Retention temperature dependence

The relative retention is dependent on (1) the nature of the stationary and mobile phases and (2) the column operating temperature. 

Relatively high (typically 980—1200°C) temperatures are required to decompose spent acids at reasonable burner retention times. Temperatures depend on the type of spent acid. A wide variety of spent acids can be processed in this way, but costs escalate rapidly when the sulfuric acid concentration in spent acid (impurity-free basis) falls below about 75%. A few relatively uncontaminated spent acids can be reused without decomposition by evaporating the excess water in concentrators, or by mixing in fresh sulfuric acid of high concentration. Weak spent acids are frequently concentrated by evaporation prior to decomposition. 

Although decarbonylation of supported metal carbonyl clusters sometimes occurs almost without changes in the metal frames, the chemistry is complex and only partially understood. When decarbonylation takes place at elevated temperatures (depending on the support), migration and aggregation of the metal inevitably occur, and these processes are less well understood than the decarbonylation with near retention of the metal frame. 

Temperature variation may also be a relevant factor in flowrate stability. Since the viscosity of the solvent is temperature dependent, wide swings in the ambient temperature can directly affect pump performance. The direct effects of temperature on pump performance usually are far smaller, however, than the effects on retention and selectivity therefore, control of column temperature is generally sufficient to obtain high reproducibility. 

In the reactions with phosphonio-a-methoxycarbonyl-alkanides, the products of type 261 derived from 1,3-cycloaddition can rearrange to the tautomeric lif-pyrazolo-triazole (87MI2). The reaction of 3-diazopyra-zoles and 3-diazoindazole with acyl-substituted phosphonium ylides led to pyrazolo-triazine and indazolo-triazine derivatives 266 instead of the expected triazole compounds (8IJHC675). In this case, the ylides, which can exist as phosphonium enolates, possess nucleophilic and electrophilic centers in a /8-relationship, giving [7 + 2] or [11 -I- 2]cycloaddition reactions. With dimethylsulfonio-a-aroyl-methanides, very complex, temperature-dependent mixtures were obtained, in some cases with sulfur retention (87MI3). 

Acidic and basic compounds often show more complex behavior with non-linear van t Hoff plots and with an increased retention at high temperatures in some cases. This is due primarily to the impact of temperature on the various equilibrium constants at play in the solutions [25], All equilibrium constants are temperature dependent. When the solute has multiple equilibrium forms, the retention depends on the fraction of the solute in each form, with the neutral form being more highly retained on the reverse phase HPLC column. The p/f of water is also temperature sensitive, with the pH of a neutral solution shifting to a lower pH as the temperature increases. 

The classical FEE retention equation (see Equation 12.11) does not apply to ThEEE since relevant physicochemical parameters—affecting both flow profile and analyte concentration distribution in the channel cross section—are temperature dependent and thus not constant in the channel cross-sectional area. Inside the channel, the flow of solvent carrier follows a distorted, parabolic flow profile because of the changing values of the carrier properties along the channel thickness (density, viscosity, and thermal conductivity). Under these conditions, the concentration profile differs from the exponential profile since the velocity profile is strongly distorted with respect to the parabolic profile. By taking into account these effects, the ThEEE retention equation (see Equation 12.11) becomes ... 

An increase in the temperature was found to reduce the retention times and the plate count. This had a net effect of decreasing the resolution and increasing the peak height. The temperature dependence is likely to be dependent on the thermodynamics of the separation equilibria. 

According to this equation, the retention temperature for any particular solute and column should depend only upon the ratio r/F. The retention temperature (TR) may be calculated from the isothermal retention volume by graphical integration of the above equation. 

The effect of temperature as discussed in the theory section, based on the assumptions outlined, resulted in eq. 22, where the trend in retention is dependent on the volume expansivity of the fluid, the enthalpy of transfer of the solute from the mobile phase to the stationary phase and the change in the heat capacity of the fluid mobile phase. 

J. Takacs, M. Rockenbauer and I. Olacsi, Determination of the relationship between retention index and column temperature in gas chromatography through the temperature-dependence of the net retention volume, 7. Chromatogr., 42, 19-28 (1969). 

Because of the occurrence of the excess quantities hEand sE in eqn.(3.9), the coefficients in eqn.(3.10) for the temperature dependence of the retention are a function of the stationary phase. Hence, every stationary phase may be expected to yield a different optimum temperature, at which the capacity factors of all sample components fall in the optimum range. Therefore, to make a fair comparison between two different stationary phases for a given separation problem, the (potentially different) optimum temperature should be established for each of them and the resulting chromatograms should be compared. The common practice of characterizing (and consequently comparing) stationary phases at a standard temperature is a very convenient one. Nevertheless, it may give rise to erroneous conclusions in some cases. 

Unlike the relationship between retention and composition, the temperature dependence of retention in RPLC is beyond dispute. A typical van t Hoff-type equation may be used ... 

Temperature is one of the two most important variables in GC. Retention times decrease as temperature increases because partition coefficients are temperature dependent in accordance with the Clausius-Clapeyron equation ... 

The retention index has become the standard method for reporting GC data. By definition, the members of any homologous series should differ from each other by 100 units just as the standards do. This relationship is not always exact,2 and the index is somewhat temperature dependent.3 Nevertheless, it is very popular, and McReynolds4 has published a self-consistent book of indices for 350 compounds on 77 stationary phases and at 2 temperatures. Other homologous series have also been used as standards in specific industries. 

Figure 8.19. Temperature dependence of retention volume. Reprinted with permission from W. E. Harris and H. W. Habgood, Talanta 1964,11, 115. Copyright 1964, Pergamon Journals, Ltd.

Combining Equations 1-5, the temperature dependency of the carrier gas flow rate, and integrating over the column length the following relation for the retention time in isothermal gas chromatography is obtained [10] ... 

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