Chemical Ionization with atmospheric pressure (APCI)

The chemical ionization with atmospheric pressure or APCI is a technique of Ionization in the gas phase, based on the transfer of species charged with a reactive Ion to a molecule of analyte. It is used as method of ionization for the Spectrometry mass, preferentially coupled to a chromatography in liquid phase.

History

First source APCI was developed in the Années 1970 by Horning and Al This source used a blade of ⁶³Ni like source of electron S to achieve ionization, but an electrode with discharge crowns was preferred thereafter. The APCI is a technique of ionization known as “soft”, but contrary to the électrospray, it generally gives place to a certain degree of fragmentation which is useful for the structural characterization.

Operation

Source APCI uses a pneumatic Nébuliseur with nitrogen to form a Aérosol inside a room (the spray) heated with high Température S (between 350 and 500 °C). In the spray, the effluent of the column of chromatography (solvants+analytes) is evaporated and mixes with the gas atomizer (or gas of transport) to bring the analytes towards the electrode to discharge crowns made up of a needle and a room of atomizing which is used as counter-electrode. A high potential difference (between ±3 and ±6 Kv) is applied to this electrode, causing a discharge crowns (luminous electric shock) of ~2-3 μA. This discharge is a constant source of electrons for the process of ionization in APCI: it ionizes the ambient air and creates a plasma, i.e an ionized gas, around the point of the needle. The ridicalizing ions of plasma take part then in chemical reactions which give place to the ionization of the molecules of analyte.

Formation of ions

The ridicalizing ions present in plasma in APCI+ are mainly N2•+, O2•+, H2O•+ and NO•+ and in APCI- O2•-, O•-, NO2•-, NO3•-, O3•- and CO3•-. These primary education ions react quickly (~10-6 S) and transfer their load to the molecules from solvent to produce reactive ions (R). Those react then with the analyte (M) to give ions according to the mechanisms of chemical ionization. The total time of the reaction in the source is of ~ 5×10-4 S. the primary reactions which lead to the formation of ions in an atmosphere of N2 (G) in the presence of water were recognized like a mechanism of formation of the ion hydronium. With atmospheric pressure there is sufficient water so that the principal ions present in gas of transport without the effluent of the LLC or the analyte are of the ions aggregates of type (H2O) nH+. With 200 ºC and with N2 (G) like gas of transport and with only of atmospheric water the most abundant aggregate is ion (H2O) 2H+, with (H2O) 3H+, H3O+, (H2O) NO+ and (H2O2) NO+ so present. With atmospheric pressure in the presence of the quantities traces water and of nitrogen, protonic molecular ions are made by reactions ion-molecule in the gas phase with ions aggregates of water.

However, the presence of other substances in the effluent of the LLC or the made up ones which are added in the form of liquid sheath, can also take part in reactions of chemical ionization.

The reactions in APCI are divided into: I) exchange of load, II) captures electron and III) reactions ion-molecule.

The exchange of load occurs when a reaction ion-neutral takes place and the load is transferred to the neutral. These reactions take place at the time of the collision of a reactive ion having an energy of ionization high and a molecule of sample having an energy of lower ionization. The energetics of the exchange of load is determined by the energy of ionization of analyte neutral IE (M) and the energy of recombination of the reactive ion, RE (R•+). The ionization of a analyte by reactions of exchange of load is carried out if RE (X+•) > IE (M). Then the heat of the reaction and consequently the minimal internal energy of the molecular ion of analyte are:

Eint (M+•) ≥ RE (R+•) - IE (M)

The symbol ≥ indicates the additional contribution of thermal energy. To summarize, no exchange of load is awaited if RE (X+•) < IE (M); mainly M+ ions• are awaited if RER+• is slightly above IEM and a considerable fragmentation will take place if RE (R+•) is in particular larger than IE (M). Fortunately, the differences between RE and IE are small and the data of IE of the reactive ions can be used to estimate their effect. The transfer of load can be used like a selective method of ionization. The molecular ion of benzene was used as reactive ion in APCI+ and the ion O2•- is used for the transfer of load in the negative mode.

The capture of electron to the place so of the thermal electrons are present in the source of ionization. The capture of electron is a process of resonance where an external E is built-in the orbital one of an atom or a molecule. The E are not provided by the reactive ions, they move freely in gas with a thermal energy. When a neutral molecule interacts with an E having a raised kinetic energy, a positive radical ion is generated (by I.E.(internal excitation)). If the E have less energy than the IE of the neutral molecule, the capture of electron can take place. The resulting ions have kinetic energies between 0 and 2 eV. The energetics of the capture of electron is determined by the electronic affinity of the neutral molecule. As IE of a molecule east governs by the atom with lowest IE in a neutral, EA of a molecule is basically determined by the atom with highest electronegativity. Molecules having electronegative functional groupings, like groupings carbonyls combined with double linkings, halogenous groupings nitro or atoms then are ionized by resonant capture of electron, producing molecular anions, or by dissociative capture of electron producing of the ions negative fragments.

In APCI, the majority of reactions ion-molecule are based on chemistry acid-bases in the gas phase. In the positive mode, the carrier dominating of the load is the protonic solvent +. Because of the high abundance of solvent and the strong binding energies ion-molecule (typically 1 eV for the molecules bound by bridges H), ion S2H+ is also regarded as important (the broader complexes are regarded as less abundant). Then in APCI+ the reactive ions are acids. The common reagent strongest is CH5+ which is not selective towards the molecules of the sample. H3O+, CH3OH2+ and CH3CNH+ are intermediate acids. NH4+ is a selective weak acid. For the operation in the negative mode, the reactive ions are bases, OH- is a strong base, CH3O- and CH2CN- are intermediate bases and CH3COO- and Cl are weak bases.

References

Random links: