Ionization, chemical reagent

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

A structure-function study of a proton pathway in the y-class carbonic anhydrase from Methanosarcina thermophila was conducted in the work of Tripp and Ferry (2000). Four enzyme glutamate residues were characterized by site-directed mutagenesis. It was shown that Glu 84 and an active site residue, Glu 89, are important for CO2 hydration activity, while external loop residues, Glu 88 and Glu 89 are less important. Glu 84 can be substituted for other ionizable residues with similar pKa values and, therefore, participates in the enzyme catalysis not as a chemical reagent but as a proton shuttle. 

In GC/MS ionization is usually either by electron impact or chemical ionization. Chemical ionization takes place at higher pressures than electron impact ionization (about 1 torr) and results from the collision of the sample molecules with the reagent ions. The reagent ions are themselves produced by electron impact ionization and so chemical ionization is often a two stage process. Chemical ionization produces sample ions close in molecular weight to that of the uncharged molecule (often M-Gf or M+CHj if methane is being used as the... 

Chemical ionization. Chemical ionization spectra result from ion-molecule reaction between the ionic products of a high pressure reagent gas, commonly methane, with a low pressure sample gas. Because of the low abundance of the sample, almost all of the initial ionization by electron impact is of the reagent gas. When methane is ionized at a source pressure of 1 mm Hg, the normal El products CHl and CHs react with neutral CH4 molecules producing a plasma in which CH5 (48% 2) and C2H5 (41% 2) are the principal species available for further ion-molecule reaction ... 

GC/MS analysis of PFBOA-derivatives can be performed by either El or chemical ionization. Chemical ionization is performed in positive ion mode (PICI) using methane as a reagent gas. In this analysis, abundant protonated [M + H]+ ion of acetaldehyde-derivatives at m/z 240, diacetyl-mono derivatives at m/z 282 and of internal standard o-chlorobenzaldehyde derivatives at m/z 336, form, and an abundant formation of [M + H—18]+ ion of acetoin-derivatives at m/z 282, is observed. Mass spectra of acetaldehyde, diacetyl monooxime, acetoin and o-chlorobenzaldehyde PFB-derivatives recorded in the PICI analysis of a standard solution are reported in Figure 1.19. 

Where high resistivity particles are met in practice, the electrical operating characteristics and the difficulties of the reverse ionization phenomenon can be mitigated by the injection of chemical reagents into the flue gases ahead of the precipitator to modify the electrical resistivity. (Other reagents can be used to increase the cohesive properties of easily re-entrained materials if necessary to minimize re-entrainment.)... 

It would be superfluous to review here the story of e aq in the radiation chemistry of aqueous solutions. High energy radiations cause ionizations and the free electrons so generated dissipate their excess energy and are eventually trapped in solvation shells. The discovery of hydrated electrons showed that electrons in water were chemical entities (as distinct from possessing purely physical characteristics) in having diffusion properties, size and sphere of influence, associated ion atmosphere, and reaction rate parameters all of which are comparable to normal chemical reagents. 

Figure 9.3 Schematic of chemical ionization. A reagent gas is present in the source block and is initially ionized by a beam of electrons, which then interacts with sample analyte molecules also in the vapour phase to form stable molecular ion adducts that are repelled into the mass analyzer. The mass of the reagent gas adduct must be corrected for during the analysis.

Ion Trap Analysis with Liquid Chemical Ionization (Cl) Reagents USEPA Method 521... 

Isobar can be a source of positive bias in the determination of Pu isotopic abundance by mass spectrometry despite prior chemical separation of the two elements. In this respect, eluting Pu before U is advantageous (see section Isotopic Analysis by Thermal Ionization Mass Spectrometry, procedures (b)-(d)). Great attention must be placed anyway to minimize sources of blanks due to low levels of natural uranium in most chemical reagents and mass spectrometry filaments. It is therefore still customary to perform Pu measurement by alpha spectrometry in parallel with mass spectrometry. 

Chemical Ionization The absence of a prominent molecular ion in El mass spectra may favor the use of chemical ionization methods coupled with GC separations. The spectra resulting from different chemical ionization modes, reagent gases, and mass analyzers can vary significantly. Positive ion chemical ionization (PICI) of TATP has been reported using methane, isobutene, and ammonia. 

Figure 42. Chemical ionization spectrum (reagent gas isobutane) of TMS-clenbulerol...

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