Ions/ionization thermal, alkali metals

Several theories have been proposed to explain the mechanisms involved in an AFID system (31). In general, thermal energy is required to atomize a particular alkali metal salt. The alkali metal atoms formed ionize and are subjected to an electric field. This produces a current proportional to the number of ions. The presence of halogen, phosphorus, and even nitrogen enhance the signal. The system is complex and does not lend itself to a complete theory as intricate surface phenomena are possible. In addition, there is speculation that photochemical processes occur and realization that combustion products formed in the flame can interact to form a multitude of species compound the difficulty. It has been proven that the process does depend on thermal energy and not strictly speaking on the products of combustion. For this reason many researchers prefer the term thermionic ionization. 

The importance of the work function and temperature of the surface, the ionization potential for positive ion emission, and the electron affinity for negative ion emission are well established for conditions in which the S-L equations are valid. Experimentally, the IP and EA are also important for thermal emitters. For example, the alkali metals all have low IPs and are emitted in good yields from the zeolites impregnated with the corresponding alkali metal. The halide and perrhenate anions all have high EAs and are emitted in good yield from certain of the rare earth oxides. The temperature is also quite important, but possibly not for the same reasons as for the S-L conditions. Under S-L conditions a higher temperature is more likely to strip an electron or to add an electron to an atom. 

Anionic Poiymerization of Cyclic Siloxanes. The anionic polymerization of cyclosiloxanes can be carried ont in the presence of a wide variety of strong bases such as hydroxides, alcoholates, or silanolates of alkab metals (1,102,110). Commercially, the most important catalyst is potassiiun silanolate (98). The activity of the alkali metal hydroxides increases in the following seqnence LiOH < NaOH < KOH < CsOH, which is also the order in which the degree of ionization of their ion pair increases (136). Another important class of catalysts is tetraalky-lammonium and tetraalkylphosphonium hydroxides, and silanolates (137-139). These catalysts imdergo thermal degradation when the polymer is heated above the catalyst decomposition temperature (typically >150°C), producing volatile... 

Rapid-heating (or flash evaporation) techniques have been explored with ionization methods such as El and Cl in the analysis of thermally labile compounds [5,6], Their principles are based upon that if the analyte is rapidly heated, intact molecules may evaporate before decomposition takes place. Davis et al. reports that, by this technique, alkali metal attachment (cationization) of sodium benzoate and sodium acetate occirrs, giving rise to cationized molecular ions (e.g., [M + Na]+) and other cluster ions similar to those produced by desorption ionization processes [7, 8], In these ejqreriments, the molecular ions were formed by electron impact of salt molecules or clusters of sample molecules and salts in the gas phase. 

In experiments aiming at a search for emitters, which surface-ionize organic compounds more efficiently [20] (based on the understanding that the work func tion of the surface is greatly increased by the chemisorption of electronegative gases [21]), a considerable increase in alkali metal ion currents is always observed, when chlorine (or fluorine) is introduced into a mass spectrometer with a thermal ionization source. This phenomenon is well interpreted as the so-called stimulated surface ionization [22] in which the exothermic reaction on the surface is responsible for the ionization. This is another interesting example of RSI. 

As for all mass spectrometric techniques, Umitations arise in part from the possibility of generating gas-phase ions. Usually, molecules are vaporized and allowed to react with metal ions. In early studies, alkali metal cations were produced by thermionic emission from a filament coated with an appropriate melt of alkali metal oxide (or carbonate) with silica and alumina. Most recently, electrospray ionization (ESI), one of the atmospheric pressure ion sources mentioned previously, has been used to form gas-phase adducts directly, avoiding some problems associated with volatility, thermal stability, and so on [105],... 

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