IRMPD dissociation

One would like to draw quantitative conclusions about the relative abundances of the conformations from the sizes of the peaks in Fig. 2 (upper trace), but this is xmjustifiable for several reasons. For one thing, the one-photon optical absorption strengths of the different peaks as calculated quantum-mechanicaUy do not correspond accurately to the effective multiphoton absorption cross sections appropriate to the IRMPD process [8]. For another, as clearly demonstrated by Prell et al. [48] among others, the time course of IRMPD dissociation can differ widely for different... 

BIRD. The first requirement is that since BIRD is a very slow-heating method the observation time has to be long (seconds or minutes). Second, the pressure has to be low (<10-6 torr) so that the risk of dissociation due to collisions with the background gas is negligible. There are close similarities between BIRD and another slow-heating method, IRMPD. 

The activation step can alternatively be performed without gas by means of infrared multiphoton dissociation (IRMPD) or electron capture dissociation (BCD) (Chap. 2.12.2). Both IRMPD and BCD, solely require storage of the ions during their excitation by photons or electrons, respectively. It is one of the most charming properties of FT-ICR-MS/MS that even the accurate mass of the fragment ions can be determined. [216,217]... 

Figure 13. Infrared emission following the dissociation of CH2 = CFC1. (a) A spectrum taken at 10/is after 193-nm photolysis of 3-mTorr precursor by the CS method [47], (b) Emission produced following IRMPD of the precursor, in this case at 10 /is delay at a pressure of 40 mTorr using the SS method [108], Emission from HF (v1 — 1-5) is observed, and in both sets of measurements (with lower SNR than shown in the figure), data were taken at a combination of pressure of precursor and delay times such that rotational, as well as vibrational, relaxation was negligible. Figure 13a reproduced with permission from Ref. 47.

Figure 14. Spectra of the products of IRMPD of CH2CFC1 taken under conditions of higher decomposition yield than in Figure 13, brought about by addition of 1.6Torr Ar to 40mTorr of the precursor. At the time of observation, 45

Use of IRMPD to dissociate precursor silanes can initiate rather complex processes. The mechanism presented in equation 116 can explain the formation of both ground-state singlet SiH2 and excited-state singlet 1D Si atoms396. This mechanism is analogous to the phenylcarbene rearrangement397 and has not been found for vibrationally unexcited... 

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). 

Zuckermann et al. [119] studied IR multiphoton dissociation (IRMPD) of RDX and HMX in a supersonic jet. A C02 laser was used for dissociation, with a pulsed frequency-doubled dye laser tuned to 280nm to excite the A 22(v = 1) X2 (v = 0) transition... 

Two other ion activation methods were developed to replace the gas molecules as targets by laser beams (photodissociation or infrared multiphoton dissociation IRMPD) or by electron beams (electron capture dissociation ECD). These two methods can be applied to ions that are trapped during their excitations by photons or electrons, respectively. Thus, they are most often used with ion trap or ICR analysers because the residence time and the interaction time are longer. 

The fragmentation of peptides can also be obtained by FTICR instruments. Besides the most commonly used activation method, namely CID, the activation can alternatively be performed without gas by infrared multiphoton dissociation (IRMPD) and electron capture dissociation (ECD). These methods fragment peptide ions in the ICR cell by emitting a laser beam or electron beam, respectively. 

Fragmentation of peptides can also be observed with FTICR instruments. Infrared multiple photon dissociation (IRMPD) and electron capture dissociation (ECD) have been introduced as two alternative dissociation methods to the low-energy CID method. The IRMPD method produces many fragments that make the spectrum very complex and difficult to interpret. Some of the fragment types observed with IRMPD are b and y type ions or these ions that have lost ammonia or water. However, most of them are not these types of fragment ions. 

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