Part VII. Verifying the Analytical Method's Accuracy

Investigation 30: Extraction Yield for Danshensu

Let’s review our progress in developing a method for determining the concentration of hydrophilic and lipophilic compounds in Danshen. In Part IV we concluded that the optimum solvent is 80% methanol and 20% water (by volume), that the optimum extraction temperature is 70°C, and that the optimum microwave power is 800 W. In Part V we determined that the extraction yields for danshensu, lithospermic acid, salvianolic acid A, cryptotanshinone, tanshinone I, and tanshinone IIA increase at longer extraction times and for larger solvent-to-solid ratios, and that the extraction yields for rosmarinic acid and for dihydrotanshinone are not affected significantly by variations in extraction time and the solvent-to-solid ratio. We also learned in Part V that the analytes do not share a common optimum extraction time or solvent-to-solid ratio. Finally, in Part VI we used a global desirability function to show that an extraction time of 7.50 min, and a solvent-to-solid ratio of 35.0 mL/g allows for at least a 95% recovery of each analyte's optimum extraction yield.

Having optimized our method, we turn our attention to two additional crucial steps when developing an analytical method: verifying that the analytical method works and applying the method to a range of different samples. Here, in Part VII, we examine ways to verify our analytical method; in Part VIII we apply the analytical method to samples of Danshen from different natural locations and to samples from a controlled cultivation.

To verify our analytical method we need to show that the experimental extraction yields agree with the extraction yields predicted by the empirical models used to generate the response surfaces in Figures 10–15.

Investigation 30. In Part V we found that the empirical model for the extraction of danshensu is

\[EY=0.575+0.0225A+0.00905B-0.00125{ A }^{ 2 }-0.000165{ B }^{ 2 }+0.000100AB\]

where EY is the extraction yield (in mg/g), A is the extraction time (in min), and B is the solvent-to-solid ratio (in mL/g). Using this model, calculate danshensu's predicted extraction yield for an extraction time of 7.50 min and a solvent-to-solid ratio of 35.0 mL/g. Is your predicted extraction yield consistent with the data in Table 2 and your response to Investigation 25?