ESI is an extremely versatile ionisation technique. ESI is a sensitive technique, enabling molecular-weight information to be obtained on very small amounts of sample. Samples can be ionised in either positive or negative ionisation modes, allowing for the detection of a positively charged ion or negatively charged ion.
ESI can be used to analyse low (Mr >50 Da) to high (Mr <100,000 kDa) mass, and is suitable for both polar, ionic and organometallic compounds. It is equally useful for the analysis of biological molecules including peptides, proteins, saccharides and oligonucleotides. The sample must dissolve in water, methanol or acetonitrile.
ESI is a soft ionisation technique and usually only molecular ions are produced, usually as their [M + H]+, [M + NH4]+ or [M + Na]+ ions ion positive ionisation mode or [M-H]‑ or [M + Cl]‑ in negative ionisation mode, as examples. Ions which are able to hold more than one charge will form ions of the type [M + nH]n + and will appear on the spectrum at [M+nH]/n (mass spectrometry reports an m/z value where m is the ion and z is the number of charges). If greater fragmentation is required (e.g. for structure elucidation purposes), a technique called tandem mass spectrometry (MS/MS) or another called MSe can be applied post-ionisation.
The ESI technique
In ESI, a pure sample is injected into a stream of solvent, which is sprayed through a needle. A voltage potential of 1 to 4 kV is applied to the needle which forms a spray containing charged droplets. As the solvent evaporates, the charge density upon the surface of the solvent-sample droplet increases until a charged sample ion is expelled. The ions then enter the mass analyser whilst neutral molecules are lost. With a Time-of-Flight (TOF) analyser, the time taken for the ion to travel to the detector can be recorded very precisely, and the mass-to-charge ratio (m/z) can be measured to better than 5 ppm accuracy.