To evaluate possible solvents, a 3.00-g sample of powdered Danshen is placed in a 100-mL flask and soaked for 20 min at room temperature using 60.0 mL of a suitable solvent. Following the initial soaking, the sample is transferred to a microwave oven and extracted for 5.00 min at a temperature of 60°C and a microwave power of 600 W.
For the choice of solvent, consider ethanol, methanol, and water, as well as mixtures of water with ethanol or methanol, and predict how effective each is at extracting hydrophilic or lipophilic compounds. Why is a non-polar solvent, such as hexane, not a useful option for a microwave extraction? What limits, if any, might the choice of solvent place on the choice of temperature or microwave power?
The chromatograms obtained using methanol, ethanol, and water as the solvent are shown in Figures 3–5, respectively.
Consider the data in Figures 3–5 and explain any trends you see in the relative extraction efficiencies using these three solvents. Are your results consistent with your predictions from Investigation 11? Why or why not? Which solvent is the best choice if you are interested in analyzing hydrophilic analytes only? Which solvent is the best choice if you are interested in analyzing lipophilic analytes only? Which solvent is the best choice if you are interested in analyzing both hydrophilic and lipophilic analytes?
The temperature used to extract the samples in Figures 3–5 is limited by the boiling point of methanol, which has the lowest boiling point of the three solvents. Mixtures of methanol and water allow for higher boiling points, so it is worth exploring mixtures of these solvents. In addition, water and methanol have complimentary properties as solvents for microwave extractions: water is better than methanol at absorbing microwave radiation, but methanol is more efficient than water at converting absorbed microwave energy into heat.
Propose a set of experiments that will effectively and efficiently allow you to determine the optimum mixture of methanol and water for this extraction. What range of methanol/water mixtures will you explore? How many samples will you run? Explain the reasons for the range of mixtures and the number of samples you selected. In describing the solvent mixtures, report values as percent methanol by volume (e.g. 55% methanol by volume).