Application Note
Application Note: Rapid Refractive Index Determination For Pharmaceutical Actives
Few people today would dispute that the use of Mie Theory provides the only route to accurate particle size analysis when measuring fine particles using laser diffraction. The international standard for laser diffraction specifies that "for particles smaller than about 50 microns the Mie Theory offers the best general solution" [1]. However, this can often be a difficult requirement to meet, as Mie Theory requires users to specify the refractive index (RI) of the material under study. If the user is measuring inorganic compounds there is ample data available in standard texts such as the CRC Handbook [2] or on the Internet. Pharmaceutical materials present more of a problem, as it is extremely unlikely that the RI for a new active pharmaceutical ingredient (API) will be known. In such situations the temptation is to default to using the Fraunhofer Approximation which, although wrong, is considered to be better than "guessing" a RI value. However, this can lead to large errors in the assessment of the fine particle fraction, a parameter vital in assessing the solubility and therefore bioavailability of a given API.
Click here to download the complete application note in pdf format.
