Physicochemical assessments of parenteral lipid emulsions: light obscuration versus laser diffraction

The United States Pharmacopeia (USP) has proposed a new Chapter <729> entitled 'Globule Size Distribution in Intravenous Emulsions' that is intended to identify methods for analyzing the stability of lipid emulsions. We studied the differences between particle-sizing instruments when analyzing the physicochemical stability of a parenteral nutrition mixture compounded with intravenous lipid emulsion, known as an all-in-one mixture. As the growth of lipid droplets, i.e. coalescence, signals an irreversible change in emulsion stability, we focused our investigation on the large diameter tail (>5 microm) of the globule size distribution. Of the four proposed methods, droplet size was studied over a range of mixture stabilities using a low osmolality parenteral nutrition formula employing both light scattering and light obscuration techniques. In addition, the same mixtures were also freshly prepared, and then spiked with a known amount of 5 microm latex spheres. The response obtained from the light obscuration technique was linear and detected both unstable and latex-spiked mixtures in every case for droplets or particles >5 microm. The results of the laser diffraction method were non-linear and overestimated, was less sensitive or missed entirely, globules or particles in the large diameter tail of the dispersion. The results demonstrate that light obscuration is superior to laser diffraction in identifying unstable intravenous fat emulsions.