Photostability sample depth

In contrast, penetration into solids is normally restricted to a depth much less than 1mm. Only the top most layers of nifedipine and molsidomine tablets showed photodegradation (Thoma K, Aman W. In preparation) (4). This can easily be seen with nifedipine tablets where photodegradation results in coloration. After 12 hours, in samples stored in a photostability testing cabinet, no further degradation could be observed, as shown in Figure 3. Microscopic examination of these samples showed this coloration was restricted to a depth of less than 400 pm, as shown in the microphotograph of Figure 4. 

Because semisolid drugs require a different handling than solid drugs, it is important to consider the experimental conditions for in vitro photostability testing. Due to the limited penetration depth of the incident radiation, it is important to provide samples of constant thickness. As noted previously, the "volume effect" can affect the result obtained. Furthermore, the sample area must also be kept constant if repeatable results are to be obtained. 

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