In a standard experiment, the sample is loaded in the balance and isolated in a chamber. The chamber temperature is carefully monitored, alongside the pressure/concentration of the gas phase. Change in mass given by the balance has three contributions: the change in sample mass given by uptake in the sample, a change in net buoyant force given by the change in density of the gas phase and a drag force of a gas flow on the sample pan.

Buoyancy Force

The buoyancy force is small for experiments at ambient pressure and temperature but can play a role at high pressure (>a few bar) where corrections may be required.

Drag Force

The drag force is only relevant when a dynamic mode is used, as the gas flow impinges on the balance parts and may need to be corrected through a blank. A way of minimizing both drag and buoyancy corrections is to construct the balance symmetrically, having a sample and a reference pan in identical configurations in the flow path.

Advantages of Gravimetric Methods

A well-known benefit of gravimetric methods is that they typically require a low amount of sample: as little as 1-5 mg can be used to obtain reliable results for single component measurements. Conversely, the upper range of mass/volume is similarly much higher, a few grams and 10s of cm3 can be measured. Note that the mass range and accuracy depend on the performance and stability of the balance – SMS DVS balances have a total mass and resolution of 1 g / 0.01 µg for the low mass balance and 5 g / 0.1 µg for the high mass balance.

Experimental Design

• High temperature treatment under vacuum or inert gas is often required to dry and remove any residual molecules from the sample. In a gravimetric system, sample drying/activation can be followed directly and quantified.
• The probe molecule pressure can be in controlled in a pure or in a carrier configuration. Both static and dynamic modes can be used. The uptake is measured directly as a function of time, yielding the kinetics of mass transfer by default.
• Co-adsorption conditions (mixing of multiple probes) can be easily achieved in the fluid phase. Together with the ability to independently change total pressure and sample temperature, gravimetry is highly flexible when it comes to complex sorption programs: dosing or mixing of sequential components (co-adsorption/competitive adsorption), flowrate changes, temperature and pressure swings, in-situ activation under inert carrier or vacuum, stability testing, etc. SMS DVS systems are all gravimetric systems designed around the SMS UltraBalance design. Most DVS instruments are symmetric in design, except the DVS Intrinsic, DVS Endeavour and DVS Dual Vacuum.

• In a standard volumetric experiment, the sample is weighed into a closed vial. If the material needs drying or activation, the vial is then taken to high temperature under inert gas or vacuum for a pre-defined time.
• The vial is re-weighed to determine the activated sample mass. It is then quickly transferred to the sorption device and connected to the dosing volume. This reference cell has a known volume and is separated from the sample cell by a valve.
• All air is removed from the two volumes using a vacuum pump. Then, the volume of the sample cell must be determined, a procedure which is performed by expanding a non-sorbing gas like helium – essentially performing a pycnometry experiment.
• Once the free cell volume is calculated, the reference volume is pressurized to a known pressure and the valve separating the cell is opened. The pressure decays as the gas is taken up by the sample and its decay is monitored to estimate whether the sample has reached equilibrium.
• Adsorbed amount can then be calculated for that pressure through an equation of state, which is followed by sequential doses of the probe molecule in increasing amounts.

Advantages of Volumetric Methods

• Volumetry is a straightforward method to measure pure single component sorption as it has few moving parts. Its simple construction also means that extreme conditions of low/high temperature and high pressure are easier to implement.
• It is often used for routine characterization experiments, and it can be parallelized by having the reference volume shared between two or more sample cells.
• The geometry and gas path of the sample cell can be easily changed, which means volumetry is easier to integrate with other measurements – XRD, FTIR, microscopy, etc.

Challenges and Considerations

• The indirect measurement of sorption uptake through pressure has several important consequences. There must be enough sample in the cell to produce a measurable pressure drop. This means that minimum sample requirements are higher, usually 50-100 mg, while maximum sample possible is limited by the volume of the system, usually up to 1 g.
• Another consequence is that volumetry can only operate in a static dosing mode. The amount of adsorbate dosed should be known in advance or optimized. It should also be noted that each subsequent dose after the first propagates pressure errors in previous doses, such that the uptake uncertainty compounds as more points are recorded.
• Since it is an enclosed system, changing the temperature also changes the pressure in the system. Isobars are therefore not commonly possible on volumetric systems. Mixtures can be potentially dosed. However, as there is no knowledge of the partial pressure of each component in the headspace after the sorption process, assumptions must be made for an uptake calculation.

Method Modes

• Chromatographic/breakthrough methods are inherently flow/dynamic methods. A pure mode of introduction can theoretically be used, though systems most often work in a carrier mode to simplify total pressure control over the column.
• The probe can be introduced as a step change in inlet concentration, a mode which is often termed “frontal” or “breakthrough” due to a concentration front being generated and passing through the column until it breaks through to the other side. Thereby a mass balance can be performed to determine the amount of probe that has been retained in the column – which can be assigned to uptake on the surface and in pores.
• The probe can also be introduced as a pulse of concentration, similar to chromatography. This mode of operation is often known as “inverse chromatography”, since we are characterizing the sample in the column rather than the gas phase. The time for the pulse to fully elute is termed “retention time”. In all cases, the shape of the resulting concentration curve provides parameters about equilibrium uptake, transport through the bed and strength of the probe molecule’s interactions with the material.

Experimental Process

• In a common experiment procedure, a column of desired size is packed with a sample of the material. The column is weighed before and after packing to determine the exact sample mass.
• If thermal activation is required, the column can be purged under vacuum or an inert gas while heated, with activated mass determined again.
• The column is then loaded in the sorption analyzer. The volume of the system must be determined for an accurate experiment, a term known as “dead volume” or “non-sorbing retention time”, which is commonly done with a front or a pulse of a non-interacting gas such as helium or methane.
• The probe pulse or front is then passed through the column, with sensors at the outlet monitoring the flowrate and concentration. The mass balance is then performed.

Challenges and Considerations

• The number of parameters which must be precisely controlled and measured at both column inlet and outlet – temperature, pressure, concentration, flowrate – make this method complex to accurately set up.
• In general, the method development takes longer while single point experiments are faster to perform. Most systems also require a larger amount of sample than previous methods, 100s of mg to a few grams, through the set-up can be scaled up to kg ranges to simulate industrial processes.
• Sample morphology plays a role, with fine powders and crystals often leading to blockages and over pressurization, requiring preliminary shaping. On the other hand, there is a large flexibility in the type of sensors used to monitor concentrations, and to set-up hyphenated techniques around the material in the column itself.

Additional Capabilities

Chromatographic/breakthrough methods are one of the few methods which can be used to determine true multicomponent adsorption data: e.g., individual uptakes of two or more components in a mixture and surface interaction parameters. The shape of the eluted peak or breakthrough curve encodes information about the kinetics of adsorption and mass transfer through the bed, which can be crucial for process scale-up.

The iGC-SEA and the BTA Frontier systems are SMS flagship dynamic chromatographic and breakthrough analyzers, respectively.