Sedimentation analysis of aqueous microsuspensions based on near infrared transmission measurements during centrifugation. Determination of a suitable amount of gelling agent to minimise settling in the gravitational field

The behavior of model drug suspensions containing 0.5-3.8% of sodium carboxymethylcellulose was investigated under centrifugation with different rotor accelerations (12-128 g), using a particle separation analyzer, the LUMiFuge 114, with a timed near infrared simultaneous transmission profile monitoring of the initial interface sinking rate. Initial sedimentation velocities obeyed a power law as a function of their centrifugal acceleration. The proposed simple power law considered the ideal case of a linear relationship between settling rates and acceleration. Sedimentation velocities calculated for acceleration under gravity were of the same order of magnitude as those observed in conventional sedimentation testing using graduated cylinders. Suitable gelling agent concentrations were estimated for the required suspensions to remain homogeneous over given periods, i.e. ad hoc toxicological suspensions and permanent suspensions. The analysis of separation under accelerated conditions is a useful guide in formulation research, minimizing dependence on long-term stability testing and allowing an early detection of gravitational sedimentation problems.