Infrared-multiphoton irradiation

Farneth s group used infrared multiphoton irradiation of VCP with a CO2-TEA (transversely excited atmospheric) laser to form cyclopentene in low yield (-32%). Unfortunately, the remainder of the VCP isomerized to a mixture of cyclic and acyclic dienes. ... 

Gauthier, J.W. Trautman, T.R. Jacobson, D.B. Sustained Off-Resonance Irradiation for Collision-Activated Dissociation Involving FT-ICR-MS. CID Technique That Emulates Infrared Multiphoton Dissociation. Anal. Chim. Acta 1991,246,211-225. 

FT-ICR instruments are also capable of performing MS" experiments. The most popular method of ion activation is sustained off-resonance irradiation (SORI), where ions are excited to a larger cyclotron radius using rf energy, undergo collisions with a neutral gas pulsed into the cell and dissociate. Other methods are available, including infrared multiphoton dissociation (IRMPD)65 and electron capture dissociation (ECD)66 which is of particular value in glyco-peptide analysis (Section VIA). 

Output from both gated continuous wave and pulsed carbon dioxide lasers has been used to desorb ions from surfaces and then to photodissociate them in a Fourier transform ion cyclotron resonance mass spectrometer. Pulsed C02 laser irradiation was most successful in laser desorption experiments, while a gated continuous wave laser was used for a majority of the successful infrared multiphoton dissociation studies. Fragmentation of ions with m/z values in the range of 400-1500 daltons was induced by infrared multiphoton dissociation. Such photodissociation was successfully coupled with laser desorption for several different classes of compounds. Either two sequential pulses from a pulsed carbon dioxide laser (one for desorption and one for dissociation), or one desorption pulse followed by gated continuous wave irradiation to bring about dissociation was used. 

ESI mass spectra after the addition of 49 (or 50) to 51(PF6)2 show the hexameric clusters 51 (49)6 and 51 (50)6 as the dominating signals (Figure 3.24). These ions were irradiated in an infrared multiphoton dissociation experiment (I RMPD) with IR radiation from a C02 laser at different time intervals. The observed fragmentation behavior was exactly that expected for the hexameric capsule. 

Selective isomerization of the ( )-alkene to (Z)-alkene has also been carried out via infrared multiphoton excitation [70]. Upon IR laser irradiation at MOOO/cm of 2-butene 19a and 2-pentene 19b, essentially quantitative contrathermodynamic conversion (i>to-Z) can be achieved (Sch. 9). It is concluded that small differences in cross-section may be amplified in the multiphoton up-pumping process. However, the quantum yield was too small to measure (<10-6). 

Gauthier, J.W., Trautman, T.R. and Jacobson, D.B. (1991) Sustained off-resonance irradiation for collision-activated dissociation involving Fourier transform mass spectrometry. Collision-activated dissociation technique that emulates infrared multiphoton dissociation. 

Other fragmentation techniques have been introduced [108]. Some of these, e.g., sustained off-resonance irradiation (SORl) and infrared multiphoton dissociation (IRMPD), provide similar fragmentation as in CID, i.e., preferential backbone cleavages at the peptide amide bond (b- and y-ions). Others like electron-capture dissociation (ECD) [109-110] induce different fragmentation reactions, i.e., the formation of c- and z -ions due to cleavage of N-C bonds. 

Initial results with the fragmentation of multiple-eharge protein ions in FT-ICR-MS were achieved, either by in-source CID [159], sustained off-resonance irradiation (SORI) [160], or infrared multiphoton dissoeiation (IRMPD) [161], and later on with electron-capture dissociation [109-110]. 

Another ion activation method that is well suited for identification and sequence analysis of phosphopep-tides in the positive and negative polarity modes is infrared multiphoton dissociation (IRMPD).105,112 In this technique, phosphopeptides are irradiated with 10.6 pm photons emitted from a C02 laser. The phosphate group behaves like a chromophore for these photons, allowing evenly distributed cleavages in the peptide chain and more sequence coverage than the CID technique.105... 

At the same time Kaldor, et al. (25) reported the observation of infrared multiphoton photodissociation of UFg using a one color CF4 laser irradiation source (16 pm) coincident with V3. A dissociation threshold and yield were estimated to be in the range of those found for SFg. The preliminary report was recently followed by a more extensive paper from the Exxon group (26) in which infrared multiphoton excitation and dissociation in the V3 mode was further described. In addition, two color excitation (16 ym) and dissociation (10.6 ym) experiments similar to those described by Wittig were described. 

Fragmentation of peptide and protein ions in FT-ICR mass spectrometry may be induced by sustained off-resonance irradiation collision-induced dissociation (SORI-CID) [28], infrared multiphoton dissociation (IRMPD) [29,30], blackbody infrared radiative dissociation (BIRD) [31,32], surface-induced dissociation (SID) [33,34], and electron capture dissociation (ECD) [35,36]. These techniques are true MS/MS techniques in which the precursor ion is isolated prior to fragmentation. Additional techniques in which ions are not isolated but fragmented before they... 

When NH3 and ND3 were photolyzed in the infrared region (CO2 laser) electronically excited fragments were also observed [84, 85]. In the infrared multiphoton dissociation (IRMPD) of ND3, the imidogen radical is formed in both the spin-forbidden (X and the spin-allowed (a A) states [86, 87]. In a CO2 laser pyrolysis experiment, in which a mixture of SFg, CF4, and NH3 was irradiated, ground-state NH radicals were produced [88]. 

Figure 16.1. Typical ranges of activation times associated with various activation methods in tandem mass spectrometry. EID, electron-induced dissociation SID, surface-induced dissociation CA, collisional activation (beam), typical conditions used in a beam-type tandem mass spectrometer ICR-SORI, ion cyclotron resonance sustained off-resonance irradiation cw IRMPD, continuous-wave infrared multiphoton dissociation. (Reproduced from Ref. 15 with permission from John Wiley Sons.)...

Gauthier, J. W. IfeAurrMAN, T. R. Jacobson, D. B. Sustained off-resonance irradiation for CAD involving FTMS CAD technique that emulates infrared multiphoton dissociation. Anal Chim. Acta 1991,246, 211 225. 

Vibrationally excited [CF30] ions (generated from the dissociation of CF3OOCF3 using an electron beam pulse) were trapped whilst irradiating with infrared light from a CO laser. Multiphoton dissociation results in the following reaction [1034] ... 

Various forms of radiation have been used to produce ions in sufficient quantitites to yield neutral products for subsequent analysis. In principle, it should be possible to use intense beams of UV below ionization threshold for this purpose. To date, however, efforts to collect neutrals from resonant multiphoton ionization (REMPI) have not succeeded. In one experiment, 1 mbar of gaseous -propyl phenyl ether was irradiated at room temperature with a 0.1 W beam of 266 nm ultraviolet (from an 800 Hz laser that gives 8 n pulses) concurrent with a 0.5 W beam at 532 nm. The beams were intense enough not only to ionize the ether in the mass spectrometer, but also to excite it so that it expels propene. After several hours of irradiation < 10% of the starting material remained. Production of carbon monoxide and acetylene (decomposition products of the phenoxy group) could be detected by infrared absorption spectroscopy, but the yield of neutral propene (as measured by NMR spectroscopy) was infinitesimal. 

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