Dynamic Vapour Sorption(DVS) or Gravimetric Vapour Sorption(GVS)

Dynamic Vapour Sorption analysis is a widely used gravimetric technique for the study of vapour interactions with solids. In many cases, the vapour of interest is water, because of the presence of ambient humidity during manufacturing, stockage and use of materials, from foods and drugs to plastics, paper, chemicals, and geological samples. The measurement of water sorption isotherms is now a fundamental characterisation technique in many of these fields.

The technique involves:

• A sample held in a temperature controlled chamber.
• A sensitive microbalance, to measure the absorption and desorption of the vapour.
• A stream of carrier gas with controlled vapour content. Usually this is generated by controlled mixing of a pure (dry) carrier flow with a vapour-saturated flow.

Measurements are typically performed with constant pressure and total flow rate, to minimise the buoyancy and flow force effects. The measurements are normally performed at a fixed temperature, to obtain moisture sorption isotherm curves by allowing the sample to equilibrate at a series of different vapour concentrations. The measurement is often very sensitive to temperature, requiring very stable temperature control, although repeating at several different temperatures is commonly used to probe the thermodynamics of the sorption process.

The dynamic method is particularly useful where the sample may release vapours or gases when it interacts with the analysis vapour. This is very often the case with foods, which often release moisture or volatile organic compounds, and also for many organic pharmaceutical compounds, which can release solvents trapped during synthesis. In some cases, the sample can lose up to half its weight due to this effect. The continuous flow of gas eliminates these contaminant vapours, which can lead to false results if allowed to accumulate, as in a static measurement method, where there is no flow of gas.

Gravimetric Vapour Sorption instruments of this type, such as the Hiden Isochema IGAsorp or the multisample Projekt Messtechnik SPS instruments are very cost-effective and allow rapid measurement of moisture sorption isotherms.

Depending on the application, the sample size may vary from a few milligrams, as for a novel pharmaceutical material, to tens or hundreds of grams for building materials.