Atmospheric-pressure laser ionization

The laser light sources used in APLI have power densities which allow multiphoton ionization via stable electronic states of the molecule or atom.

The required power density has to be sufficiently high, so that in the lifetime of the first reached electronic state, which is in the range of a few nanoseconds, a second photon can be absorbed with a reasonable probability.

Thus APLI utilizes light with a photon energy of around 5 eV which corresponds to a wavelength of about 250 nm, which is in the ultraviolet (UV) part of the electromagnetic spectrum.

UV tunability and discrete energy states of analyte allow improved ionization with reduced background signal.

In comparison to the single photon ionization (APPI) with vacuum ultraviolet light (λ = 128 nm) APLI is much more sensitive, in particular in applications with liquid chromatography (LC-MS).

[6] The selectivity of APLI is one factor contributing to the selectivity, but under LC conditions, APPI suffers from another effect: The VUV light utilized by APPI does not penetrate deep into the ion source geometry, because the solvents used by LC, which are present as vapor in the ion source, strongly absorb the VUV light.

APLI Ionization Mechanism: A molecule M is brought from the electronic ground state into an electronically excited state A by the absorption of a photon if the energy of the photon fits to the energy of an excited state. The molecule relaxes afterwards or by the absorption of a second photon at sufficiently high photon fluxes, the ionization potential is reached: One electron is removed from the molecule and a radical-cation is formed. For efficient ionization by the absorption of two photons a high density of electronic states in the intermediate region is necessary.
Absorption cross sections of nitrogen, oxygen and some common LC-Solvents at the ionization energies of APPI (10 eV) and APLI (5 eV). The light utilized by APPI is strongly absorbed by substances in the ion source (oxygen and solvent vapor)