| Several different modifiers were used to find what organic or combination of organics produced the best results. The modifiers that were used in the experiments were around 10 volume percent in carbon dioxide. The specific organics used as modifiers were isopropanol, ethanol, methanol, ethyl acetate, acetone, and combinations of equal amounts by volume of isopropanol and ethanol, isopropanol and methanol, ethanol and methanol, ethyl acetate and isopropanol, as well as a 9.4% and 0.6% combination of hexane and isopropanol. The ones that worked best were isopropanol and ethanol. Combining the two in a 50:50 mixture and 10% v/v gave the best results. Good results were also achieved using a combination of methanol and ethanol 50:50, and isopropanol and methanol 50:50. Figure 18 depicts the various data points obtained from the before mentioned results. Ethyl Acetate was used by itself at 10% v/v and no separation was observed. Methanol worked and the retention times were noticeably reduced, but the separation and the resolution were drastically worse. A 50:50 combination of methanol and isopropanol was used which combined the lower retention times of methanol and the better separations of isopropanol. The hexane modifier did not work well at all at supercritical conditions so no comparison of results in various publications and in this work was possible with isopropanol and combinations of isopropanol and ethanol. This figure includes results from sub-critical experiments. The effect of pressure and modifier is summarized in Table 5.
Figure 18. Effect of Pressure on Peak Resolution, Various Modifiers |