Biopharmaceutical researchers face two low key challenges in obtaining accurate particle size with their protein solutions: small sample volumes available for testing and lack of accurate viscosity values for calculating the diffusion coefficient during Dynamic Light Scattering measurements.
What does light scattering have to do with viscosity? Dynamic Light Scattering is a common technique used to measure the size of submicron particles as well as the size distribution.
The accuracy of the measurement is dependent on the accuracy of the user's supplied viscosity data. For the sake of the calculation, researchers often assume that the viscosity of the solution is the same as the buffer (~1 mPa-s). This easily introduces a significant error, espcially as concentrations increase.
During a Dynamic Light Scattering measurement, the Brownian motion of the particals cause the light to scatter at various intensities which are then analyzed to determine the diffusivity of the Brownian motion and as a result, the size is based on the well-known Stokes-Einstein equation. The diffusion coefficient is a component in determining the speed of the Brownian Motion, making viscosity a key component in determining particle size. The potential errors are significant as a 1% error in the viscosity would lead to a 1% error in particle size. Customers typically face an average of 10 to 30% error in their measurements.
As a result, measuring accurate viscosity prior to performing a Dynamic Light Scattering measurement will optimize the measurement data.
While accurate viscosity can solve the % error in Dynamic Light Scattering measurements, many continue to face the issue of sample volume when it comes to viscosity measurements. As a result, the m-VROC™ viscometer, has been thoughtfully constructed to be the solution to a prevalent issue many biopharmaceutical researchers face. m-VROC™ can easily conduct multiple tests with 50 µL of sample at varying temperatures and shear rates.
In instances where even 50 µL of sample is too much to spare, RheoSense has developed a technique which allows for measurements to be conducted with just 20 µL of sample and allowing recovery of approximately 50% of the clean, uncontaminated sample.