The moisture content of food has been measured for many years to control microbial growth and assess shelf-life. This measurement, called Water Activity, is also known as Equilibrium Relative Humidity (ERH).

Pharmaceutical oral solid dosage forms also have an ERH that is below the threshold for microbial growth. However, the active ingredient and also the other ingredients (excipients) of tablets and capsules can change physically and chemically as a result of moisture sorption or desorption.

Relequa is a simple system which monitors the ERH of tablets and capsules by allowing a sample to come to equilibrium when exposed to ambient humidity and the resulting measured relative humidity is the ERH value of the sample. The sample will adsorb water at a relative humidity above the ERH and, conversely, will lose water at a relative humidity below the ERH.

Relequa follows this dynamic process in real time and is designed to test Equilibrium Relative Humidity (ERH) of tablets and capsules within high density polyethylene (HDPE) containers. The built-in 30 ml HDPE bottle has a small head space to allow rapid measurement of the internal %RH and has a workable volume for handling samples of tablets and capsules.

Ideal as a first ‘look see” overview for water activity, moisture sorption studies are easily performed by simply adding tablets or capsules and screwing on a cap. The software has options for observing changes in the internal humidity in real time and downloading data.   Moisture sorption and desorption by the sample is continuous and, after a period, determined by the activity of the sample, an equilibrium is established at its ERH.

Knowing the ERH is important in deciding if a material is likely to adsorb or lose moisture at a given %RH. Based on the ERH of samples, manufacturing processing conditions can be determined to control processing and packing.

Typical other applications are, testing the effectiveness of desiccants and the impact of temperature changes during sample shipment.

Tests using Relequa are simple and empirical requiring no calculations. Relequa is compact and completely portable. The test system is applicable for use both as an aid for in-process development and as a process analytical tool.

Relequa has been used to gain valuable information and insights into the behaviour of moisture in final and finished products exposed to different environmental circumstances.