Electrospray ionization

Electrospray ionization ( ESI) is one technique for the generation of ions by using the electro-spray method. The concept of electrospray ionization goes back to work by Dole ( 1968) and was established by John B. Fenn 1984 ( Nobel Prize in Chemistry in 2002).

Principle

In the electrospray an analyte solution is passed through a metal capillary, at the head of a voltage is applied. By the tension in the formation of an electric field between the capillary and a counter electrode. The electric field penetrates the analyte solution and move into their ions located electrophoretically on the counter electrode. It forms at the tip of the capillary, an excess similarly charged ions that repel each other and exit via the formation of a Taylor cone ( Taylor cone ) as a fine aerosol ( about 10 micron droplet size ) from the capillary. A neutral carrier gas such as nitrogen is often used to support the atomization of the solution and evaporation of the solvent. Due to the evaporation of the solvent, the droplet size reduces, while the density of the electric field is increased on the droplet surface. When the radius of the droplet is smaller than the so-called Rayleigh limit, the droplets disintegrate due to the repulsion of like charges ( Coulomb explosion ) into smaller droplets. For the formation of free ions in the gas phase, several conceptual models exist. The Charge Residue Model (CRM, the charged residue model ) assumes that ultimately remain tiny drops of about 1 nm in diameter, containing only an ionized analyte. When Ion Evaporation Model ( IEM, ion emission model) it is assumed that already from larger charged drops free ions are emitted into the gas phase. The ions are directed by the potential difference between Sprayerkapillare and orifice in the MS.

The type of voltage applied to the capillary, sets the charge of the ions generated. By a positive voltage, positively charged ions are generated, and negatively charged by a negative voltage ion.

Properties

In the electrospray ionization is a gentle method of ion generation, and in the sensitive molecules and non-covalent aggregates can be ionized. Typically quasi-molecular ions are detected ( [M H] with a positive voltage, [ MH ] with a negative voltage). A characteristic phenomenon in the ESI is the formation of adduct ions with components of the eluent or the 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 generally coupled with ion traps, quadrupole - TOF or analyzers.

Application

Electrospray is a technique for generating ions used in mass spectrometry (LC / MS). It belongs as well as the APCI ionization methods in which the generation of the ions takes place under atmospheric pressure. It is the preferred Ionisationverfahren for analysis of biomolecules, since it is very gentle to the analyte and hardly leads to fragmentations.

Fields of application are such as ESI, the determination of the molecular mass and sequence analysis of proteins and oligonucleotides, and the quantitative determination of drugs, pesticides, inter alia, small molecules.

In contrast to MALDI- ionization is an advantage of the ESI ionization in the coupling to an LC system. The coupling allows the analysis of complex samples, as the LC (usually a RP-HPLC ), a separation of the analytes. Furthermore, often multiple charged molecular ions generated in the ionization of this, on the one hand, the measurement of very large molecules, such as whole proteins, on the other hand the introduction of tandem mass spectrometry (MS / MS ) measurements, such as for the de novo sequencing of peptides possible. A disadvantage of ESI is that it to contaminants such as soaps, and salts is more sensitive than MALDI. The problem of the different needs of each mass spectrometric method is taken into account in the preparation of samples.