ESI – Electrospray ionization
The analyte is dissolved and forced through a narrow needle held at high voltage. A fine spray of charged droplets emerges from the needle. These droplets are attracted to the entrance of the mass spectrometer due to high opposite voltage at the mass analyzer’s entrance. As they enter the mass spectrometer, the droplets are dried using a stream of inert gas, resulting is gas-phase ions that are accelerated through the analyzer towards the detector.
ESI is conducive to the formation of singly charged small molecules, but can also produce multiply charge species of larger molecules. Multiple charging makes it possible to observe very large molecules.
Many solvents can be used in ESI and are chosen based on the solubility of the compound of interest, the volatility of the solvent and the solvent’s ability to donate a proton. Better sensitivity is obtained when a volatile organic solvent is added.
ESI is generally used with multiply charged ions with quadrupoles and quadrupole ion traps. These instruments are more readily configured as tandem mass spectrometers for mass-selecting and fragmenting single components of a mixture.
Liquid technique is compatible with online chromatographic methods such as RP-HPLC, anion exchange chromatography and capillary electrophoresis.
Usage
ESI is a method routinely used with peptides, proteins, carbohydrates, small oligonucleotides, synthetic polymers, and lipids.
| Advantages | Disadvantages |
|---|---|
| Practical mass range up to 70000 Da | Salts and ion-pairing agents reduce sensibility |
| Good sensitivity - femtomole | Complex mixture can reduce sensitivity |
| Softest ionization method | Simultaneous mixture analysis can be poor |
| Easily adaptable to LC | Multiple charging can be confusing |
| Easily adaptable to tandem mass analyzers such as ion traps and triple quadrupoles | Sample purity is important |
| Multiple charging analysis allows high mass ion analysis | Carryover from sample to sample |
| No matrix interference |
NanoESI
nanoESI has a very small needle positioned close to the entrance of the mass analyzer, resulting in more efficient ion transmission. Effusing the sample at very low rates allows for high sensitivity. The end result of this rather simple adjustment is increased efficiency, which includes a reduction in the amount of sample needed.
Since nanoESI droplets are smaller, the amount of evaporation necessary to obtain ion formation is much less. As a consequence, nanoESI is more tolerant to salts and other impurities. Less evaporation means that impurities are not concentrated down as much as they are in ESI.