified half of what it appears to.
HPLC measures how much
High-performance liquid chromatography pushes a dissolved sample through a column packed with a stationary phase. Different molecules stick to that phase to different degrees, so they emerge at different times. A detector at the far end — usually watching ultraviolet absorbance around 214 nm, where the peptide bond absorbs — records what comes off and when.
The output is a trace with a peak for each distinct substance. Integrate the area under each peak, and the main peak as a fraction of the total is the purity percentage.
What HPLC is good at:
- Finding out that there is more than one thing in the vial.
- Separating molecules that are nearly identical in mass but differ in shape or polarity — a D-residue where an L-residue belongs, for instance, which weighs exactly the same and comes off the column at a different time.
- Putting a number on the proportion.
What HPLC cannot do:
- Tell you what the main peak is. A single clean peak proves the sample is one substance. It says nothing about which substance.
Mass spectrometry measures what
Mass spectrometry ionises the molecule and measures its mass-to-charge ratio. For peptides this normally means electrospray ionisation, which produces a series of multiply charged ions that resolve to one molecular weight.
Compare that observed mass against the theoretical mass calculated from the molecular formula. They should agree closely. Where they do not, the size of the gap identifies the problem:
- +16 — an oxygen, usually on a methionine.
- +18 on a cyclic peptide — a ring that never closed, since forming the bond releases water.
- +114 and multiples — trifluoroacetate adducts from purification.
- −71, −97, −113 — the residue masses of alanine, proline, and leucine or isoleucine: a chain missing a residue.
- −1 — deamidation, an asparagine or glutamine side-chain amide turned to an acid.
What mass spectrometry cannot do:
- Tell you the proportion. Ionisation efficiency varies by molecule, so peak height in a mass spectrum is not a reliable measure of how much of each species is present. It is an identity tool, not a quantity tool.
Why both, always
Because the two failure modes are independent, and each instrument is blind to the one the other catches.
A sample can be 99.8% pure and entirely the wrong molecule — HPLC passes it, mass spectrometry fails it. A sample can be the right molecule at 60% purity — mass spectrometry finds the expected mass and reports nothing wrong; HPLC finds the other 40%.
Neither instrument is checking the other's work. They are checking different things, and a certificate showing only one has answered only one question.
And then we do it twice
Both measurements are run by an independent third-party laboratory before the material reaches us, and both are run again in our own laboratory when it arrives. Only then does it ship.
The order is not incidental. The third party goes first. A supplier that tests its own material and then sends it out for confirmation already knows what answer it is hoping for — and the whole value of an independent test is that it was taken before anyone had an opinion. How we verify.
All products supplied by Alpha Aminos are for laboratory research use only and are not for human or animal consumption. Nothing on this page describes or is intended to describe an effect of any compound. No statement here has been evaluated by the Food and Drug Administration, and nothing here is intended to diagnose, treat, cure, or prevent any disease.