J. Soc. Cosmetic Chemists 21 259-269 (1970) ¸ 197o Society of Cosmetic Chemists of Great Britain Sampling and assessment of mixtures for cosmetics and pharmaceuticals powder j. A. HERSEY* Presented at the sy•nposium on "Powders", organised by the Pharmaceutical Society of Ireland and the Society of Cosmetic Chemists of Great Britain, at Dublin, on 18th April 1969. Synopsis--This paper discusses a planned approach to the SAMPLING of PHARMA- CEUTICAL TABLETS in which samples are taken from the MACHINE to ensure satis- factory sampling performance from each station throughout the tabletting run. The purpose of sampling must be considered before deciding on the sampling plan and sample size. An INDEX of HOMOGENEITY has been applied to results previously described by an index of MIXING using, as a basis, the randomized state. •he degree of homog•neity is of the utmost importance to the mixture consumer, where the completely randomized mixture may not be sufficiently homogeneous for his purpose. Where possible, a parallel has been drawn between the COSMETIC and pharmaceutical POWDER fields. INTRODUCTION Powder mixing is an important unit operation in both pharma- ceutical and cosmetic industries. In pharmacy the preparation of unit solid dosage forms, such as tablets and capsules, requires a considerable understanding of the powder mixing process in order that a high degree of homogeneity between products may be attained, especially when highly potent drugs have to be dispensed in a large bulk of inert diluent. The formulation of powder cosmetics is a no less arduous task where pigments must be so dispersed that on application to the skin, a layer of single particles must present a homogenous colour reaction. *Department of Pharmaceutics, The School of Pharmacy, University of London. Now at Pharmaceutical Division, Sandoz A.G., CH-4002, Basle, Switzerland. 259

260 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Whilst the actual operation of powder mixing is tremendously important, it is not considered within the scope of this paper. Many misconceptions exist and problems arise which may be attributed to incorrect sampling or to erroneous assessment in the quality control of powder mixtures. This paper attempts to cover this field with special reference to the pharma- ceutical industry, where possible similarities or differences with the cosmetic industry are made. SAMPLING OF POWDER MIXTURES The sampling of a powder mixture is a complex problem about which much has been written. The number of samples to be taken, the size of each sample, the purpose of the sample and the method by which they are to be taken must all be considered. The number of samples to be taken depends upon the acceptable sampling error. Sampling is simply taking a number of parts from a mass such that these parts illustrate the qualities of the mass. In order that these samples represent accurately the qualities of the mass it is necessary to divide the mass into a number of parts and examine all of these parts. Such an exercise would clearly defeat the object of sampling which is to examine a relatively small part of the mass and conclude from this examination the qualities of the mass within certain acceptable limits of error. The larger the number of samples taken, the smaller will be the sampling error and this will facilitate more accurate statistical evaluation of the quality of the mass. Where possible variations are known to exist, a sampling plan may be devised to include all such variations. For example, consider a 33-station multiple-punch rotary tablet machine. The possible variations may be considered as the weight of tablet produced in each of the 33 stations and segregation of the tablet ingredients due to vibration in the hopper feeding all these stations. There will thus be an individual random variation from each station depending upon the machine setting superimposed on a cyclic variation caused by repeated filling of the hopper in which the materials are segregating. With a small batch size, it may be necessary to take 33 consecutive tablets at the beginning and end of the tablet run as the samples. With larger batches of 106 tablets it may only be necessary to examine at intervals of 102 or 103 in order to examine the probable variations. Where it is impossible to have a preconceived knowledge of variations, then