SYMPOSIA PAPER Published: 01 January 2001

Assay Variance in a Granular Formulation Can Predict Mixture Homogeneity


In granular formulations, the formulator often wishes to spray a small amount of an active ingredient onto a granule. An accurate amount and uniform distribution are the key to product performance in the field. Once product is formulated, the QC lab can provide information on average active ingredient amount. Seldom is there reliable information on active ingredient distribution. Active ingredient distribution can be estimated by observing assay variance. The greater the mixture homogeneity, the less the assay variation and vice versa. Also, the greater the homogeneity of a mixture, the less assay variance is affected by sample size. That is, variation varies inversely with sample size and therefore the number of particles sampled. Or, σ2 = k/N where σ2 is the variance, k is a constant and N is the number of particles sampled. The value of k will vary inversely with mixture homogeneity. In other words, the value of k will approach zero with a completely homogeneous material or a large enough sample. One can determine mixture homogeneity by determining the variance. Using a single sample size and utilizing the binomial theorem σ2 = (A2/pN)(l-p) where A = assay percent active, p = fraction of particles with chemical, and N = number of particles sampled, k from the previous equation is then (A2/p)(l-p). This paper discusses the theory and practicality of this equation.

Author Information

Goss, GR
Oil-Dri Corporation, Chicago, IL, USA
Hendrix, C
, South Charleston, West Virginia, USA
Baldwin, HM
Oil-Dri Corporation, Chicago, IL, USA
Price: $25.00
Contact Sales
Reprints and Permissions
Reprints and copyright permissions can be requested through the
Copyright Clearance Center
Developed by Committee: E35
Pages: 59–67
DOI: 10.1520/STP10434S
ISBN-EB: 978-0-8031-5451-3
ISBN-13: 978-0-8031-2877-4