We begin this series by clarifying key terms related to sorption variables and how probe molecules are introduced into a system. This foundation sets the stage for a broader look at common measurement techniques, including their benefits and limitations.

Throughout this series, we will use the terms “gas” and “vapor” interchangeably to refer to molecules in the gaseous phase used to probe the material—also known as the sorbate. Strictly speaking, a gas is a fluid that cannot be liquefied by increasing its pressure (i.e., a fluid above its critical temperature).

Several commonly used experimental techniques measure gas and vapor sorption, including:

• Volumetric (or Manometric): Measures gas uptake by tracking pressure changes as gas is adsorbed by a sample.

• Gravimetric (Dynamic Vapor Sorption or DVS): Measures changes in weight to determine gas or vapor uptake by a sample.

• Chromatographic (or Breakthrough): Measures the flow of a probe molecule through a column, tracking variables to determine the molar mass balance.

These techniques have wide practical applicability: first, in determining fundamental material properties such as surface area, porosity, surface energy, hygroscopicity, and contact angle; and second, in assessing a material’s performance for applications that rely on sorption behavior—such as heterogeneous catalysis, API stability, water harvesting, and gas separation.

The data obtained from different techniques can vary, and a variety of factors should be considered when choosing the most suitable method. Each technique has its own set of advantages and limitations that influence this decision. Factors that guide this choice include:

• Measurements on large or small sample volumes, or samples with very high or low surface area and porosity.

• Sample sensitivity to temperature, vacuum, and pressure changes.

• The importance of mimicking process and real-world conditions—multiple gases/vapors, presence of air and humidity, constant or varying temperature and pressure, etc.

• The importance of determining sorption kinetics, equilibrium times, diffusion, and general time-resolved data.

• The complexity, cost, and ease of use of the instrument.

• The number of samples and gases/vapors to be analyzed simultaneously.

• The interest in probing the material’s fundamental properties, such as density, surface area, pore size distribution, surface energy, specific gas-material interaction properties, cohesion and adhesion, etc.

• Options to combine multiple measurements at the same time (hyphenations), e.g., XRD, Raman, IR, calorimetry, etc.