Effect of Temperature and Pressure on Carotenoid Separations

The solubility of a substance in a supercritical fluid is the sum of two factors: the volatility of the substance (which is a function of temperature) and the solvating effect of the supercritical fluid (which is a function of fluid density). Hence, solubility is controlled experimentally by selecting appropriate temperatures and pressures which are important for controlling retention in SFC. These parameters influence the solvating power, efficiency and selectivity. At constant pressure, an increase in temperature decreases the density and consequently the solvating power. The temperature also influences the diffusion coefficients of the solutes in the supercritical fluid. With increasing temperature, diffusion coefficients increase and higher chromatographic efficiency results. However, one needs to exercise caution in this regard as carotenoids are thermally labile and will decompose at high temperatures. With pure carbon dioxide, the capacity factor (kR) increases in proportion to the temperature

20 25 30 35 40 45 50 55 Temperature TO

20 25 30 35 40 45 50 55 Temperature TO

Figure 3 Dependence of kR values of carotenes on temperature at constant pressure (25 MPa). Column, Spheri-5 ODS-5A; mobile phase, CO2-methanol (80:20v/v); flow rate, 3.0 mL min~1; detection, 450 nm. Squares, all-trans a-carotene, circles, all-trans ^-carotene. Reproduced with permission from Aubert etal. (1991).

(at constant pressure) and can be explained by the resulting decrease in the density of the mobile phase. On the other hand, as shown in Figure 3, when the carotenes are eluted with carbon dioxide containing 12% methanol, kR decreases with increasing temperature between 22 and 55°C, while the resolution between the all-trans a- and ^-carotenes decreases as the temperature is increased. Although the optimum temperature for this separation is between 22 and 25°C, and the mobile phase is therefore subcritical, this is of little consequence because there is no discontinuity in the physical properties of the fluid at the critical point.

At constant temperature, an increase in pressure produces an increase in density. Increasing the pressure increases the mobile-phase viscosity, thus decreasing the mass transfer term C and the diffusion coefficient. In SFC, density (or pressure programming) is the primary method for developing separation. It is analogous to temperature programming in GC, and eluent composition programming in LC. Increasing the pressure at constant temperature leads to decreased capacity factors, which can be explained by the enhanced solubility of the solutes with increasing density of the mobile phase. Figure 4 illustrates the dependence of kR values of carotenes on pressure at constant temperature. In this work, the pressure was varied between 100 and 250 bar, at 22°C and the capacity factors were observed to decrease less rapidly at pressures above 200 bar. This is probably due to the nonlinearity of the P-T curve of carbon dioxide, which at 22°C becomes less compressible. In practice, it is preferable to avoid working at pressures

100 150 200 250

Pressure (bar)

Figure 4 Dependence of kR values of carotenes on pressure (P) at constant temperature (22°C). Column, Spheri-5 ODS-5A; mobile phase, CO2-methanol (80:20v/v); flow rate, 3.0 mL min"1; detection, 450 nm. Open squares, all-transa-caro-tene; circles, cis a-carotenes; triangles, all-trans ^-carotene; filled squares, cis ^-carotenes. Reproduced with permission from Aubert etal. (1991).

100 150 200 250

Pressure (bar)

Figure 4 Dependence of kR values of carotenes on pressure (P) at constant temperature (22°C). Column, Spheri-5 ODS-5A; mobile phase, CO2-methanol (80:20v/v); flow rate, 3.0 mL min"1; detection, 450 nm. Open squares, all-transa-caro-tene; circles, cis a-carotenes; triangles, all-trans ^-carotene; filled squares, cis ^-carotenes. Reproduced with permission from Aubert etal. (1991).

where the fluid is very compressible, as the pressure drop along the column is associated with a density gradient and reduced efficiency.

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.

Get My Free Ebook


Post a comment