Infrared multiphoton dissociation
Infrared multiphoton dissociation (IRMPD) is a technique used in mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 to fragment molecules in the gas phase usually for structural analysis of the original (parent) molecule.

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
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 of multiple infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s. The parent ion becomes excited into more energetic vibration
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

al states until a bond(s) is broken resulting in gas phase fragments of the parent ion.

IRMPD is most often used in Fourier transform ion cyclotron resonance
Fourier transform ion cyclotron resonance
Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer for determining the mass-to-charge ratio of ions based on the cyclotron frequency of the ions in a fixed magnetic field...

 mass spectrometry.

IRMPD spectroscopy

By applying intense tunable IR lasers, like IR-OPO
Optical parametric oscillator
An optical parametric oscillator is a parametric oscillator which oscillates at optical frequencies. It converts an input laser wave into two output waves of lower frequency by means of second order nonlinear optical interaction. The sum of the output waves frequencies is equal to the input wave...

s or IR free electron laser
Free electron laser
A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...

s, the wavelength dependence of the IRMPD yield can be studied. This IRMPD 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.

See also

  • Infrared spectroscopy
    Infrared spectroscopy
    Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...

  • Tandem mass spectrometry
    Tandem mass spectrometry
    Tandem mass spectrometry, also known as MS/MS or MS2, involves multiple steps of mass spectrometry selection, with some form of fragmentation occurring in between the stages.-Tandem MS instruments:...

  • Photodissociation
    Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule....

  • Blackbody infrared radiative dissociation
    Blackbody infrared radiative dissociation
    In mass spectrometry blackbody infrared radiative dissociation is a method for fragmenting gas phase ions for tandem mass spectrometric analysis . BIRD uses the light from black body radiation to thermally excite the ions until a bond breaks...

  • Electron capture dissociation
    Electron capture dissociation
    Electron-capture dissociation is a method of fragmenting gas phase ions for tandem mass spectrometric analysis . ECD involves the direct introduction of low energy electrons to trapped gas phase ions...

  • Collision-induced dissociation
    Collision-induced dissociation
    In Mass spectrometry, Collision-induced dissociation , referred to by some as collisionally activated dissociation , is a mechanism by which to fragment molecular ions in the gas phase. The molecular ions are usually accelerated by some electrical potential to high kinetic energy and then allowed...

     is another, more common, method to fragment gas phase ions.
  • Isotope separation
    Isotope separation
    Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...

    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 separate 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.
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