Introduction To USP <922>: General Chapter For Water Activity Measurement

By Michael Lally, Application and Validation Specialist at Lighthouse Instruments

The recently published USP <922> General Chapter [1] describes the specific methods that pharmaceutical companies will use to calibrate, qualify, and use water activity instruments in their testing protocols. It is water activity measurements, not total water content, that correlate to critical product quality attributes. USP <922> includes a description of tunable diode laser absorption spectroscopy which is the only method to reliably and accurately measure water activity when the sample contains other volatile compounds. This article summarizes the importance of water activity and how laser-based headspace analysis can be used for water activity determination.