Effect of Column Temperature

The temperature, T, at which an analysis is carried out has an important influence on retention. At constant pressure the retention factor, k, may be either increased or decreased (Figure 1) by increasing the temperature, because of the combined influences of solubility in the stationary and mobile phases, and on vapour pressure, so that resolution may be tuned by adjusting T. A rigorous expression which accounts for the variation of SFC retention with temperature at constant pressure is:

where is the fugacity coefficient of the solute in the mobile phase, p is the density of the supercritical fluid, c is the standard concentration and C is a constant. AH® and AS® are respectively the partial molar enthalpy and entropy of solution of the solute in the stationary phase.

The left-hand side of eqn [1] is obtained as follows: experimental values of k; values of p/c® from the

Figure 1 Capacity ratios of pyrene as a function of column end pressure (Pe) and temperature (T). Three-dimensional network shown in two perspectives. Conditions: 25 cm x 4.6 mm i.d. packed column, 10 ^m silica particles; n-pentane. (Reprinted with permission from Lee ML and Markides KE (eds) (1990) Analytical Supercritical Fluid and Extraction. Provo, UT, USA: Chromato-graphy Conferences, Inc.)

Figure 1 Capacity ratios of pyrene as a function of column end pressure (Pe) and temperature (T). Three-dimensional network shown in two perspectives. Conditions: 25 cm x 4.6 mm i.d. packed column, 10 ^m silica particles; n-pentane. (Reprinted with permission from Lee ML and Markides KE (eds) (1990) Analytical Supercritical Fluid and Extraction. Provo, UT, USA: Chromato-graphy Conferences, Inc.)

IUPAC (International Union of Pure and Applied Chemistry) equation of state for CO2; and values for are calculated from the Peng-Robinson equation of state. This relationship has been tested for a number of hydrocarbon solutes. Figure 2 shows the individual contributions and total of the left-hand side of eqn [1] for fluorene. It can be seen how the addition of the steepening curves for both (ln p/ce) and ln to the curve of ln k, with its maximum, gives a total which is approximately a straight line when plotted against 1/T.

The dependence of retention on temperature at constant density is simpler and is predicted via the van't Hoff equation:

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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