The role of the medium in the effective-sphere interpretation of holographic particle characterization data

Abstract

Holographic particle characterization measurements can be interpreted with the effective-sphere model to obtain the size, refractive index and porosity of porous and irregularly shaped colloidal particles.

Quantitative Differentiation of Protein Aggregates From Other Subvisible Particles in Viscous Mixtures Through Holographic Characterization

“… 2 , 3 , 24 , 25 Their measured refractive indexes are intermediate between that of the particle’s matrix and that of the medium. 15 , 16 HVM has been demonstrated to yield accurate characterization data for such inhomogeneous and aspherical particles 2 , 16 , 24 – 26 through the effective medium theory of light scattering. 15 , 27 In this case, the population of particles near the medium refractive index is naturally identified with protein aggregates, as distinct from emulsion droplets.…”

“…Changes in the properties of the medium due to added excipients can influence the results of HVM measurements, most notably for porous particles whose pores are perfused with the medium. 15 , 16 The effective refractive index reported by HVM for such particles is intermediate between the refractive index of the medium and the refractive index of the porous particles’ matrix material. This mechanism was invoked in the discussion of protein aggregates’ properties presented in Figure 2 .…”

“…Both of these trends are consistent with predictions of the Maxwell Garnett effective medium theory for light-scattering by inhomogeneous media. 15 , 16 , 24 , 25 , 27 …”

“…Medium refractive index, however, does not influence holographic characterization of compact objects such as oil droplets, metal particles, and air bubbles, whose intrinsic light-scattering properties are not influenced by the medium. 15 , 16 Changes in the medium’s refractive index do change the signature of porous objects such as protein aggregates, whose measured refractive indexes track changes in the index of the medium. Such changes also can be used to distinguish protein aggregates from other, compact homogeneous contaminants.…”

Label-free Viability Assay using Holographic Video Microscopy

“…This technology has similarly been used to distinguish bacteria from plastic beads and oil in water 43 since each of those species is composed of materials of different refractive indexes. A study of porous silica particles showed that THC is able to detect subtle changes in particle composition 44,45 . The ability to detect small changes in particle composition is due to the effective sphere model and the effective medium theory 44,46 .…”