Infrared multiphoton dissociation

Infrared multiple photon dissociation (IRMPD) is a technique used in mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original (parent) molecule.^[1]

Infrared multiple photon dissociation

Acronym	IRMPD
Other techniques
Related	Blackbody infrared radiative dissociation Electron capture dissociation Collision-induced dissociation

How it works

An infrared laser is directed through a window into the vacuum of the mass spectrometer where the ions are. The mechanism of fragmentation involves the absorption by a given ion of multiple infrared photons. The parent ion becomes excited into more energetic vibrational states until a bond(s) is broken resulting in gas phase fragments of the parent ion. In the case of powerful laser pulses, the dissociation proceeds via inner-valence ionization of electrons.^[2][3]

IRMPD is most often used in Fourier transform ion cyclotron resonance mass spectrometry.^[4]

Infrared photodissociation spectroscopy

By applying intense tunable IR lasers, like IR-OPOs or IR free electron lasers, the wavelength dependence of the IRMPD yield can be studied.^[5][6] This infrared photodissociation spectroscopy allows for the measurement of vibrational spectra of (unstable) species that can only be prepared in the gas phase. Such species include molecular ions but also neutral species like metal clusters that can be gently ionized after interaction with the IR light for their mass spectrometric detection.^[7]

Analytical applications

The combination of mass spectrometry and IRMPD with tunable lasers (IR ion spectroscopy) is increasingly recognized as a powerful tool for small-molecule identification.^[8] Examples are metabomics, where biomarkers are identified in body fluids (urine, blood, cerebrospinal)^[9] and forensic sciences, where isomeric designer drugs were identified in seized samples.^[10]

Isotope separation

Due to the relatively large differences in IR absorption frequencies that are due to different resonance frequencies for molecules containing different isotopes, this technique has been suggested as a way to perform Isotope separation with difficult-to-separate isotopes, in a single pass. For example, molecules of UF₆ containing U-235 might be ionized completely as a result of such a laser resonance, leaving UF₆ containing the heavier U-238 intact.

See also

• Infrared spectroscopy
• Tandem mass spectrometry
• Photodissociation
• Blackbody infrared radiative dissociation
• Electron capture dissociation
• Collision-induced dissociation is another, more common, method to fragment gas phase ions.

References

1. Little DP, Speir JP, Senko MW, O'Connor PB, McLafferty FW (1994). "Infrared multiphoton dissociation of large multiply charged ions for biomolecule sequencing". Anal. Chem. 66 (18): 2809–15. doi:10.1021/ac00090a004. PMID 7526742.
2. Talebpour A, Bandrauk AD, Yang J, Chin SL (1999). "Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse". Chemical Physics Letters. 313 (5–6): 789–794. Bibcode:1999CPL...313..789T. doi:10.1016/s0009-2614(99)01075-1.
3. Talebpour A, Bandrauk AD, Vijayalakshmi K, Chin SL (2000). "Dissociative ionization of benzene in intense ultra-fast laser pulses". Journal of Physics B: Atomic, Molecular and Optical Physics. 33 (21): 4615–4626. Bibcode:2000JPhB...33.4615T. doi:10.1088/0953-4075/33/21/307.
4. Laskin J, Futrell JH (2005). "Activation of large ions in FT-ICR mass spectrometry". Mass Spectrometry Reviews. 24 (2): 135–67. Bibcode:2005MSRv...24..135L. doi:10.1002/mas.20012. PMID 15389858.
5. Polfer NC, Oomens J (2007). "Reaction products in mass spectrometry elucidated with infrared spectroscopy". Physical Chemistry Chemical Physics. 9 (29): 3804–17. Bibcode:2007PCCP....9.3804P. doi:10.1039/b702993b. PMID 17637973. S2CID 23485300.
6. Fielicke, André (2023). "Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy". Chemical Society Reviews. 52 (11): 3778–3841. doi:10.1039/D2CS00104G. ISSN 0306-0012.
7. Gruene P, Rayner DM, Redlich B, van der Meer AF, Lyon JT, Meijer G, Fielicke A (2008). "Structures of Neutral Au7, Au19, and Au20 Clusters in the Gas Phase". Science. 321 (5889): 674–6. Bibcode:2008Sci...321..674G. doi:10.1126/science.1161166. hdl:11858/00-001M-0000-0010-FC2A-A. PMID 18669858.
8. Martens J, van Outersterp RE, Vreeken RJ, Cuyckens F, Coene KL, Engelke UF, et al. (January 2020). "Infrared ion spectroscopy: New opportunities for small-molecule identification in mass spectrometry - A tutorial perspective". Analytica Chimica Acta. 1093: 1–15. doi:10.1016/j.aca.2019.10.043. hdl:2066/212342. PMID 31735202.
9. Martens J, Berden G, van Outersterp RE, Kluijtmans LA, Engelke UF, van Karnebeek CD, et al. (June 2017). "Molecular identification in metabolomics using infrared ion spectroscopy". Scientific Reports. 7 (1): 3363. doi:10.1038/s41598-017-03387-4. PMC 5469762. PMID 28611404.
10. van Geenen FA, Kranenburg RF, van Asten AC, Martens J, Oomens J, Berden G (February 2021). "Isomer-Specific Two-Color Double-Resonance IR²MS³ Ion Spectroscopy Using a Single Laser: Application in the Identification of Novel Psychoactive Substances". Analytical Chemistry. 93 (4): 2687–2693. doi:10.1021/acs.analchem.0c05042. PMC 7859929. PMID 33470107.