Electron ionization negative ions produced

Negative Ion Chemical Ionization Negative ions are produced under ci conditions by electron capture. Under the higher pressure conditions of the ci ion source, electrons, both primary (those produced by the filament) and secondary (produced during an ionization event), undergo collisions until they reach near-thermal energies. Under these conditions, molecules... 

Negative ions can be formed by electron capture due to the presence of thermalized electrons produced by Penning ionization or by surface Penning ionization. Negative ions can also be obtained by reactions of analyte molecules with negative ions formed from atmospheric water and oxygen to produce the deprotonated molecule. 

The use of negative ions produced by the electron capture ionization may constitute another alternative for increasing sensitivity. Stapleton and Bowie [226] applied this method to the study of the structures of the nitrobenzoyl derivatives of various natural amino acids. All the species studied led to clear-cut and interpretable fragmentations, enabling the structure of the amino acid in question to be determined. 

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

Electron Capture Detector In the electron capture detector (ECD), a beta emitter such as tritium or 63Ni is used to ionize the carrier gas. Electrons from the ionization migrate to the anode and produce a steady current. If the GC effluent contains a compound that can capture electrons, the current is reduced because the resulting negative ions move more slowly than electrons. Thus, the signal measured is the loss of electrical current. The ECD is very sensitive to materials that readily capture electrons. These materials frequently have unsaturation and electronegative substituents. Because the ECD is sensitive to water, the carrier gas must be dry. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

There have been fewer studies of the reactions of M ions with potential ligand molecules. Laser ablation, which has been the major ionization source for the production of bare metal ions, produces very few negative ions. Electron impact with low-energy electrons (12 eV) of metal carbonyls has been used to produce [Co(CO)4]- and Fc( CO)4 from Co2(CO)8 and Fe(CO)5. Collision-induced dissociation of these two anions produced Co- and Fc, which could be isolated. Both Co- and Fe were reacted with H2S, aliphatic thiols, aromatic thiols, CS2, and disulfides (153). Reactions with H2S gave the metal monosulfide anion [MS]-, which reacted with H2S by two pathways. 

We have recently described another spectroscopic rnethod for observing IM reactions at atmospheric pressure that utilizes the photodetachment-modulated electron capture detector (PDM-ECD) as a means of monitoring the negative ions either consumed or produced in an IM reaction. The reaction of interest is made to occiu in a steady-state flow-through reactor in which ionization of the buffer gas is continuously caused by a Ni-on-Pt foil beta emitter. A chopped light beam of... 

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