[5 Reversed Phase Chromatography

When methanol-H2O-CO2 mixtures are used in re-versed-phase separations with an octadecyl poly-siloxane stationary phase, the efficiency of the separation increases significantly compared to the same conditions without the addition of CO2. For example, the reduced plate height for naphthalene at a reduced velocity of 10 decreased from 11 to 4 for a mobilephase composition change involving the addition of 0.50 mol fraction CO2 to a 0.7 : 0.3 mol ratio meth-anol-H2O mixture. The addition of 0.5 mol fraction CO2 to the 0.7 : 0.3 mol ratio methanol-H2O shortens the analysis time of a separation of polynuc-lear aromatic hydrocarbons by factors of 2.5 and 8 at linear velocities of 0.15 (reduced velocity = 5) and 0.35 (reduced velocity = 17) cms-1, respectively, compared to that for the methanol-water mixture at the same linear velocities.

Figure 5 shows the variation in the band dispersion of pyrene using the 0.70 : 0.30 mol ratio methanol-

Figure 5 Variation of plate height with mobile-phase linear velocity for pyrene at 204 atm for different mobile-phase conditions: triangles, 0.70 : 30 mol ratio methanol-H2O at 26°C, k' = 3.77; circles, 0.70 : 0.30 mol ratio methanol-H2O at 60°C, k' = 2.02; diamonds, 0.49 : 0.21 : 0.30 mol ratio methanol-H2O-CO2 at 26°C, k' = 1.82; squares, 0.49 : 0.21 : 0.30 mol ratio methanol-H2O-CO2 at 60°C, k' = 0.92. (Reproduced with permission from Lee and Olesik (1994).)

Figure 5 Variation of plate height with mobile-phase linear velocity for pyrene at 204 atm for different mobile-phase conditions: triangles, 0.70 : 30 mol ratio methanol-H2O at 26°C, k' = 3.77; circles, 0.70 : 0.30 mol ratio methanol-H2O at 60°C, k' = 2.02; diamonds, 0.49 : 0.21 : 0.30 mol ratio methanol-H2O-CO2 at 26°C, k' = 1.82; squares, 0.49 : 0.21 : 0.30 mol ratio methanol-H2O-CO2 at 60°C, k' = 0.92. (Reproduced with permission from Lee and Olesik (1994).)

H2O mixture, with 0.30 mol fraction CO2 added at room temperature and with 0.30 mol fraction CO2 added at 60°C. As expected, the band dispersion decreased and the optimum linear velocity increased when CO2 was added to the mobile phase. The increase in temperature also yields substantial gains in efficiency, especially at high linear velocities.

Figure 6 shows the improved separation time which is possible by adding CO2 and increasing the temperature to 60°C at the same linear velocity for the separation of 16 polynuclear aromatic hydrocarbons. The separation required approximately 40 min with 0.70: 0.30 methanol-H2O approximately 15 min when 0.30 mol fraction CO2 was added and less than 7 min when this mixture was heated to 60°C. However, when 30% CO2 was present in the mobile phase and the temperature was 60°C, the selectivity of the separation degraded substantially.

Also, due to the low viscosity of the meth-anol-H2O-CO2 multiple columns can be placed in series to increase the overall efficiency of a chro-matographic separation. For the same linear velocity, Figure 7 compares the separation of a coal tar sample using one and four ODS columns. Substantial gains in efficiency are obtained by placing the columns in series.

However, the addition of liquefied CO2 is not optimal to improve the performance of every separation. When CO2 is mixed with H2O, carbonic acid is for med. The acidic nature of this mobile phase can have a significant impact on the retention factor of ioniz-able compounds. Also CO2 and H2O are not highly miscible at room temperature without the addition of a co-solvent. Even with the addition of co-solvent, the total amount of CO2 that is miscible in the mixture is controlled by the amount of H2O present. For example, a 0.70 : 0.30 mol ratio methanol-H2O mixture is immiscible with more than 0.50 mol fraction CO2. To eliminate many of these problems, other liquefied gases can also be used to modify the fluidity of the mixture.

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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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