Desolvation process

Thermogravimetry (TG) is a measure of the thermally induced weight loss of a material as a function of the applied temperature [45]. Thermogravimetric analysis is restricted to studies that involve either a mass gain or loss, and it is most commonly used to study desolvation processes and compound decomposition. The major use of TG analysis is in the quantitative determination of the total volatile content of a solid. When a solid can decompose by means of several... 

Differential thermal analysis (DTA) consists of the monitoring of the differences in temperature existing between a solid sample and a reference as a function of temperature. Differences in temperature between the sample and reference are observed when a process takes place that requires a finite heat of reaction. Typical solid state changes of this type include phase transformations, structural conversions, decomposition reactions, and desolvation processes. These processes may require either the input or release of energy in the form of heat, which in turn translates into events that affect the temperature of the sample relative to a nonreactive reference. 

Differential scanning calorimetry can also supply valuable information regarding solvate species, and it is particularly useful with respect to temperature and energetics of the desolvation process Two samples of the developmental compound L-706000-001T were shown to be chemically identical, and each contained two moles of water. The XRD powder patterns for the two samples were found to be quite different, demonstrating the existence of polymorphism... 

TG analysis also represents a powerful adjunct to DTA or DSC analysis, since a combination of either method with a TG determination can be used in the assignment of observed thermal events. Desolvation processes or decomposition reactions must be accompanied by weight changes, and can be thusly identified by a TG weight loss over the same temperature range. On the other hand, solid-liquid or solid-solid phase transformations are not accompanied by any loss of sample mass and would not register in a TG thermogram. 

Fig. 9.6. In LSIMS and FAB, sample-matrix cluster ion formation and desolvation processes occur on a longer time scale. Adapted from Ref. [24] by permission. John Wiley Sons, 1995.

Buffer systems are generally neutral or mildly acidic/basic aqueous solutions appropriate concentrations of buffer (> 10 mM) are generally employed to avoid fluctuations in pH during the desolvation process aqueous solutions at pH 6-8 with 10-100 mM buffer are typical [6, 7]. 

Certain solid phases, on the other hand, cannot be obtained (even as microcrystalline powders) by crystallization experiments, but instead can be generated only by other types of preparation procedure. Some types of preparation processes commonly (or in some cases inherently) yield microcrystaUine products, including (1) preparation of materials directly from solid-state chemical reactions (see Sect. 6.6), (2) preparation of materials by solid-state desolvation processes (see Sect. 6.4), (3) preparation of materials by solid-state grinding (mechanochemical) processes (see Sect. 6.2), and (4) preparation of materials directly by rapid precipitation from solution (as opposed to crystallization) (see Sect. 6.7). Again, structure determination from powder XRD data may represent the only opportunity for determining the structural properties of new solid phases obtained by such processes. 

Structure Determination of Materials Prepared by Solid-State Dehydration/Desolvation Processes... 

In a dilute phase, atoms or ions are transported from far away, and, on arrival at the crystal surface, they are incorporated into the crystal. There is a desolvation process involved in crystallization in the solution phase or in CVT. (For an explanation of desolvation, please see Section 3.4.) The essential role of the driving force is mass transfer grovyth temperature is much lower than that in melt grovyth, and the solid-liquid interface tends to be smoother than that in the condensed phase. 

The sample to be analyzed is introduced to the ESI source by means of a flow stream from an HPLC instrument. The sample flows through a stainless-steel needle and then, sprays out in the form of a mist whose droplets hold peptide ions and mobile phase of HPLC. Peptide ions are separated from the mobile phase and subsequently, transferred into a mass analyzer either by a heated capillary or a curtain of nitrogen gas. Desolvation process can be carried out by a vacuum system. 

The synthesis of 94 is noteworthy, because no templating interactions seem to be required to form the rotaxane in preparative yields The slipping approach performed in the melt seems to offer universal access to otherwise not obtainable rotaxanes. The absence of solvent not only assists this slipping by guaranteeing high concentrations, but also eases the reaction because no desolvation processes need to take place. 

Conversely, in conventional reverse-phase HPLC, very high water content is required to retain polar analytes. The high water content in turn hinders the ionization and desolvation process during LC-MS (Hsieh and Chen, 2005 Xue et al., 2006). Therefore, HILIC allows one to elute highly polar analytes with small amounts water and maintain good LC-MS sensitivity (Hsieh and Chen, 2005). In a recent... 

Two main mechanisms have been proposed for how the resulting droplets yield desolvated ions. Dole proposed the charge residue or solvent evaporation (emission) model in which ion formation is the result of an ion-desolvation process.9,11,12 The droplets, produced by electrostatic dispersion in the liquid at the capillary tip, lose solvent molecules (aided by the curtain or nebulizer gas, usually nitrogen), and eventually produce individual ions (Fig. 3). 

Changing the solvent from polar to less polar solvents effects not only the electron transfer but also the back-electron transfer. Back-electron transfer rate constants are in less polar solvents larger than those in polar solvents, which can reasonably be interpreted in terms of desolvation process and loose in ion pair formation. The transient absorptions of the pyrrolidino fullerene radical anions are slightly blue-shifted compared to that of Qo (Qo 1076 nm, derivatives radical anions 991-1002 nm) [179],... 

Experimental problems with TGA are usually connected with sample preparation for instance, homogeneous or very disperse particle sizes may yield different results, while the presence of humidity adsorbed on the surface of the particles may mask or alter the response. Deliquescent or highly hygroscopic samples yield poorly reproducible results because it can be difficult to discriminate between removal of wetting solvent and removal of structural solvent. It is useful to accompany DSC experiments with TGA experiments. Heat absorption in a DSC plot may correspond to solvent loss and not to a phase transition (see above). Importantly, as shown below, a desolvation process may sometimes induce the formation of another polymorph or pseudo-polymorph not otherwise attainable. 

Weber C, Coester C, Kreuter J, Langer K (2000) Desolvation process and surface characterisation of protein nanoparticles. Int J Pharm 194 91-102... 

Here (D, A)SK, (D, A )SK and (D, A)SK are used to designate a collision donor-acceptor complex, (D+, A )FC the photoexcited donor-acceptor complex (of the Franck-Condon type), and (D + A )s a radical ion pair where both components are separated by solvent. The frequency v, of the exciting radiation is generally different for the donors, D, complexes, DA, and acceptors, A. Each step in scheme (79) is probably reversible, and the position and rate equilibruim establishment (of which the solvation and desolvation processes are the slowest) determine the rate and effectiveness of product formation. A review of donor-acceptor interactions and of the properties of DA bonds was published, for example, in refs. [299 and 300] and in the original communication [301],... 

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