Nephelometry / Turbidity
Nephelometry / turbidity is a quick and non-invasive technique for the detection of relative changes in aggregate and particle content in a liquid sample.
A laser light passes through the sample and either the scattered light is detected, e.g., at an angle of 90° to the laser source (= nephelometry) or the transmitted light is detected at an angle of 180° to the laser source (= turbidity). Typically, a laser wavelength in the range of 320–800 nm is used in order not to be absorbed by formulation components, such as proteins, peptides, DNA/RNA, formulation excipients (e.g., amino acids), and water. In both cases, the clarity and degree of opalescence of samples is measured according to a setup described in current pharmacopoeias (Ph. Eur. 2.2.1 and USP <855>) and assigned based on comparative measurements against reference formazin suspensions.
Nephelometry / turbidity measurements are nonspecific, but highly useful for a relative comparison of samples, e.g., for rapid screening of protein aggregation during formulation development.
The technique requires limited sample preparation and is non-destructive. Although nephelometry / turbidimetry does not provide information about size, concentration, or nature of protein aggregates or particles, the technique is often used to detect relative changes in the aggregation status. Because of its high sensitivity, nephelometry / turbidity is capable of detecting the formation of particles in liquid samples very early during forced-degradation or stability studies. It should be noted, however, that turbidity can also originate from other factors, such as high protein concentration, and does not necessarily reflect the presence of aggregates or particles.
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S. Zölls, R. Tantipolphan, M. Wiggenhorn, G. Winter, W. Jiskoot, W. Friess, A. Hawe, Particles in therapeutic protein formulations, Part 1: overview of analytical methods., J. Pharm. Sci. 101  914–35. doi:10.1002/jps.23001.