New USP Chapter <922> Water Activity

On March 30, 2018 the United States Pharmacopeia, USP, posted the announcement on a new general chapter <922> Water Activity. Deadline for input for preparing the draft is April  30, 2018. The final proposal is estimated to be published in Pharmacopeial Forum (PF) 44(6) in November 2018.

The USP General Chapters—Physical Analysis Expert Committee proposes this new chapter for suppliers and manufactures of excipients, drug substances, drug products, veterinary products, regulatory agencies, and manufacturers of water activity instruments.
Water activity (Aw) is the effective concentration of pure water in a raw material or product. When water is strongly bound to a solute or surface, it does not contribute to the Aw. It has been established that bacterial growth is correlated with water activity, not water content. Substances (e.g. food) obtaining higher Aw values tend to support more microorganisms.

According to USP, the "new general chapter is intended to provide guidance for performing measurements of water activity (Aw)". Furthermore, the chapter "will specify methods for qualification of instruments, calibration of instruments, methods for performing water activity measurements, and reporting of results".

The preliminary outline is as following: 

  • Introduction
  • Background
  • Application of Water Activity Measurements
  • Types of Water Activity Meters:
    •    Electronic Hygrometer (Resistance hygrometer  / Capacitive hygrometer)
    •    Psychrometric (wet bulb temperature) 
    •    Dew Point  (Chilled-mirror hygrometer / Thermocouple) 
    •    Hair hygrometer  
    •    Freezing-point depression sensor 
    •    Near infrared spectrometry 
  • Operation and Calibration of Water Activity Meters
    •    Standard Saturated Salt Solutions 
    •    Calibration of Water Activity Meters (Dew Point Hygrometer / Electronic Hygrometer / Near Infrared spectrometry)
    •    Precision / Repeatability Assessment (Performance Qualification)  
    •    Measurement of Water Activity (Sample Preparation / Calibration Check / Measurement Procedure / Reporting Results)
  • Assay Verification and Validation 
  • Troubleshooting  
    •    Dew Point Hygrometer  
    •    Electronic Hygrometer   
    •    Near Infrared spectrometry
  • Glossary
  • References

According to the USP the new general chapter <922> will be developed by a Joint Subcommittee with representation from General Chapters—Physical Analysis, General Chapters—Microbiology, and the Non-botanical Dietary Supplements Expert Committees. In addition, the Joint Subcommittee will assess the impact of the new general chapter on the existing General Chapter <1112> "Microbiological Attributes of Nonsterile Pharmaceutical Products—Application of Water Activity Determination" and work with the General Chapters—Microbiology Expert Committee to update <1112> to align with the new general chapter <922>.

For more information please see the general anoouncement on the USP website.

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