Water Activity Determination Of Oral Solid Dosage Products Using Frequency Modulation Spectroscopy

By Allison Alix, Application Scientist, Lighthouse Instruments

Controlling water content in oral solid dosage (OSD) products, and dry pharmaceutical products in general, is essential to maintaining efficacy and safety. Water activity (Aw) is a measurement of the free or loosely bound water available to participate in unwanted chemical, physical, and biological reactions. Measuring the water activity at multiple time points during the product life-cycle will correlate to changes in critical quality attributes such as degradation of the active ingredient, changes in the dissolution or disintegration rate, and changes in physical properties such as hardness or friability.

The Lighthouse Instruments FMS-Water Activity Analyzer is a non-destructive headspace gas analyzer used to monitor the headspace water partial pressure in sealed, optically transparent containers. This analyzer employs a near infrared (NIR) laser that is tuned to match the specific transition frequency of the water molecule (~1400 nm). During a measurement session, the laser is passed through the headspace region (i.e. the region located above the product) and the laser frequency is repeatedly scanned over the absorption feature. Successive scans are averaged to improve the signal to noise ratio and the resulting amount of light absorbed after exiting the container is detected by a photodetector. The absorption of the light is proportional to the water vapor partial pressure in the headspace of the product-package system . The water activity associated with the headspace of a sealed product-package system is then determined by taking the ratio between the water partial pressure measured in the container headspace and the water partial pressure observed above pure water at the same temperature; therefore, a direct measurement of water activity can be made using the FMS-Water Activity Analyzer. Finally, because the water activity of a sample is highly dependent on temperature, the FMS-Water Activity Analyzer is equipped with a temperature-controlled sample chamber. Measurements can be acquired at temperatures ranging from 19°C to 45°C.