Controlled Stress on a Substance

To obtain information on the stability of a substance and to confirm the applicability of the testing method for stability tests, mainly in the field of pharmacy, controlled amoimts of stress are applied to the substance. For this, the term stress test is generally used. 

Usually, 20-50 mg of a pure active ingredient or ca. 100-500 mg of plant extracts, drugs or mixtures of active ingredients are weighed out. These are stored either as the dry substance or after addition of 10 ml solution. 

For comparison, a sample containing 10 ml water as well as the substance (extract etc.) is stored at 80 °C. After the predetermined storage time, the samples are worked up, i.e. undissolved substance must be brought into solution and acidic and basic solutions must be neutralized. Drugs and extracts are usually subjected to an often complex sample preparation before the preparations for TLC can be started. 

Examples of the TLC investigation of stressed samples are shown in Fig. 92 (dex-panthenol) and in the testing procedme Identity and Purity of Various Flavonoid-Containing Plant Extracts is given in Section 9.4.2. 

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