Part III. Extracting Analytes From Samples

Investigations 7–9: Solvent Extraction of Danshen

The chromatogram in Figure 1 was obtained using samples of the eight analytes purchased from commercial sources. Because the analytes are available in pure form, there was no need to complete an extraction prior to injecting the standard sample into the HPLC; however, to analyze samples of Danshen, we first must extract the analytes from its roots using a suitable solvent.

Investigation 7. Brewing coffee is nothing more than a simple solvent extraction, which makes it a useful and a familiar model for considering how a solvent extraction works. There are a variety of methods for brewing coffee that differ in how the solvent and the coffee are brought together. Investigate at least five of the following methods for preparing coffee: Turkish, French Press, Aeropress, Chemex, Pour Over, Stovetop, Vacuum Pot, Espresso, and Cold Brew. In what ways are these methods similar to each other and in what ways are they different from each other? What variables in the extraction process are most important in terms of their ability to extract caffeine, essential oils, and fragrances from coffee?

The most common method for extracting an analyte from a natural material—such as the roots, stems, and leaves of a medicinal plant—is to place a powdered sample in a suitable solvent and allow it to steep for 60 min at or near the solvent's boiling point. After filtering, the solid residue is extracted a second time and the two extracts combined to give a final sample [9].

Investigation 8. Why might a combination of high temperature, a lengthy extraction time, and the need for two extractions be undesirable when working with a medicinal plant such as Danshen?

Microwave-assisted solvent extractions are a promising method for addressing the limitations of a traditional solvent extraction because they use shorter extraction times and use smaller volumes of solvent [10]. In this case study we will develop a microwave-assisted solvent extraction for the quantitative analysis in Danshen of the four lipophilic and the four hydrophilic compounds identified earlier.

Investigation 9. What variables might we choose to control if we want to maximize the microwave extraction of Danshen's constituent compounds? For each variable you identify, predict how a change in the variable's value will affect the ability to extract from Danshen a hydrophilic compound, such as rosmarinic acid, and a lipophilic compound, such as tanshinone I.

[9] For a review of methods used for the quantitative analysis of Danshen, including different methods for extracting its active constituents, see "Advancement in Analysis of Salaviae miltiorrhiza Radix et Rhizoma (Danshen)," the full reference for which is Li, Y-G.; Song, L.; Liu, M.; Hu, Z-B.; Wang, Z-T. J. Chromatogr. A. 2009, 1216, 1941-1953 (DOI).

[10] For additional information on microwave extractions, see "Analytical-scale microwave-assisted extraction" the full reference for which is Eskilsson, C. P.; Björklund, E. J. Chromatogr. A 2000, 902, 227–250 (DOI), and "Standardizing the World with Microwaves," the full reference for which is Erickson, B. Anal. Chem. 1998, 70, 467A–471A (DOI). The microwave ovens used for solvent extractions essentially operate in the same manner as microwave ovens found in the home, although they are designed to provide more control over the microwave"s settings, and to handle better the harsher chemical environment found in a laboratory.