Chemical ionization category

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

Ionization methods such as electron impact, chemical ionization, desorption chemical ionization, and negative-ion chemical ionization are all based on ionization of gas-phase samples and, thus, fall within the first category of gas-phase ionization. 

Ionization methods such as electrospray, ionspray, and atmospheric pressure chemical ionization, in which the analyte is sprayed at atmospheric pressure into an interface to the vacuum of the MS ion source, constitute the category of the atmospheric pressure ionization. 

API The atmospheric pressure ionization (API) source is the most common category of source for LC-MS analysis, in which ionization is performed outside of the high-vacuum region of the mass spectrometer. Electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources are both examples of API sources. 

Interferences in the flame can be classified into four categories chemical, ionization, physical and spectral. 

Pesticides of this category (Fig. 10, Table 3) do not ionize significantly in aqueous systems and vary widely in their chemical composition and properties (i.e., water solubility, polarity, molecular volume, and tendency to volatilization). 

Semi-metals or metalloids are a very small group of elements found in the periodic table of elements along the zig-zag line that distinguishes metals from non-metals and is drawn from between boron and aluminum to the border between polonium and astatine. Elements to the upper right of this line are nonmetals while metals are to the lower left. Together with metals and nonmetals, metalloids form one of the three categories of chemical elements as classified by ionization and bonding properties [1, 2]. 

Electron spectroscopy can be divided Into several categories. These would Include X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), Ultraviolet Photoelectron Spectroscopy (DPS) and Auger Electron Spectroscopy (AES). Other electron spectroscopies Include Penning Ionization and Ion neutralization. XPS uses soft X rays as the... 

When the electric field E in the colloid is uniform, the conductivities can be simply determined from the measured current densities I, as a = I/E. The transition from LC into the IC regime can be somewhat arbitrarily set at a, = 10- n lm-l LC colloids can be prepared by chemical attachment to the particles of charge control agent (cca) or other ionizable species or by irreversible physical adsorption of cca onto particle surfaces. Typical physically adsorbed colloids will have some cca remaining in solution at dynamic equilibrium and, as a result, will fall generally into the category of IC colloids. 

Generally, we can calculate the hydrogen ion concentration or pH of an acid solution at equilibrium, given the initial concentration of the acid and its value. Alternatively, if we know the pH of a weak acid solution and its initial concentration, we can determine its K. The basic approach for solving these problems, which deal with equilibrium concentrations, is the same one outlined in Chapter 14. However, because acid ionization represents a major category of chemical equilibrium in aqueous solution, we will develop a systematic procedure for solving this type of problem that will also help us to understand the chemistry involved. 

Other surface reactions also can be anticipated. Surface acidic or basic structures can exist in an ionized and/or neutral form. The ratio of the ionized to neutral form will depend strongly on a material s environment and previous history. Also, any compound with ionic charges can complex or couple with many ions and some neutral molecules even neutral polymers can have strong, rather specific interactions with ions (15). Finally, molecules such as S02 and NH3 commonly are found in the urban and/or laboratory environment. Such reactive molecules can, again, completely alter the nature of a surface. Of course, some reactions are purposely performed on surfaces to alter their properties. A large number of grafting and chemical reactions come under this category. Such reactions are described elsewhere (3). 

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