Ion source

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An ion source , also known as an ion gun , is used to generate ions .

Types of ionization

Electron impact methods

Ion source of a mass spectrometer, suitable for electron impact and chemical ionization
Electron impact ionization (EI, also electron impact , English electron ionization )
The impact ionization with electrons is the most widely used ionization method. Electrons from a filament are accelerated by an electric field and sent through the gas-borne molecules to be ionized. The acceleration voltage is 5 to 200 volts, typically 70 V, where the ionization cross section is maximum for most molecules. When the electrons collide with the molecules, another electron is knocked out, creating a radical cation . This is mostly unstable and breaks down into smaller mass fragments, one of which remains charged. If the molecule was already a radical, the collision product is usually stable. Before being separated, the charged particles are accelerated and focused in a further electric field that is generated by a voltage of a few thousand volts . The sizes and frequencies of the fragments are cataloged in databases for a given substance accelerating voltage of the electrons and can be used for identification.
Chemical ionization (CI)
In chemical ionization, a gas is supplied that is excited / ionized by electron impact ionization. The reaction gases used are nitrogen , noble gases or hydrocarbons with up to five carbon atoms. The ions formed from the gas then react with the analyte and ionize it. The degree of fragmentation is lower than with electron ionization. This method is also often used in conjunction with a gas chromatograph . Because of the low fragmentation, chemical ionization is suitable, for example, in biochemical analysis for determining the sequence of peptides or for detecting drugs .
Gentle ionization
With soft ionization, gases with a low ionization energy of 10 to 14 eV are used to ionize the sample gases (e.g. xenon , argon or mercury ). For this purpose, an ion cloud of the ionization gas is generated by electron bombardment. By making a selection via aprons, only single ionized atoms of the ionization gas reach the sample gas to be ionized. A fragmentation of the molecules of the sample gas is thereby almost completely prevented. Since different gases with the same number of masses can also be ionized differently from the ionizing gases, differentiation of isomers is possible through analytical comparisons.

Field ionization methods

Field ionization (FI)
In field ionization, molecules are ionized in an electric field with a high field strength. High field strengths can be achieved if the anode consists of a metal wire with a point, or better if the metal wire is provided with numerous points (micro-needles made of graphite dendrites ). A voltage of several thousand volts is applied between the anode and the cathode as a counter electrode. The sample is supplied in a gaseous state. The ionization takes place both in the space between the electrodes and on the surface of the anode. With this method, the molecules are only slightly fragmented. This also makes it easy to examine mixtures. One example is the investigation of pyrolysis products of high molecular weight substances.
Field desorption (FD)
The basic structure of field desorption does not differ from field ionization. The difference is that a solid or liquid analyte is previously adsorbed on the emitter surface (anode). This eliminates the need for a separate evaporation process. The ionization takes place here by field-induced surface reactions. Because of the gentle ionization, this method is suitable for the analysis of poorly volatile or thermally unstable substances, for example from natural product chemistry. In addition, mixtures and inorganic salts can be investigated well.
Liquid Injection Field Desorption Ionization (LIFDI)
With LIFDI, the pressure difference between normal pressure and vacuum causes a liquid analyte to be sprayed directly onto the graphite dendrite via a capillary and ionized there very gently in a high electronic field, as with FI. As the analyte is supplied through a capillary, this method is particularly suitable for analyzing air-sensitive substances.

Particle bombardment methods

Liquids and solids can be bombarded with fast atoms or ions, whereupon ions dissolve. Come atoms are used, i.e., the method FAB (engl. Fast Atom Bombardment , Fast Atom Bombardment), wherein ion SIMS (engl. Secondary ion mass spectrometry , secondary ion mass spectrometry ). In addition to the secondary ions, uncharged particles (secondary neutral particles) are also generated. If these are re- ionized with laser light, for example, and then analyzed, it is called secondary neutral particle mass spectrometry (SNMS). Devices that enable a spatial resolution of less than 50 nanometers are called NanoSIMS . These methods are u. a. used in surface chemistry.

Spray methods

Electrospray ionization
Electrospray ionization (ESI) involves spraying solutions of charged or polar substances, ionizing them and then drying the droplets so that ions of the analyte remain. This method is particularly suitable for larger molecules such as proteins.
Atmospheric Pressure Chemical Ionization
Chemical ionization under the atmospheric pressure (engl. Atmospheric Pressure Chemical Ionization , APCI) works similarly ESI, except that the solution of the analyte prior to the ionization is evaporated. The solvent molecules are ionized on a pointed electrode at atmospheric pressure. The method is also suitable for less polar analytes.

Photoionization methods

One-photon ionization
With single photon ionization (SPI), the ionization energy is overcome by a single photon whose wavelength is in the vacuum ultraviolet range (VUV). These high-energy photons can be generated either as synchrotron radiation or with discharge lamps. A widespread special case is atmospheric pressure photo ionization (APPI).
Resonance-enhanced multiphoton ionization
The resonance-enhanced multiphoton ionization (engl. Resonance Enhanced Multi Photon Ionization , REMPI) based on the near simultaneous absorption of multiple ultraviolet photons, which by means of a laser to be produced, the total energy above the ionization energy is to be ionized of the molecule. There is also an atmospheric pressure variant for this ionization method, Atmospheric Pressure Laser Ionization (APLI).

The two photoionization methods under atmospheric pressure are mainly used when coupling mass spectrometers with LC systems . The eluent is first evaporated and then ionized by photons. The photons are emitted perpendicular to the molecular beam.

Matrix-assisted laser desorption / ionization
In matrix-assisted laser desorption / ionization (MALDI), the analyte is mixed with a large excess of a substance called a matrix and cocrystallized (integration of the analyte into the crystalline matrix). The matrix has the property of absorbing energy when bombarded with a laser of a certain wavelength (for example a nitrogen laser 337 nm). The intact analyte is vaporized by the bombardment with the laser. A charge carrier provided by the matrix, e.g. B. a proton is bound.

Other methods

  • Thermal ionization ( TIMS, thermal ionization mass spectrometry ) is used in solid-state mass spectrometry . The sample (sample amount ng to µg depending on the substance) is applied to a tungsten filament, for example. Current is sent through the filament, heating it up and vaporizing the applied sample. Some of the evaporated atoms are ionized because the kinetic energy of the atoms is so high that they can ionize each other through collisions.
  • An inductively coupled plasma ( inductively coupled plasma , ICP) breaks up most of the compounds into their elements and mainly simply positively charged ions are created. This method is therefore mainly used in mass spectrometry with inductively coupled plasma in inorganic element analysis and trace analysis .
  • In the ionization by corona discharge (engl. Glow discharge ionization , GDI), the sample is ionized by a signal generated by an arc plasma.

application

The ions generated can e.g. B. accelerated in particle accelerators, their mass determined in a mass spectrometer or used for doping semiconductors by ion implantation .

The following are decisive for the requirements for an ion source:

  • the types of atoms or molecules to be ionized
  • the desired charge states
  • Permissibility or even desire for fragmentation of molecules
  • desired ion current
  • Energy distribution of the ions.

For some applications the output energy of an ion source is sufficient and the ion beam can be used directly. Other areas of application, especially nuclear and particle physics, require higher energies, for which the ions have to be accelerated in a particle accelerator .

Designs

Examples of ion source designs are:

Web links and literature

Individual evidence

  1. ↑ Teach / Me instrumental analytics. Retrieved December 9, 2010 .
  2. ↑ Demtröder, Experimentalphysik 3 Atome ,olekule und Festkörper, Volume 3, Springer DE, 2009, ISBN 3642039111 , p. 36, Google Books