Column temperature decrease

Also, Jensen (86) studied the influence of column temperature upon the retention of TGs retention was found to increase as column temperature decreased, and linear plots between log k and 1 IT were obtained. 

FIGURE 11.18 In a distillation column, temperature decreases with height in the column. The less volatile components condense and fall back to the flask, but the more volatile ones continue up the column into the water-cooled condenser, where they condense and are recovered in the receiver. 

It follows that die separation of cadmium must be carried out in a distillation column, where zinc can be condensed at the lower temperamre of each stage, and cadmium is preferentially evaporated. Because of the fact that cadmium-zinc alloys show a positive departure from Raoult s law, the activity coefficient of cadmium increases in dilute solution as the temperature decreases in the upper levels of the still. The separation is thus more complete as the temperature decreases. 

In all modes of chromatography, high sample loads distort peak shapes and cause an overall decrease in efficiency due to column overload. Sample loads may be increased by using organic solvents to enhance the solubility of the sample or by using higher column temperatures to lower the viscosity of... 

Temperature has an influence on the retention and consequently on the capacity factors of carotenoids in HPLC columns. Usually, as the column temperature increases, the retention decreases however, in a polymeric C30 column, after an initial decrease of the t values of cis isomers of carotenoids, the retention of cis isomers actually increases at temperatures above 35°C. This different behavior can be explained by the increased order and rigidity of the C30 stationary phase at lower temperatures that in turn induce preferential retention of long, narrow solutes as the trans isomer and partial exclusion of bent and bulky cis isomers. The greater chain mobihty and less rigid conformation of the C30 at higher temperatures may increase the contact area available for interaction with the cis isomers and also may lower... 

The separation number, section 1.4, is dependent on the nature of the stationary phase, the column length, column temperature, and carrier gas flow rate. The separation number tends to increase as the coluam temperature decreases. At a... 

Another approach to increase HPLC speed is the use of higher temperatures. The viscosity of a typical mobile phase used in reversed-phase separation decreases as the column temperature is increased. This allows an HPLC system to operate at a higher flow rate without suffering too much from increased back pressure. Zirconia-based packing materials provide excellent physical and chemical stability. They have been used successfully for high-throughput bioanalysis at elevated temperatures.9... 

Most modern HPLC instruments include a column oven that can thermostat the column to at least 100°C. A typical HPLC analysis can be done in half the time by elevating the column temperature from ambient to 50 or 60°C. At temperatures above 100°C, it is not uncommon to decrease analysis time by a factor of 5.26 Also, re-equilibration time for the column is much shorter, so it is possible to achieve ultra-fast gradient analysis with HTLC. 

In addition to the above strategies, the use of higher column temperatures is another approach that may decrease analysis time and improve sample throughput. The relationship between the chromatographic retention factor, k, and separation temperature is shown in Equation 13.1 ... 

Column temperature (T) Increased temperature decreased retention time Increased temperature sharper peaks effect on N difficult to define check maximum and minimum temperature before using column... 

As measured, the Hs term means that a smaller particle size reduces the distance between particles, and thus the spreading due to diffusion of sample molecules is minimized. From Equations 5.18 and 5.19, a decrease in H value is achieved by increasing the diffusion speed (elevating the column temperature,... 

An increase in temperature decreases the viscosity and hence increases the EOF. Thus, for a given voltage, more rapid analysis is possible. Temperature also affects the solute partitioning between the mobile and stationary phases and therefore the chromatographic retention. The distribution of the solute between the mobile and stationary phases is a function of its solubility in the liquid phase and adsorption on the solid stationary phase. This is characterized by the distribution ratio K defined as the ratio of the concentration of the solute in the stationary phase to its concentration in the mobile phase. Retention factors are influenced by increasing column temperature because of the increased partition into the mobile phase according to the Van t Hoff equation ... 

In order to study further the favorable aspects of in situ acid catalyzed hydrolysis, experiments were performed at different temperatures so as to evaluate the dependence of rate on temperature. Solutions of aldlcarb were passed through a jacketed column around which water at 30, 40, or 50°C was circulating. The ion exchange bed (5 cm x 0.70 cm) contained 2.0 g of Bio-Rad AG MP-50 strong acid cation exchange resin (iT ", 100-200 mesh), and the solution flow rate was approximately 1.0 ml/mln. The percent of Initial aldlcarb remaining at the end of the column for each temperature decreased from 76% at 30 C to 56% at 40 C and 35% at 50°C. Future temperature studies will be done in order to evaluate the practicality of temperature control in a detoxification filter unit. 

The situation described in Equation 9.1 is reversed at a reduced temperature. The overall column efficiency decreases rather dramatically for most samples, but successful separations are still practical with the correct choice of parameters. The reduced longitudinal diffusion in the first term means that the optimal flow rate shifts to lower flow rates. The increased viscosity of the mobile phase requires lower flow rates as well. While at high temperatures one often operates the HPLC at flow rates many times the optimal value, in subambient work, it is best to sacrifice speed and work close to the optimal flow rate. 

Thus, a low diffusion coefficient for the analyte in the mobile phase increases efficiency with regard to the A term but decreases efficiency with respect to the C , term. On balance, a higher diffusion coefficient is more favourable. Higher column temperatures reduce mass transfer effects because the rate of diffusion of a molecule in the mobile phase increases. 

As column temperature increases the degree of resolution between two components decreases because the degree of interaction with the stationary phase is reduced as the vapour pressure of the analytes increases. Lower temperatures produce better resolution. 

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