MALDI vs ESI: How the Mass on a Peptide Certificate Was Measured
Most peptide certificates include a line such as “MS: conforms” or a single observed mass next to a theoretical one. Few say how that mass was measured, yet the choice of MALDI vs ESI ionisation shapes what the spectrum can reveal and what it can quietly miss. For QC staff confirming identity across a multi-vial order, knowing which technique produced the number is the difference between a strong identity check and a rough one. This guide compares the two, with an eye on what each means for incoming inspection and lot records.
Getting a peptide into the gas phase
A mass spectrometer weighs ions, not molecules sitting in a vial. Before anything can be measured, the peptide has to be lifted into the gas phase and given a charge without being broken apart. MALDI and electrospray are the two soft ionisation methods that made routine peptide mass spectrometry possible, and they solve that problem in very different ways.
MALDI in brief
Matrix-assisted laser desorption/ionisation mixes the sample with an excess of a small, UV-absorbing organic acid (common choices include alpha-cyano-4-hydroxycinnamic acid for peptides and sinapinic acid for larger proteins). The mixture dries into crystals on a metal target. A pulsed laser hits the spot, the matrix absorbs the energy and carries the peptide into the gas phase, mostly as a singly protonated ion, written [M+H]+. It is usually paired with a time-of-flight analyser.
ESI in brief
Electrospray ionisation pushes a solution through a fine, high-voltage capillary. The spray forms charged droplets that shrink as solvent evaporates until individual ions are released. Peptides typically pick up several protons, so one compound appears as a series of peaks at different mass-to-charge values ([M+2H]2+, [M+3H]3+ and so on). Software then deconvolutes that envelope back into a single neutral mass.
MALDI vs ESI side by side
| Feature | MALDI | ESI |
|---|---|---|
| Sample form | Dried spot co-crystallised with matrix | Liquid, often straight from an LC column |
| Typical charge state | Mostly +1 | Several charges per molecule |
| Spectrum appearance | One dominant peak near the mass plus one proton | A ladder of peaks that must be deconvoluted |
| Coupling to chromatography | Usually offline | Direct, as LC-MS |
| Tolerance of salts and buffers | Moderate | Low; signal is easily suppressed |
| Low-mass region | Crowded by matrix ions | Generally clean |
What each technique can resolve
Because MALDI ions carry a single charge, the analyser has to separate them at their full mass. For a small peptide that is easy. As mass rises, resolving adjacent isotope peaks and small shifts becomes harder on many instruments, and a reported value may be an average across an unresolved cluster.
ESI spreads the same molecule across lower mass-to-charge values. At a charge of +3, a 3,000 Da peptide appears near m/z 1,001, a region where a modern analyser separates isotope peaks comfortably. That is why an ESI measurement on a good instrument can distinguish shifts of about one dalton, such as a deamidated species (+0.98 Da) or a C-terminal free acid in place of an amide (the same +0.98 Da difference). On a lower-resolution MALDI spectrum of a larger peptide, those species may hide inside the main isotope envelope.
Monoisotopic versus average mass
Well-resolved spectra let the lab report the monoisotopic mass, the peak made only of the lightest isotopes. Poorly resolved spectra give an average mass. The two can differ by a dalton or more for mid-sized peptides, so a certificate should make clear which one the theoretical figure was calculated as. A mismatch between the two conventions can look like an identity failure when it is only a bookkeeping error.
Where each method struggles
- MALDI and fragile groups: the laser pulse can knock off labile modifications, so the observed mass may reflect a fragment rather than the intact molecule.
- MALDI reproducibility: signal depends on how the crystals formed, which varies from spot to spot. That makes intensities unreliable for comparing amounts.
- ESI and salts: non-volatile buffers, residual salts and heavy counter-ion loads compete for charge and suppress the peptide signal. Sodium and potassium adducts also appear (about 22 and 38 Da above the protonated ion, respectively), and a careless reading can mistake them for impurities.
- ESI and TFA: trifluoroacetic acid in the mobile phase sharpens chromatography but reduces electrospray response, which is why LC-MS methods often use formic acid instead.
Identity, not purity
Neither technique is a purity method in the way UV-detected HPLC is. Mass spectrometry signal reflects how readily each species ionises, and two compounds present in equal amounts can give very different peak heights. A clean-looking mass spectrum confirms that the expected molecule is present; it does not tell you what fraction of the vial it represents.
LC-MS with ESI narrows that gap. Because it measures masses continuously as peaks leave the column, it can show whether the main HPLC peak contains one mass or two, which is the most direct way to expose co-elution. MALDI, as a spot measurement, does not offer that time dimension.
Using the technique in lot records
For a lab logging a large or repeat order, the mass line on a certificate becomes more useful with a few extra details recorded beside the lot number:
- The ionisation technique (MALDI or ESI) and, where given, the analyser type.
- The observed mass and the theoretical mass, with a note of whether each is monoisotopic or average.
- Whether the full spectrum or only a cropped label was supplied.
- Whether identity was confirmed by LC-MS across the main peak or by a single offline spot.
If two lots of the same compound show different identity evidence, the difference may be in the method rather than the material. Consistent technique across lots makes trend comparisons far easier.
Bulk Peptides sends its products for third-party HPLC and purity testing. Certificates are available for some products rather than all, and the cap and crimp colour on each vial points to the certificate that applies, which keeps multi-lot receipts traceable.
Which one should you ask for?
For a quick identity check on a short, well-behaved peptide, a clean MALDI spectrum showing the expected [M+H]+ is reasonable evidence. For larger sequences, for anything where a one-dalton difference matters, or where co-elution is a concern, ESI-based LC-MS gives stronger evidence. In either case, the full spectrum is more informative than a single labelled number.
This article is about analytical characterisation of research material. Bulk Peptides products are supplied only for in-vitro and laboratory investigation and are not for human or veterinary use.
