Activated Ion ECD

An alternative approach is plasma ECD [86], in which electrons (0.1-15 eV) are collided with pulsed nitrogen gas prior to the trapping of ions in the ICR cell. The induced plasma conditions result in significant increase in ECD efficiency. A single plasma ECD mass spectrum of carbonic anhydrase ca 29 kDa) showed peaks corresponding to cleavage of 183/253 N-Ca bonds c/116/258 by activated ion ECD. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

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). 

Selenium forms a volatile derivative, piazselenol, which can be subjected to GC analysis (Scheme 5.39). Young and Christian [612] treated selenium with 2,3-diaminonaph-thalene at pH 2.0 and extracted the resulting piazselenol into -hexane. With the use of an ECD, down to 5 10-I° g of selenium could be detected. The procedure, applied to the analysis of selenium in human blood, urine and river water, led to results equivalent to those obtained by neutron activation analysis. Similarly, Nakashima and Toei [613] performed the reaction of selenium (as selenious acid) with 4-chloro-o-phenylenediamine at pH 1 and extracted the derivative into toluene. They reported a detection limit of 0.04 jug. Shimoishi [614] analysed the content of selenium in metallic tellurium by this method. The sample was dissolved in aqua regia, followed by reaction with 4-nitro-o-phenylenediamine and extraction into toluene. Down to 10 ng of selenium could be determined using only a few milligrams of sample. Common ions did not interfere even when present in a large excess. Selenium in marine water was determined after the same derivatization step [615],... 

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. 

Electron capture dissociation (ECD) has recently been developed as an alternative activation method and is now widely used [24,25], The ECD activation method is applied to multiply charged positive ions submitted to a beam of low energy produced by an emitter... 

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. 

Therefore, ECD is a powerful tool for structural analysis of peptides and proteins that is complementary to the other ion activation methods. However, ECD is not compatible with instruments such as ion traps or QTOF. As a consequence, ECD analysis of peptides and proteins is typically performed on FTICR mass spectrometers. 

To bring the power of ECD to ion trap analysers, a new ECD-like activation method has been developed [58]. This method, which is called electron transfer dissociation (ETD), uses gas-phase ion/ion chemistry to transfer an electron from singly charged aromatic anions to multiply charged ions. The mechanism of this method and the observed fragmentation pathways are analogous to those observed in ECD. 

Fragmentation paths yielding the c and z ions after activation of a multiply protonated peptide by ECD. 

Figure 2.2 Morse potential energy curves for the neutral and negative-ion states of anthracene. The vertical electron affinity VEa, adiabatic electron affinity AEa, and activation energy for thermal electron attachment E are shown. The two Ea are 0.68 eV and 0.53 eV observed in ECD data. There will be nine other negative ion curves, yielding a total of thirteen anion curves, four each for the different C—H bonds and a polarization curve. Some of these will be accidentally degenerate.

Dissociative electron capture is observed with hyperthermal electrons in NIMS electron impact experiments. In order for dissociative electron capture to take place with thermal electrons, there must be a dissociative pathway that is accessible by the thermal activation of the neutral molecule or a low-lying negative-ion state. The quantity D(R — Le) — Ea(Le) must be less than about 1.0 eV. This limit has been established empirically. Two types of dissociative thermal electron attachment have been observed in NIMS and ECD. The first occurs by unimolecular dissociation in which there is only one temperature region for many compounds. In the original work a low-temperature low-slope region was observed but unexplained. We now believe this could represent the formation of a molecular ion with an electron affinity of about 0.1 eV. The exact nature of this ion is not known, but it could represent stabilization to an excited state. In Figure 4.8 ECD data are plotted for several... 

Figure 4.8 ECD data plotted as In ftp versus 1,000/7. These alkyl halides dissociate via activation of the molecule. They are designated DEC(l) for dissociative electron capture via activation of the molecule. The slope multiplied by R is equal to the activation energy in the high-temperature region. The low-temperature data were originally not explained, but could be an indication of a low molecular electron affinity. The curves were fit using both dissociation and molecular ion formation. Data from [16-19].

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

In many cases the mere observation of a parent negative ion in mass spectrometry or ion mobility spectrometry is evidence of the positive electron affinity of a molecule. The ECD kinetic model is applicable to the ions observed in NICI experiments so the same quantities measured in the ECD can also be measured with this technique. There is a large body of NIMS data taken at two temperatures for compounds significant to those used in environmental chemistry that can be analyzed to obtain approximate electron affinities and activation energies [10]. 

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