Solvent evaporation system

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

Precipita.tlon. An ink may also be caused to dry by precipitation of its binder rather than by evaporation of solvent. This can be accompHshed by a dding a diluent, such as water in the form of steam or humidity, to a hygroscopic solvent ink system, which causes the solubiHty of the resin in the ink film to decrease sharply and causes it to precipitate when its tolerance for the diluent is reached. Eurther drying is accompHshed by absorption of the solvents into the stock and then by evaporation. Another form of precipitation setting is the quick-set mechanism. This utilizes resins held in solution in a relatively poor solvent, by means of a small amount of an exceUent solvent (called a sweetener) blended with it. When the ink film is printed on the paper, an amount of the solvents is absorbed reducing the content of the sweetener solvent to a point which causes the resins to precipitate and the ink to set. 

Infrared and Microwave Inks. These ate inks which have been formulated to absorb these radiant energies. The energy causes the inks to heat and dry through the partial evaporation of solvent. Absorption of the ink into a porous substrate can also be part of the overall drying mechanism with these inks. They have not found wide commercial success due to the variabiHty of the it absorption with ink color and the energy inefficiency of microwave systems in drying nonwater-based inks. 

FIGURE 11.20 Densitogram of alkaloid fraction from Fumaria officinalis herb extract chromatographed in system silica/PrOH -i- AcOH -i- CHjClj (4 1 5) (a) fraction introduced with applicator with evaporation of solvent (b) fraction introduced from the edge of the layer with the eluent distributor. 

FIGURE 11.21 Densitograms of PLC of Taxus baccata fraction from preparative column (silica/aqueous methanol) containing unknown taxoid (Tax 1) introduced to the layer with a set of capillaries with simultaneous evaporation of solvent from the starting band. System silica/CH2Cl2 + DX + Me2CO + MeOH (84 10 5 1). Plates double developed (a) 0.3 ml of fraction introduced (b) 0.5 ml of fraction introduced (c) 1 ml of fraction introduced to the layer. 

Solvent evaporation system (rotary evaporator, water-bath and vacuum line)... 

These are the most important components of paint. Coatings can be either clear or opaque (containing pigments) and either solvent- or water-based. With solvent-based paints, after application, most of the solvent is lost through evaporation. These solvent-based coatings are mostly alkyds (name derived from alkyl/acid) or modified alkyd resins. Normally they contain 30% polymer solids higher solids content is limited by the increasing viscosity of the system. 

To identify plasticisers in simple systems, often an IR spectrum of a methanol or acetone extract (after evaporation of solvent) will enable the positive identification of plasticiser type. For more complex systems it is necessary to initially separate the extractable fraction from the sample on for example a molecular weight basis (molecular size in solution). 

Mixtures of PEI/epoxy monomer and PEI/BPACY monomer were prepared by dissolving PEI and thermoset monomers in methylene chloride at room temperature and evaporating the solvent. The epoxy monomer/PEI solution was heated on a hot plate to 140°C, and stoichiometric amounts of DDS were added while stirring for about 7 min. For the catalyzed system, 2-methyl imidazol catalyst was also added. 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate, which is much finer than the particle beam system but similar to that formed during thermospray. A cross-flow of heated nitrogen gas is used to facilitate the evaporation of solvent from the droplets. The resulting gas-phase sample molecules are ionized by collisions with solvent ions, which are formed by a corona discharge in the atmospheric pressure chamber. Molecular ions, M+ or M , and/or protonated or de-protonated molecules can be formed. The relative abundance of each type of ion depends upon the sample itself, the HPLC solvent, and the ion source parameters. Next, ions are drawn into the mass spectrometer analyzer for measurement through a narrow opening or skimmer, which helps the vacuum pumps to maintain very low pressure inside the analyzer while the APCI source remains at atmospheric pressure. 

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