Analysis of the effects of particle size and densification on NIR spectra

A novel method for quantifying the median particle size of a dry powder using standard normal variate (SNV) transformed, near infrared (NIR) spectra is presented in this manuscript. The findings are of particular interest since the SNV transformation is traditionally used to remove physical information such as particle size information. Eight sieve cuts of dibasic calcium phosphate were densified in a 100 ml graduated cylinder by tapping using a tap density tester. NIR absorbance spectra spanning 1101-2200 nm were collected from each sieve cut in the diffuse reflectance mode in situ between each set of taps. The relationship of the untreated log(1/R) spectra (R is the reflectance), zeroed spectra and SNV transformed spectra to median particle size and logarithmically transformed number of taps was investigated using multiple linear regression (MLR) for 1100 wavelengths spanning 1101-2200 nm. Summations of normalized, SNV transformed absorbance intensities from 1473 to 1520 nm and 1914 to 1933 nm were also investigated by MLR. A quadratic model was developed to explain these summations as a function of the median particle size since the effect of densification was minimal. (C) 2008 Elsevier B.V. All rights reserved.