Reference · Updated 11/4/2024
Mass spectrometry identity confirmation
How ESI-MS and MALDI-TOF confirm peptide identity by molecular mass, why purity and identity are separate analytical questions, and how to read a mass spec result on a COA.
HPLC purity and mass spectrometry identity answer two different questions. Purity asks: how homogeneous is this sample? Identity asks: is the dominant component actually the sequence it is labeled as? A Certificate of Analysis that reports purity without an accompanying identity confirmation has only answered half the question.
Why purity alone cannot confirm identity
A chromatographically pure, single sharp HPLC peak only demonstrates that the sample is dominated by one species relative to detectable impurities. It does not by itself demonstrate what that species is. Two different peptides of similar hydrophobicity could, in principle, produce a comparably clean-looking trace. Confirming the molecular identity requires a technique that measures mass directly, independent of chromatographic behavior.
Electrospray ionization (ESI-MS)
In electrospray ionization, a dilute solution of the sample is sprayed through a charged capillary, producing charged droplets that evaporate down to bare, multiply charged ions of the peptide. The mass spectrometer measures the mass-to-charge ratio (m/z) of these ions, and because a peptide can pick up more than one charge, the resulting spectrum often shows a series of peaks corresponding to different charge states of the same molecule.
Software deconvolutes this charge-state series back into a single calculated molecular mass, which is then compared against the theoretical mass computed from the peptide's amino acid sequence. ESI is commonly coupled directly to an HPLC system (LC-MS), allowing purity and identity data to be collected from the same run.
MALDI-TOF as an alternative method
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mixes the sample with a light-absorbing matrix compound, crystallizes it on a metal plate, and ionizes it with a pulsed laser. The resulting ions are accelerated down a flight tube, and their mass-to-charge ratio is calculated from time of flight — heavier ions arrive later. MALDI-TOF tends to produce mostly singly charged ions, giving a comparatively simple spectrum with a clear molecular ion peak, and is a common method for quick single-point mass confirmation.
Reading the result on a COA
A useful mass spec result on a COA states the observed mass (or m/z with charge state noted), the theoretical/calculated mass for the labeled sequence, and the deviation between them, usually a fraction of a mass unit or a small percentage. Instrument accuracy varies by method and calibration, so a small deviation is expected; a mismatch that corresponds to a different amino acid composition, a missing or extra residue, or an unexpected chemical modification is a genuine identity discrepancy, not measurement noise.