A new method of determining the settling rates of suspensions has been studied using a specially constructed absorptiometer and a sealed source of p-excited characteristic X-radiation. The X-radiation source was immersed in the suspension, and the attenuation of the radiation beam, which passed vertically through a portion of the suspension column, was used to measure the suspensoid concentration in that portion of the column which was between the source and the bottom of the suspension-containing cell. The method described is suggested as a nondestructive means of determining the sedimentation rates of pharmaceutical suspensions which contain elements with a high atomic number.
HE CONVENTIONAL method of measuring the 'settling rate of a suspension consists of recording changes in the position of the suspensionsupernatant interface as it changes with time. Changes in the system which occur prior to the appearance of a visible separation of the phases are not detectable by this method. Other methods of measuring settling rates require sampling techniques that are likely to disturb the suspension system and necessitate the manipulation of the suspension a t given intervals by the investigator.
This study is concerned with the development of a nondestructive method of measuring settling rates. The method makes it possible to make determinations with a minimum of effort, continuously or after any desired period of time and reveals changes in the distribution of suspended particles which are not apparent in the observation of the interface. The method utilizes an absorptiometer and a sealed source of Xradiation.
Other absorptiometers used to measure sedimentation rates are reported in the literature (1-5). These devices use conventional X-ray machines or low energy 7-emitting isotopes as sources of radiation. The use of low energy ?-emitting isotopes is often precluded by their high cost, short half lives, and complex decay schemes which limit their usefulness where monoenergetic radiation is desirable. Sources of tubeproduced X-rays are bulky and expensive. 8-Excited characteristic radiation provides an alternative which consumes no power and has a stability that is unaffected by changes in circuitry or environmcnt.