Electrospray ionization

from Wikipedia, the free encyclopedia
Electrospray (nanoSpray) ion source

Electrospray Ionization ( ESI ) is a technique for generating ions using the electrospray method. The term electrospray ionization goes back to the work of Malcolm Dole (1968) and was established by John B. Fenn in 1984 (Nobel Prize in Chemistry 2002).

principle

Schematic representation of an electrospray in positive mode

In electrospray ionization, an analyte solution is passed through a metal capillary, at the tip of which a voltage is applied. The voltage creates an electric field between the capillary and a counter electrode. The electric field penetrates the analyte solution and the ions in it move electrophoretically towards the counter electrode. At the tip of the capillary, an excess of ions of the same charge forms, which repel each other and emerge from the capillary as a fine aerosol (about 10 µm drop size) via the formation of a Taylor cone . A neutral carrier gas such as nitrogen is often used to aid in nebulizing the solution and evaporation of the solvent. Due to the evaporation of the solvent, the droplet size decreases while the density of the electric field on the droplet surface increases. If the radius of the drops becomes smaller than the so-called Rayleigh limit , the drops disintegrate into smaller droplets because of the repulsion of charges of the same type ( Coulomb explosions ). There are several models for the formation of free ions in the gas phase. The Charge Residue Model (CRM, model of the charged residue) assumes that in the end tiny drops of about 1 nm in diameter are left that contain only one ionized analyte molecule. In the ion evaporation model (IEM, ion emission model) it is assumed that even larger charged droplets emit free ions into the gas phase. The generated ions are directed into the MS by the potential difference between the sprayer capillary and the orifice (a counter electrode with an opening for the ions to fly through).

The polarity of the voltage applied to the capillary determines the charge of the generated ions. A positive voltage generates positive charged ions and a negative voltage generates negatively charged ions.

properties

Electrospray ionization is a gentle method of ion generation that can also ionize sensitive molecules and non-covalent aggregates. Typically, quasi-molecule ions are detected ([M + H] with positive voltage; [MH] with negative voltage). A characteristic phenomenon in ESI is the formation of adduct ions with components of the eluent or buffer ([M + Na] , [M + NH ] ). This adduct formation is also possible with components of the sample matrix.

Another characteristic phenomenon is the formation of multiply charged ions. ESI is mostly coupled with ion trap , quadrupole or TOF analyzers.

application

Electrospray ionization is an ion generation technique used in mass spectrometry ( LC / MS ). Mass spectrometry using an electrospray ion source is sometimes referred to as electrospray mass spectrometry (ESMS). Like APCI, it is one of the ionization processes in which the ions are generated under atmospheric pressure. It is the preferred ionization method for the analysis of biomolecules, as it is very gentle on the analyte molecule and hardly leads to fragmentation.

Areas of application for ESI are e.g. B. the determination of the molecular mass, the analysis and sequencing of proteins and oligonucleotides and the quantitative determination of drugs , pesticides and the like. a. small molecules.

In contrast to MALDI ionization , one advantage of ESI ionization is the online coupling to an LC system. The coupling enables the analysis of more complex samples, as the LC (mostly RP-HPLC ) separates the analytes. Furthermore, with this ionization method, multiply charged molecular ions are often generated, which on the one hand enable the measurement of very large molecules, such as. B. whole proteins, on the other hand the implementation of tandem mass spectrometry (MS / MS) measurements, z. B. for de novo sequencing of peptides made possible. A disadvantage of ESI is that it is more sensitive to contaminants such as soaps and salts than MALDI. The problem of the different requirements of the individual mass spectrometric methods is taken into account in the sample preparation .

literature

  • Herbert Budzikiewicz: Mass Spectrometry, 4th Edition Wiley-VCH Weinheim New York Chichester Singapore, 1998 pp. 28-30, ISBN 3-527-29381-7 .