Part VII. Verifying the Analytical Method's Accuracy

Investigation 33: Spike-Recovery Studies

Although the results of Investigation 31 and Investigation 32 provide confidence in our analytical method, we have not yet established that the concentrations we report represent accurately the actual concentrations of our analytes in Danshen. The possibility of thermal degradation during the microwave extraction, for example, is a potential concern not addressed in Investigation 32.

Establishing our method's accuracy is complicated because we do not know, a priori, the actual concentrations of the hydrophilic compounds and the lipophilic compounds in any particular sample of Danshen; indeed, their concentrations certainly vary from plant-to-plant and from field-to-field, particularly plants and fields in different geographic regions, and likely are affected by methods of cultivation. Nevertheless, we can establish our method’s accuracy by analyzing a sample before and after adding a known amount of the analytes of interest [17].

Investigation 33. Explain why analyzing a sample before and after adding a known amount of an analyte allows you to evaluate a method's accuracy. Figure 20 shows the chromatogram for a sample of Danshen spiked prior to the microwave extraction with known amounts of each analyte, the concentrations of which are shown in Table 5. Using this data and your results for the unspiked sample in Investigation 31, how confident are you in the accuracy of our analytical method?

Table 5. Concentrations of Spikes Added to Danshen
analyte Cspiked
analyte Cspiked
danshensu 0.500 dihydrotanshinone 0.500
rosmarinic acid 2.500 cryptotanshinone 1.000
lithospermic acid 2.500 tanshinone I 1.000
salvianolic acid A 0.500 tanshinone IIA 2.500

[17] You can read more about spike recoveries in Chapter 15 of Analtyical Chemistry 2.0.