Ion source chemical ionization

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

Representative mass spectral conditions (negative chemical ionization) ion source temperature, 150°C ionizing current, 0.20 mamp electron energy, 70 eV methane reagent gas (source pressure 0.5 to 1 torr). 

Fig. 7.1. Schematic layout of a chemical ionization ion source. Adapted from Ref. [14] by permission. Springer-Verlag Heidelberg, 1991.

The most common ion sources used in GC/MS are clcciron-impaci ionization and chemical ionization, Ion sources for mass spectrometry are discussed in detail in Section 20IT The most common mas.s analyzers are quadrupole and ion-trap analyzers. These analyzers arc described in Sections llB-2 and 20C-.T Time-of-flighl mass analyzers are al.so used, but not as frequently as quadrupolcs and ion traps. 

The detectors used in HTGC are, apart from the highly versatile flame ionization detector (FID), the phosphorus/nitrogen-selective alkali-flame ionization detector (AFID), the atomic emission detector, the inductively coupled plasma (ICP)-mass spectrometer, and, last but not least, mass spectrometers with electron impact and chemical ionization ion sources (EI/CI-MS). 

FIGURE 21.2 Atmospheric pressure chemical ionization ion source with counterflow introduction. ... 

Incos 2400 or equivalent with combined EI/CI (electron inpace/chemical ionization ion source)... 

Electron transfer dissociation (ETD) is an ECD-like method with most of the same characteristics [21]. Like ECD, ETD yields abundant peptide backbone c- and z-type ions while often retaining such labile groups as peptide O/TV-glycosylation and phosphorylation [22]. Unlike ECD, ETD can be performed in the presence of an RF field. Here, anions created in a chemical ionization (Cl) source (see Section 2.1.7) are used as electron donors but the fragmentation pathways are essentially the same as for ECD. Commercial linear QIT instruments have recently become available with the ETD option. 

Chemical ionization (Cl) sources (48, 49) use electron bombardment of a reagent gas at higher pressures than normally found in a mass spectrometer ion source, i.e., torr. Sample ionization follows via an ion-molecule reaction, often accompanied by a proton transfer to yield a quasi-molecular ion ... 

Prosek et al. also in 2004 [6] described construction of a new online computer-controlled TLC-MS approach for collection of sample and programmed loop injection into an LCQ MS" ion trap mass spectrometer with an atmospheric pressure chemical ionization (APCI) source. The elution head was positioned manually, and knives on the bottom cut around the zone of interest. One pump applied eluent onto the plate and a second pump removed it. Morlock and Schwack [4] stated that this interface was more laborious and less sensitive than the ChromeXtractor because the recovery rate was less than 10%, and its use was not reported further in the literature. 

The type of ion source used in an SID experiment depends on the projectile ion under study. For instance, if volatile, small organic compounds are of interest then an electron ionization (El) and/or chemical ionization (Cl) source will be sufficient. However, if one wishes to study biological compounds then a spray or desorption ionization method will be required, such as electrospray (ESI), atmospheric pressure chemical ionization (APCI), desorption chemical ionization (DCI), or matrix-assisted laser desorption ionization (MALDI). 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

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