Purity, changes monitoring

The heat eapaeity of high-purity thorium wires ca. 150 ppm non-metallic elements) has been measured from room temperature to 1273 K, using an eleetrieal pulse-heating method. The sample was held in vacuo in a furnace, at 1273 K, and cooled slowly at ca. 40 K hr during which it was periodically pulsed electrically and the rate of temperature change monitored. The heat capacity was evaluated at about 25 data points for each of ten pulses used, but the oidy a table of the fitted values at round temperatures are given. The uncertainty in is estimated by the authors to be 2% at most. 

In many cases, biotechnology-derived products may have many components that have biological activity. The aim should be to devise controls that monitor the various components so as to retain a consistent potency and purity. For example, the USP monograph for erythromycin [9] indicates that the principal component is erythromycin A and that the percentage of erythromycin A, erythromycin B, and erythromycin C is not less than 85.0% and not more than 100.5%. Within these parameters, the relative ratios of erythromycins A, B, and C may change. This is not always the case for biotechnology-derived products, however. For example, the USP monograph for amoxicillin [10] allows for only one active component. 

Phase solubility and thin layer chromatography are two very commonly employed techniques which will verify purity. Not only do these techniques indicate initial suitability of a standard, but they also help to monitor changes in standard purity with time which could render the standard no longer acceptable. Of the two, phase solubility is the most reliable from the standpoint of accuracy and the fact that it can be employed in most laboratories. On the other hand, when phase solubility data indicate differences in bulk purity, it is good practice to establish whether the change is purity or stability related. Phase solubility is not readily employed in this capacity however, thin layer chromatography can provide excellent supplemental data. There are additional techniques, but these two can provide considerable information with relative ease and low cost. 

The above statement has lot of details in reference to what is a SIM The statement starts with method validation (refer to Chapter 9). Next, most methods need to be specific (specificity, resolution of active from related substances, peak purity), reproducible (precision), quantitative (recovery, linearity, LOD, LOQ), and able to monitor a change in chemical, physical, and/or microbiological properties of drug products over time (refer to sections on stability testing and mass balance). 

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