Atmospheric-pressure laser ionization

The laser ionization at atmospheric pressure ( atmospheric pressure English laser ionization, APLI ) is an atmospheric pressure ionization method in mass spectrometry (MS), wherein the molecules by resonance enhanced Mehrphotonenionisation ( REMPI ) are converted into ions. In this case, lasers are used as light source.

Principle

The absorption of photons electrons in the atoms or molecules of a sample can be so far suggested that they leave the atom or molecule, and so the irradiated with electromagnetic radiation sample is ionized. For gaseous samples is called this process photoionization. The energy of the absorbed photons must be at least ionization potential of the atom or molecule.

In the simplest case, a single photon to the ionization energy deposit is sufficient; This principle makes use of the atmospheric pressure photoionization (English atmospheric pressure photo ionization, APPI ) advantage. When light is irradiated with a sufficiently large power density, it can, however, lead to non- linear absorption processes in which several photons are absorbed sequentially in rapid succession and together raise the ionization energy. In this case one speaks of a Mehrphotonenionisation (English multiphoton ionization MPI).

The atmospheric pressure is a laser ionization Photoionisationsmethode which utilizes laser light sources, the power density is sufficient to Mehrphotonenionisation a stable intermediate state of the quantum molecule or atom allow. The power density must be so great that in the lifetime of the excited intermediate state ( typically a few nanoseconds) a second photon can be absorbed. Analogous to the photoionization produces a radical cation:

This process is called resonance enhanced Mehrphotonenionisation ( engl. resonance enhanced multiphoton ionization REMPI ). In APLI two photons of the same wavelength are absorbed, in this case one speaks of "1 1 REMPI ."

Since the ionization of most organic molecules, which are used as the analyte to such Photoionisationsmethode in question is less than 10 eV, used in APLI a photon energy of about 5 eV, corresponding to a wavelength of around 250 nm. This lies in the ultraviolet (UV) region of the electromagnetic spectrum.

Typical for APLI used laser systems are KrF excimer laser ( λ = 248 nm) and frequency-quadrupled Nd: YAG laser ( λ = 266 nm).

Properties

APLI has by the ionization by UV laser light some special properties:

Simple Koppelbarkeit

APLI is relatively easy to couple with existing atmospheric pressure ion sources, because only the ionizing laser light must be incorporated into the existing AP- source, which is easy to implement through quartz window.

Selectivity

To be directly ionized by 1 1 REMPI using UV laser light can, a molecule must have a matching, sufficiently stable intermediate electronic state and the two electronic transitions of the 1 1 REMPI process must be quantum mechanically allowed. APLI is therefore a selective ionization method.

In particular, polynuclear aromatic molecules fulfill the spectroscopic conditions such that APLI is an ideal ionization method for the detection of polycyclic aromatic hydrocarbons ( PAHs).

However, the selectivity is also a disadvantage when the direct ionization of the analyte with APLI is not possible. In this case, some of the analyte by chemical coupling are made available with a label molecule for APLI. If such a derivatization available, the selectivity can be extended to other classes of molecules.

High sensitivity

Compared to single-photon ionization ( APPI ) with vacuum - ultraviolet radiation ( λ = 128 nm) has APLI particularly when coupled with liquid chromatography (LC -MS), a much greater sensitivity. The reason for this is partly due to the selectivity of APLI. On the other hand penetrates the VUV light, in this case hardly in an AP - ion source, since the typical solvents used in the liquid and present in LC-MS as a vapor in the ion source, that absorb strongly. The UV light used APLI is barely absorbed by solvents and it can be produced in the entire volume of ion.

Independence of the ion generating electric fields

In contrast to other Ionsationsmethoden (eg electrospray ionization (ESI ), chemical ionization at atmospheric pressure (APCI ) ) allows the generation of ions APLI independently of electric fields as ions may be formed only in the laser beam. This allows for some special process, such as the inclusion of an ion signal as a function of ionization, which is used in the development of ion sources.