Laser ionization at atmospheric pressure

The laser ionization at atmospheric pressure ( English atmospheric pressure ionization laser , APLI) is an atmospheric pressure ionization method in mass spectrometry (MS), in which molecules by resonance-enhanced multiphoton ionization (REMPI) in ions are transferred. In this case, be laser as a light source used.

principle

APLI ionization mechanism: A molecule M in the electronic ground state X is brought into an excited electronic state A by absorption of a photon and is relaxed. By absorbing a second photon, an electron is removed and the ionization potential (IP) is exceeded, creating a radical cation

By absorbing photons, electrons in the atoms or molecules of a sample can be excited to such an extent that they leave the atom or molecule and the sample irradiated with electromagnetic radiation is ionized. In the case of gaseous samples, this process is called photoionization . The energy of the absorbed photons must at least correspond to the ionization potential of the atom or molecule.

In the simplest case, a single photon is sufficient to generate the ionization energy; This principle makes use of the atmospheric pressure photoionization (Engl. atmospheric pressure photo ionization , APPI) to Use. If light is radiated with a sufficiently high power density, on the other hand, non-linear absorption processes can occur in which several photons are sequentially absorbed in rapid succession and jointly generate the ionization energy. In this case one speaks of a multiphoton ionization (MPI).

Atmospheric pressure laser ionization is a photoionization method that uses laser light sources whose power density is sufficient to enable multiphoton ionization via a stable, quantum mechanical intermediate state of the molecule or atom. The power density must be so high that a second photon can be absorbed during the lifetime of the excited intermediate state (typically a few nanoseconds). Analogous to one-photon ionization, a radical cation is created:

{\ displaystyle \ mathrm {M {\ xrightarrow [{}] {h \ nu \ (5 \ {\ text {eV}})}} M ^ {*} {\ xrightarrow [{}] {h \ nu \ ( 5 \ {\ text {eV}})}} M ^ {+ \ cdot} + e ^ {-}}}

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

Since the ionization potential of most of the organic molecules that can be used as analytes for such a photoionization method is less than 10 eV, a photon energy of around 5 eV is used with APLI, which corresponds to a wavelength of around 250 nm. This is in the ultraviolet (UV) range of the electromagnetic spectrum.

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

properties

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

Easy connectivity

APLI can be coupled relatively easily with existing atmospheric pressure ion sources, since only the ionizing laser light has to be introduced into the existing AP source, which can easily be achieved through quartz windows.

selectivity

Absorption cross-sections of nitrogen, oxygen and some common LC solvents at the ionization energies of APPI (10 eV) and APLI (5 eV). The relatively strong absorption of the APPI light by components in the ion source (oxygen and solvent vapor) is clearly visible

In order to be able to be ionized directly by 1 + 1-REMPI with UV laser light, a molecule must have a suitable, sufficiently stable, electronic intermediate state and the two electronic transitions of the 1 + 1-REMPI process must be quantum-mechanically permitted. APLI is therefore a selective ionization method.

Polynuclear aromatic molecules in particular meet the spectroscopic conditions, so that APLI is an ideal ionization method for the detection of polycyclic aromatic hydrocarbons (PAH).

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

High sensitivity

Compared to single-photon ionization (APPI) with vacuum ultraviolet radiation (λ = 128 nm), APLI has a significantly greater sensitivity, especially when coupled with liquid chromatography (LC-MS). The reason for this lies on the one hand in the selectivity of APLI. On the other hand, the VUV light hardly penetrates into an AP ion source in this case, since the typical solvents used in liquid chromatography and which are present in the ion source as vapor in LC-MS, strongly absorb this. The UV light used by APLI, however, is hardly absorbed by solvents and ions can be generated in the entire volume.

Independence of ion generation from electrical fields

In contrast to other ionization methods (e.g. electrospray ionization (ESI) , chemical ionization at atmospheric pressure (APCI) ), APLI allows ions to be generated independently of electrical fields, since ions can only be formed in the laser beam. This allows some special methods, such as the recording of an ion signal depending on the ionization location, which is used in the development of ion sources.

swell

M. Constapel, M. Schellträger, O. J Schmitz, S. Gäb, K. J Brockmann, R. Giese, Th Benter: Atmospheric ‐ pressure laser ionization: a novel ionization method for liquid chromatography / mass spectrometry . In: Rapid Communications in Mass Spectrometry . tape 19 , no. 3 , 2005, p. 326-336 , doi : 10.1002 / rcm.1789 .
Stefan Droste, Marc Schellträger, Marc Constapel, Siegmar Gäb, Matthias Lorenz, Klaus J Brockmann, Thorsten Benter, Dieter Lubda, Oliver J Schmitz: A silica ‐ based monolithic column in capillary HPLC and CEC coupled with ESI ‐ MS or electrospray ‐ atmospheric‐ pressure laser ionization MS . In: ELECTROPHORESIS . tape 26 , no. 21 , 2005, p. 4098-4103 , doi : 10.1002 / elps.200500326 .
R. Schiewek, M. Schellträger, R. Mönnikes, M. Lorenz, R. Giese, KJ Brockmann, S. Gäb, Th. Benter, OJ Schmitz: Ultrasensitive Determination of Polycyclic Aromatic Compounds with Atmospheric-Pressure Laser Ionization as an Interface for GC / MS . In: Analytical Chemistry . tape 79 , no. 11 , 2007, p. 4135-4140 , doi : 10.1021 / ac0700631 .

Individual evidence

↑ JB Thiäner, C. Acht: Liquid chromatography-atmospheric pressure laser ionization-mass spectrometry (LC-APLI-MS) analysis of polycyclic aromatic hydrocarbons with 6-8 rings in the environment . In: Analytical and Bioanalytical Chemistry . tape 409 , 2017, p. 1737-1747 , doi : 10.1007 / s00216-016-0121-9 .
↑ Matthias Lorenz, Ralf Schiewek, Klaus J. Brockmann, Oliver J. Schmitz, Siegmar Gäb, Thorsten Benter: The Distribution of Ion Acceptance in Atmospheric Pressure Ion Sources: Spatially Resolved APLI Measurements . In: Journal of the American Society for Mass Spectrometry . tape 19 , no. 3 , 2008, p. 400-410 , doi : 10.1016 / j.jasms.2007.11.021 .

[1] JB Thiäner, C. Acht: Liquid chromatography-atmospheric pressure laser ionization-mass spectrometry (LC-APLI-MS) analysis of polycyclic aromatic hydrocarbons with 6-8 rings in the environment . In: Analytical and Bioanalytical Chemistry . tape 409 , 2017, p. 1737-1747 , doi : 10.1007 / s00216-016-0121-9 .

[DIA_paper-2] Matthias Lorenz, Ralf Schiewek, Klaus J. Brockmann, Oliver J. Schmitz, Siegmar Gäb, Thorsten Benter: The Distribution of Ion Acceptance in Atmospheric Pressure Ion Sources: Spatially Resolved APLI Measurements . In: Journal of the American Society for Mass Spectrometry . tape 19 , no. 3 , 2008, p. 400-410 , doi : 10.1016 / j.jasms.2007.11.021 .