Gas-phase methods

LC operates in the liquid phase, while MS is a gas-phase method, so it is not a simple matter to connect the two. An interface is needed to pass separated components of a mixture from the LC to the MS. With an effective interface, LC/MS becomes a very powerful analytical technique. 

J. Tranchant, IndChimBelge 32 (Spec No), 601-5 (1967) CA 70, 49146 (1969) A gas-phase method of chromatographic analysis is described for measuring stabilizers and some of their derivatives, as well as a method for the chromatographic sepn in thin layers of nitro and... 

The crystal growth of metal borides by gas-phase methods permits preparation of products at moderate T (1000-1500°C). This is an important advantage since most borides melt at high T (ca. 3000°C), which makes their crystal growth from melts difficult. In addition, the gas-phase methods lead to the formation of single crystals and solid films of incongruently melting borides. 

The gas-phase methods usually applied to the crystal growth of borides are two chemical vapor deposition (CVD) and chemical vapor transport (CVT). 

Solvation enthalpy data for neutral short-lived species, like radicals, are even more scant than for long-lived stable molecules. They can only be experimentally determined through indirect methods, namely, by comparing the enthalpies of reactions of those species in solution and in the gas phase. The former are obtained, for instance, by using the photoacoustic calorimetry technique (see chapter 13), and the latter by several gas-phase methods. 

Recently, a number of reviews on the experimental gas-phase methods applied in... 

The general field of measuring thermodynamic quantities using gas-phase methods is based simply on Eq. (3.55) where the activity of component i (Oj) is equal to the vapour pressure of i in the alloy divided by the equilibrium vapour pressure of pure i. 

Bond dissociation energies, BDE, are required in order to understand dissociative reduction processes. While BDE values of many simple hydrocarbons have been measured using various gas phase methods, for practical purposes, reactions of interest to electrochemists occur in solution. Solution-... 

Conversion in the liquid phase has the disadvantage that the carbon tetrachloride formed during the disproportionation of trichlorofluoromethane forms a complex compound with the aluminum trichloride possessing no catalytic effect, so that only a relatively small amount of trichlorofluoromethane can be converted with a predetermined amount of aluminum trichloride. The continuous gas-phase method in a tubular reactor is more practicable the temperature at which it takes place must be high enough to prevent any products from condensing on the catalyst. It is also possible to perform the disproportionation process continuously in the liquid phase in a tubular reactor, under pressure and at an increased temperature. In this case aluminum trichloride must first be activated by pretreatment (partial fluorination), since the partial fluorination of aluminum trichloride greatly reduces the tendency for complex compounds to form with the chlorinated hydrocarbon when this itself has formed. 

Post-synthesis gas-solid isomorphous substitution methods are also known [61]. Ti-beta essentially free of trivalent metals can be prepared from boron-beta. However, the gas-phase method is not efficient for Ti incorporation and could have some disadvantages such as the deposition of Ti02 [62],... 

The concept of superelectrophilic activation was first proposed 30 years ago.20 Since these early publications from the Olah group, superelectrophilic activation has been recognized in many organic, inorganic, and biochemical reactions.22 Due to the unusual reactivities observed of superelectrophiles, they have been exploited in varied synthetic reactions and in mechanistic studies. Superelectrophiles have also been the subject of numerous theoretical investigations and some have been directly observed by physical methods (spectroscopic, gas-phase methods, etc.). The results of kinetic studies also support the role of superelectrophilic activation. Because of the importance of electrophilic chemistry in general and super-acidic catalysis in particular, there continues to be substantial interest in the chemistry of these reactive species. It is thus timely to review their chemistry. 

Gas-phase methods for generating ions for mass spectrometry are the oldest and most popular methods. They are applicable to compounds that have a minimum vapor pressure of ca. 1CT6 Torr at a temperature at which the compound is stable this criterion applies to a large number of nonionic organic molecules with MW < 1000. 

Karlen et al. analyzed acetylcholine and choline by an ion pair extraction and gas phase method [134]. The gas chromatographic estimation of the isolated acetylcholine and choline was carried out after demethylation either with benzenethiolate or by controlled pyrolysis. Acetylcholine was quantitated by flame ionization detection at the nmol level. Mass fragment analysis was employed for the determination of acetylcholine in pmol amounts. 

Gas-phase methods also constitute a source of important information on basic physical properties of silylenium ions. In particular, the thermochemical behavior is well characterized (30,33,34,47,61). Thermochemical data are applied for the evaluation of relative thermodynamic reactivities of silylenium ions in some systems. For example, affinities of R3Si+ and R3C+ toward various bases may be compared as the heterolytic dissociation energies of corresponding bonds [Eq. (12)] (47,61). It was shown that... 

Electron diffraction (71PMH(3)27) is predominantly a gas-phase method. By this method, mixtures of conformations can be detected, but rotational barriers can only be estimated in special cases. Microwave spectroscopy and electron diffraction have given many valuable structural determinations in the gas phase, especially for the parent heterocycles and simple derivatives. 

Analgesics, Ancdysis by Gas Phase Methods (Thenot and Haegele). 24... 

Conformer Gas-Phase Method Aqueous-Phase Method ... 

Smaller molecules and inert gas atoms have been extensively studied using EFISH in the gas phase (see for example Miller and Ward, Ward and Miller, Shelton ). Shelton and Rice provide a comprehensive list of gaseous EFISH measurements on small molecules up to 1994. The only such result reported for molecules with donor/ acceptor substitution on a benzene ring appears to be that obtained by Kaatz et alP for pNA in 1998. In this experiment a gas mixture containing 0.075 mole fraction of pNA was used to obtain an EFISH measurement at 1064 nm at one temperature. The y (—2(B (B,a),0) of eqn (4.15) was estimated from a THG experiment and taken as the intercept on a two point plot of y versus /T. The value of was calculated from the slope. The linearity of such plots has been confirmed in the work on smaller molecules. The gas phase method differs from that used for solutions in that the extrapolation to infinite dilution is not made since the molecular density in the gas is very much smaller. Also the internal field factors are close to unity. It is usually possible to make measurements over a sufficiently wide range of temperatures to obtain the quantity (jifi/k) from the plot of F versus l/F. In the case of pNA the value of the dipole was chosen as 6.87 D. 

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