Other Methods of Ion Activation

Low-energy CID spectra are measured using triple quadrupole, ion trap, ICR or hybrid instruments. For tandem mass spectrometers in space, the collision chamber is most often a quadrupole in the RF mode only, which allows one to focus the ions that are angularly dispersed by the collision. The pressure difference between the collision cell and the rest of the analyser is obtained through differential pumping. 

At low energy, the ions excitation energy is mostly of a vibrational nature [20] because the interaction time between an ion with mass 200 Da at an energy of 30 eV with a target of a few angstroms is about 10 14 s, corresponding approximately to the bonds vibration period. 

The nature of the collision gas is more important than it is for the high-energy collisions. Normally, heavier gases such as argon, xenon or krypton are preferred because they allow the transfer of more energy. 

Another activation method, which is performed without gas, uses collisions with a solid surface [22,23], This method is called surface-induced dissociation (SID). In practice, 

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. 

It has been shown that SID spectra are very similar to high as well as to low-energy CID spectra. [128] The absence of collision gas presents an advantage of 

SID over CID because losses of resolution due to high background pressure are avoided. SID has been successfully employed for structure elucidation of proto-nated peptides, [131] and a SID mode of operation has even been implemented with a quadrupole ion trap. [132] However, apart from the quadrupole ion trap SID requires substantial modifications of the instrumental hardware circumstances that made SID lag behind the countless applications of CID. 

The energy received from multiple photon absorption may also be used to activate and dissociate otherwise stable gaseous ions [133] 

For an ion, the cross section for electron capture (EC) roughly increases with the square of the ionic charge. [137] This makes multiply charged ions as produced by electrospray ionization (ESI, Chap. 11) the ideal targets for this process, e.g.  

Note As one electron charge is neutralized upon EC, the precursor ion for ECD must at least be a doubly charged positive even-electron ion to yield a singly charged radical ion for subsequent dissociation. 

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Following the development of the hybrid Q-FTMS instruments, a linear ion trap (LIT) was used in place of the mass-filtering quadrupole and storage multipole. This hybrid-FTMS configuration is found in some commercial FTMS instruments, and an example of this instrument configuration is shown in Figure 11.20C. Ions pass from the ESI source into the LIT where they are stored, and then they are passed down to the analyzer cell. With the LIT, ion accumulation, mass selection, and dissociation (including MS") can be performed within the LIT instead of the analyzer cell. CAD is typically performed within the LIT, while other methods of ion activation, infrared multiphoton dissociation (IRMPD), and electron capture dissociation (ECD) (vide infra) are performed inside the analyzer cell. One unique... 

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). 

There are many other methods of preparing active synthetic silica-alumina catalysts. A fair catalyst can be made by impregnating dried silica gel with an aluminum compound which is easily converted to the oxide by calcination, e.g., A1(NC>3)3. A preferred impregnation technique is to soak a sodium-free silica hydrogel in a solution of an aluminum salt and to follow this with an aqueous ammonia treatment to precipitate the hydrous alumina on the silica (Thomas, 16 Ryland and Tamele, 17). It should be noted that silica hydrogel can easily be freed of sodium ions by water washing, since it is not a zeolite. Exceptionally pure silica-alumina composites can also be prepared by the hydrolysis of mixtures of ethyl orthosilicate and aluminum alkoxides (Thomas, 18). 

A thkd method utilizes cooxidation of an organic promoter with manganese or cobalt-ion catalysis. A process using methyl ethyl ketone (248,252,265—270) was commercialized by Mobil but discontinued in 1973 (263,264). Other promoters include acetaldehyde (248,271—273), paraldehyde (248,274), various hydrocarbons such as butane (270,275), and others. Other types of reported activators include peracetic acid (276) and ozone (277), and very high concentrations of cobalt catalyst (2,248,278). 

There are other methods of preparation that iavolve estabhshing an active phase on a support phase, such as ion exchange, chemical reactions, vapor deposition, and diffusion coating (26). For example, of the two primary types of propylene polymerization catalysts containing titanium supported on a magnesium haUde, one is manufactured usiag wet-chemical methods (27) and the other is manufactured by ball milling the components (28). 

In addition, some metals may be determined by other methods, including ion-selective electrode, ion chromatography, electrophoresis, neutron activation analysis, redox titration, and gravimetry. Atomic absorption or emission spectrophotometry is the method of choice, because it is rapid, convenient, and gives the low detection levels as required in the environmental analysis. Although colorimetry methods can give accurate results, they are time consuming and a detection limit below 10 pg/L is difficult to achieve for most metals. 

Given the problems inherent to the electrode measurement of in soil solution, it would seem desirable to consider other methods of estimating Eh. Theoretically, one could analyze the soil solution for the reduced and oxidized species of a redox couple, say dissolved Fe " and Fe, and use the Nernst relation (equation 7.2) to calculate Eh. Usually, however, there are difficulties with this approach. In the case of the Fe couple, Fe solubility in all but very acid soil solutions is extremely low (below detection), so that an assumption must be made about the activity of the free Fe " ion. It might reasonably be assumed that the solubility product of Fe oxide limits this activity, but Fe oxides have a rather wide range of solubilities depending on oxide crystallinity, structure, and purity. A better approach to measuring E would be to use an indicator chemical that undergoes reversible electron transfer with natural redox couples in soil solution, that is,... 

Other methods of activation of stable ions are used in structural analysis. The ions can be collided with a surface (surface-induced dissociation, SID), reacted with electrons, or subjected to photodissociation. ... 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

Other methods of activating organostannanes have been reported in literature. These methods capitalize on the use of fluoride ions and the formation in situ or not of activated hypervalent stannate intermediates. For instance, Garcia Martinez has used the hypervalent tin reagent tetrabutylammonium difluorotribenzylstannate 24 (prepared in quantitative yield from BnsSnF) in the synthesis of unsymmetrical diarylmethanes... 

The resuspended and formulated Fraction II precipitate normally contains some aggregated IgG and trace substances that can cause hypotensive reactions in patients, such as the enzyme prekail ikrein activator (186). These features restrict this type of product to intramuscular adininistration. Further processing is required if products suitable for intravenous adininistration are required. Processes used for this purpose include treatment at pH 4 with the enzyme pepsin [9001-75-6] being added if necessary (131,184), or further purification by ion-exchange chromatography (44). These and other methods have been fiiUy reviewed (45,185,187,188). Intravenous immunoglobulin products are usually suppHed in the freeze-dried state but a product stable in the solution state is also available (189). 

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