Gas pycnometry

What is pycnometry used for?

Gas pycnometry is used to determine the density of solid materials and suspensions, ranging from bulk objects to granules, powders, and slurries. By comparing the densities of different batches of the product, it is possible to deduce information about the encapsulation of air in closed pores within the sample, which may be a sign of compromised product quality. This makes gas pycnometry valuable in the research and development of polymers, catalysts, ceramics, films, coatings, and various other materials with varying degrees of porosity.

Helium pycnometry data can also be combined with specific surface area data obtained from a BET measurement. As a result, it is possible to calculate the volume-specific surface area (VSSA), which can be used to determine whether the material has nanoscale properties. Nanomaterial identification is a key part of the compliance testing of cosmetics, food, and feed, as an extended safety assessment is required when these products contain nanoparticles.

How does a gas pycnometer work?

The gas pycnometer is comprised of two connected chambers of known volume. The sample is placed into the first chamber, which is then filled with an inert gas, most often helium. The volume of gas required to fill the rest of the chamber, as well as the gas pressure, is recorded. The gas is then allowed to flow into the reference chamber, and the change in pressure is recorded.

Knowing the volume of the gas and the pressure difference, the pycnometer will automatically calculate the volume of the sample using an equation based on Boyle's law. When the mass of the sample is known before the measurement, the density can be calculated simultaneously.

Helium vs. other gasses

Pycnometry is performed with an inert gas, as any interactions between the gas and the sample would alter the results. Helium is most commonly used, but other gasses including nitrogen, carbon dioxide, and sulfur hexafluoride are also available. Gas selection should be based on which gas has the lowest propensity to interact with the sample.

Advantages and limitations of gas pycnometry

Gas pycnometry offers a highly accurate and reproducible method of measuring volume and density. The technique is suitable for a wide variety of solid sample types and is typically non-destructive.

The gas cannot, however, penetrate any closed pores the sample may contain. Instead of true volume, therefore, gas pycnometry provides the skeletal volume, meaning the combined volume of the material and the closed pores.

Need an analysis?

Measurlabs offers He pycnometry measurements as a part of nanomaterial identification and analysis by EFSA's guidelines. Other applications are also possible. You can contact our experts through the form below to describe your measurement needs and request a quote.