Solvent
The term solvent is often used as a misnomer the cause for which has evolved over many years with the development of chemistry. Basically a solvent should be considered as a liquid in which a specific substance is soluble (e.g. water is a solvent for sucrose and n-heptane is not a solvent for sugar; methylene chloride is a solvent for fat and water is not a solvent for fat.). However, due to common use of all organic liquids as solvents, the word solvent has come to be used as a collective name for all types of organic liquids e.g. hydrocarbons, chorohydrocarbons, ethers, esters, ketones, alcohols etc.; all these liquids are considered to be ‘solvents’. This leads to an apparently ridiculous statement in a recent publication “the sample of ground cement was suspended in a solvent to estimate the particle size by sedimentation”. The term solvent in this example is used as a term for an organic liquid and, in fact, the success of the test depends on the cement being insoluble in the solvent. So much for the complexity of scientific terminology. It is clear that when using the word solvent its meaning must be unambiguously indicated by the sentence construction.
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles TLC Chambers
for
evaporation. The use of a paper wick is depicted on the right-hand side of
figure 29.
The saturated
solvent vapor in the chamber not only prevents solvent evaporating from the
plate surface but partly controls the retention mechanism by surface
deactivation. The solvents are selectively adsorbed on the surface of the
stationary phase causing the solutes to interact, not with the native silica
surface, but with the silica surface covered with the most strongly interacting solvent. It should be emphasized,
however, that the equilibrium between the solvent vapor and the plate
will not be the same as the equilibrium between the solvent and the
plate. For example, for a binary mixture of solvents having concentrations of
solvent in the gas phase of c1
and c2 respectively,
where (x1) is the molar
fraction of solvent (1)
and (x2) is the molar
fraction of solvent (2)
 
Principles TLC Chambers
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Development Elution
the surface corresponding to the
area (Y). Finally, the remaining solvent (C) with the weakest interactions with
the stationary phase continues to migrate and cover the surface with a layer of
solvent (C) in the area (Z). It is seen that the distribution system, which
results from the frontal analysis of the three mobile phase components is now
quite complex. The solutes will interact during the separation process.
In the first section (X) solutes will be distributed between the ternary solvent mixture (A), (B) and (C) and the surface covered with solvent (A). In
the next section (Y) the solutes will be distributed between a binary solvent
mixture of (B) and (C) and a surface covered with solvent (B). Finally,
distribution will take place in section (Z) between pure solvent (C) and a
surface covered with solvent (C). Even this is an over-simplification, as the
composition of the mobile phase in each section will not be constant but will
decrease along the plate.
Principles Development Elution
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Gradient-Programmer High-Pressure
solvent to pass to the pump, air again dissolves in the
solvent. The solvent is filtered through a stainless steel or sintered glass
filter to remove any solid contaminants. Depending on the type of solvent
programmer that is employed, the supply from each reservoir may pass either to
a pump or to a valved blending device. Solvent reservoirs are not usually
thermostatted but, when necessary, the solvent can be brought to the column
temperature by the use of an appropriate heat exchanger. The solvent containers
are often situated in an enclosure that protects the user from toxic solvent
vapors such as chloroform or aromatic hydrocarbons. Such enclosures also
isolate the solvents from atmospheric moisture.
The Gradient Programmer
The High Pressure Programmer
There are two
basic types of solvent programmer. In the first, the solvent mixing occurs at
high pressure and in the second the solvents are premixed at low pressure and
then passed to the pump. The high pressure
HPLC Basic-HPLC Gradient-Programmer High-Pressure
Author: RPW Scott
Book:The Thermodynamics of Chromatography
Section:Thermodynamics Other-Methods Chiral-Separations Enantiomers-Separation-Ratio
nbsp;
The Effect of Temperature and Solvent Composition on the Separation Ratio of the Two Enantiomers
Curves
demonstrating the change in separation ratio of the two enantiomers with
temperature and solvent composition, calculated from equation (49) are
shown in figure (21). Despite the dominant effect of solvent composition on capacity
ratio, the effect of solvent composition on the separation ratio is
much smaller, and the dominant effect is now the operating temperature.
This stresses the importance of temperature for selectivity control in chiral
separations. It is very interesting to note that there is a temperature at
which the solvent composition has no effect on the separation ratio whatsoever
(ca 43˚C).
Figure 21.
Curves Relating the Separation Ratio of the Two Enantiomers with Temperature
and Solvent Composition
It is clear
Thermodynamics Other-Methods Chiral-Separations Enantiomers-Separation-Ratio
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Pump Twin-Headed
the piston
progresses to the right, solvent is displaced to the column system and,
simultaneously, fresh solvent is withdrawn from the solvent reservoir into the
right hand chamber. When the piston arrives at the extent of its travel, a step
in the driving cam is reached and the piston is very rapidly reversed. As a
result the contents of the chamber on the right-hand side are displaced into
the left-hand chamber. This situation is shown in the lower part of figure 8.
The transfer rate of the solvent to the left-hand chamber is 100 times faster
than the delivery rate to the column and consequently reduces the refill-pulse
very significantly. In addition, if a solvent gradient is being used and the
right-hand chamber is being filled with a solvent mixture, excellent mixing is
achieved during the refill of the left-hand chamber. It is clear, however, that
there will not be a smooth transition from one solvent concentration to the
next but will be a step-wise change.
An
HPLC Basic-HPLC Pump Twin-Headed
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC HPLC-Mobile-Phases Stationary-Phase-Interactions
the solute and the stationary phase surface. There are
three different surfaces on which a molecule can interact by sorption and three
different surfaces from which molecules of solvent can be displaced and allow
the solute molecule to penetrate. In any separation all the alternatives are
possible but it is more likely that for one particular solute, one type of
interaction will dominate. the various types of interaction are depicted in figure
43. Where there are multi-layers of solvent, the solvent that interacts
directly with the silica surface is the most polar, and consequently
constitutes the first layer. Depending on the concentration of the polar
solvent the next layer may be a second layer of the same polar solvent as in
the case of ethyl acetate. If, however, the quantity of polar solvent is
limited, then the second layer might consist of a less polar component of the
solvent mixture. If a ternary mixture of solvents is used, the nature of the
surface, and the solute
HPLC HPLC-Mobile-Phases Stationary-Phase-Interactions