Chemical ionization at atmospheric pressure

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Image of an APCI source with a corona needle

Chemical ionization at atmospheric pressure ( English atmospheric pressure chemical ionization , APCI ) is an ionization process that is usually used in mass spectrometers when coupled with HPLC . It is a form of chemical ionization at atmospheric pressure and was developed by Evan C. Horning in 1974 . The APCI method is also quite suitable for analytes that are less polar.

execution

A solution of the analyte is atomized through a capillary in a stream of nitrogen , producing an aerosol . This is passed through a heated ceramic (300–600 ° C) where the solvent is completely evaporated.

The exiting steam is fed via a needle-shaped electrode (so-called. By applying a high voltage (about 5 kV) corona needle , engl. Corona discharge needle ) into a plasma transferred. In the plasma, ions are first formed from the solvent and possibly added buffer (frequently ammonium acetate ). The ionized solvent molecules in turn ionize the analyte molecules , which are then transferred to the actual measuring apparatus. This leads to the ionization of nitrogen molecules, which then finally form protonated water clusters in a reaction cascade, which can then carry out a proton transfer to the analyte molecules.

Depending on the solvent and analyte, the following reactions are possible:

  • Protonation (as with chemical ionization, e.g. with amines )
  • Charge exchange
  • Deprotonation (e.g. with carboxylic acids , phenols )
  • Electron capture (e.g. with halogen compounds and aromatics )

Multiply charged molecules [M + n H] n + , as in electrospray ionization (ESI), are not observed.

Often APCI may in a modified ESI - ion source are performed. The method is less gentle than ESI, which means that there are more fragment ions. APCI allows the relatively high flow rates of standard HPLC to be used directly without having to discard most of the volume. The use of APCI makes sense under the following conditions:

  • The analyte is difficult to ionize with ESI (e.g. few functional groups, no nitrogen)
  • The analyte contains only a few reactive functional groups (e.g. hydrocarbons, alcohols, aldehydes, ketones, esters)
  • The sample is thermally stable and vaporizable

variants

Photoionization at Atmospheric Pressure (APPI)

In atmospheric pressure photoionization , the ionization in the APCI source is excited by vacuum ultraviolet radiation . This is particularly advantageous if the ionization with ESI or APCI is insufficient.

The APPI source is usually a modified APCI or ESI source.

APCI-GC

Atmospheric pressure GC (APGC or APCI-GC) was developed by the Hornings group back in the 1970s. In recent years it has been rediscovered for coupling gas chromatography with mass spectrometers that were actually developed for coupling with HPLC-MS. The advantage of this technology is the gentle ionization compared to EI. Many organohalogen substances only form the molecular ion or quasi-molecular ion under APGC conditions.

The APGC source is usually a modified APCI or ESI source in connection with a heated transfer line to the gas chromatograph.

See also

Individual evidence

  1. ↑ a b Manfred H. Gey: Instrumental Analysis and Bioanalytics: Biosubstances, Separation Methods, Structural Analysis, Applications . Springer-Verlag, 2015, ISBN 978-3-662-46255-3 , pp. 325 ( books.google.de ).
  2. ↑ Jörg Hau: Ionization at Atmospheric Pressure: Development, Characterization and Application of an Electrospray Ion Source for a Double Focusing Sector Field Mass Spectrometer . Springer-Verlag, 2013, ISBN 978-3-663-14609-4 , pp. 3 ( books.google.de ).
  3. ↑ EC Horning, MG Horning, DI Carroll, I. Dzidic, RN Stillwell: New picogram detection system based on a mass spectrometer with an external ionization source at atmospheric pressure . In: Analytical Chemistry . 45, No. 6, May 1, 1973, pp. 936-943. doi : 10.1021 / ac60328a035 .
  4. ↑ a b c d Aprentas: Laborpraxis Volume 3: Separation methods . Springer-Verlag, 2016, ISBN 978-3-0348-0970-2 , pp. 273 ( books.google.de ).
  5. ^ The HPLC Expert: Possibilities and Limitations of Modern High Performance Liquid Chromatography . John Wiley & Sons, 2016, ISBN 978-3-527-67762-7 , pp. 6 ( books.google.de ).
  6. ↑ Analytical chemistry: Basics, methods and practice . John Wiley & Sons, 2016, ISBN 978-3-527-69879-0 , pp. 324 ( books.google.de ).
  7. ↑ I. Dzidic, DI Carroll, RN Stillwell, EC Horning: Comparison of positive ions FORMED in nickel-63 and corona discharge ion sources using nitrogen, argon, Isobutane, ammonia and nitric oxide as reagents in atmospheric pressure ionization mass spectrometry . In: Analytical Chemistry . tape 48 , 1976, p. 1763-1768 , doi : 10.1021 / ac50006a035 .