Mass spectrometry (revision 5)
Old revision·15:52, 23 Jul 2024·DiluentDiya
| Mass spectrometry | |
|---|---|
| Measures | Mass-to-charge ratio of ions |
| Usual ionisation for peptides | Electrospray |
| Answers | What the molecule is; not how much unless calibrated |
| Analytical method infobox · conventions | |
Mass spectrometry measures the mass-to-charge ratio of ionised molecules. For peptides it is the ordinary means of confirming identity: an observed molecular mass agreeing with the calculated mass excludes most alternatives, and fragmentation can confirm the sequence itself.[1]
A purity determination by chromatography establishes that one species dominates a sample; it does not establish which species. These are separate questions and require separate determinations. A certificate reporting purity but no identity leaves the more fundamental of the two unanswered.[2]
Ionisation and mass analysis
[edit]Electrospray disperses a solution into charged droplets in an electric field; solvent evaporates and ions are released into the gas phase. It is a soft technique — the molecule survives largely intact — which is why it suits peptides and proteins. Matrix-assisted laser desorption is the other common soft method and tends to produce singly charged ions.[1]
Mass analysers differ in resolution and in mass accuracy. A quadrupole gives unit resolution and is adequate for confirming a mass to within a dalton; time-of-flight and orbital-trap analysers give sufficient resolution to determine a mass to a few parts per million, which distinguishes species a quadrupole cannot.