MALDI and ESI: Two Ways to Weigh a Peptide
A mass tells you which molecule, never how much of it
Begin with the limitation, because it governs everything downstream. Signal height in a mass spectrum is a function of how readily a species accepts charge, not of how much of that species sat in the vial. Two compounds drawn from one sample can differ enormously in that willingness. The spectrum therefore settles identity and stops there, while quantity remains the chromatogram’s job. HPLC and mass spectrometry in purity verification lays out that division in more detail.
The one-dalton problem
Identity work keeps colliding with the same obstacle. Amidation at the C terminus against the free acid: one dalton. A deamidated residue against an intact one: one dalton again, 0.98 of it to be precise, which deamidation and the 0.98 dalton shift takes up in its own right. When a certificate prints a measured figure next to a calculated figure, cases like these ask a single digit to bear a great deal of weight. Whether it can bear that weight depends on something certificates rarely state, namely how the molecule picked up charge before it entered the analyzer.
Resolution is a property of the mass-to-charge axis rather than the mass axis. Give a peptide four protons and the instrument inspects it at roughly a quarter of its molecular weight, comfortably inside the range where adjacent signals stay apart. Restrict it to one proton and the identical gap has to be teased out at full molecular weight, where resolving power has already fallen away. Electrospray handles the distinction routinely. A laser-desorption run on the same material may simply not.
Laser desorption: one clean peak, several blind spots
In MALDI, short for matrix-assisted laser desorption ionization, the analyte is crystallized together with a small organic matrix compound and a laser pulse then drives both into the gas phase, charging the analyte as it goes. What comes back is overwhelmingly singly protonated: a single dominant signal near the molecular mass plus one. Certificates favor it for exactly that reason. One labeled peak, one figure, nothing for a reader to puzzle over. Simplicity photographs well.
The costs are real, though. Fragile modifications sometimes do not survive desorption, so the spectrum can describe a fragment while appearing to describe the intact species. Matrix-derived ions crowd the low-mass end and bury anything small. And quantitation from such a run is untrustworthy as a matter of principle, since how strongly a species reports back depends on the accidents of how it crystallized. There is also no separation axis at all, because the measurement is normally an offline reading taken from a dried spot.
Electrospray: a busier trace carrying more information
ESI works from solution. The liquid is pushed through a charged capillary and emerges as ions bearing various numbers of protons, which produces not one peak but a ladder of them spread across different mass-to-charge values; software then folds that ladder back into a single molecular mass. The raw trace is harder to hand a customer than a lone labeled peak, and it says considerably more per run.
Accepting liquid flow buys a second capability that has nothing to do with resolving power. Put a column in front of the source and the detector reports, instant by instant, the mass of whatever is eluting at that instant. Two distinct masses emerging beneath one chromatographic peak leaves nothing to argue about, which is why LC-MS is the deciding experiment for co-elution.
Electrospray has its own failure mode, and it sits elsewhere. Salts and involatile buffers suppress ionization, so material carrying a heavy counter-ion load can return a poor spectrum for reasons entirely unconnected to the peptide itself. Salt form is covered in TFA and acetate counter-ions.
Reading the mass line critically
| Look for | Why it matters |
| A measured mass shown beside a calculated mass, with the assumed form stated | A calculation run on the free acid and a measurement of the amidated form will disagree by a dalton, and vice versa |
| Enough of the spectrum to reveal satellite signals | A tightly cropped label around one peak hides whatever else was present nearby |
| The ionization technique, named explicitly | This is what determines how much confidence a one-dalton agreement deserves |
Treating the document as a whole is the subject of how to read a certificate of analysis.
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