Powder Characterisation Using Inverse Gas Chromatography (iGC)

Interview conducted by Beth EllisonMar 31 2015

Dr. Daryl Williams, Director of Development in the Department of Chemical Engineering at Imperial College, and Dan Burnett, of Surface Measurement Systems, speak to AZoM about Inverse Gas Chromatography and how this established, but somewhat forgotten, the technique can be used to accurately study the solid-state properties of powders.

Could you please provide a brief introduction to the industry that Surface Measurement Systems works within?

Surface Measurement Systems works with a broad range of industrial sectors, who all share a common interest in solid-state materials usually powders. That includes pharmaceuticals, foods, natural materials, polymers, building materials, and personal care products to name but a few sectors.

Surface Measurement Systems develops and engineers innovative experimental techniques and instrumentation for the physicochemical characterization of complex solids. We are the world leaders in Dynamic Vapour Sorption technology and Inverse Gas Chromatography instrumentation and solutions.  Our products are used across many industry sectors including pharmaceuticals, personal care, foods and flavors, nanomaterials, composites, biomaterials, and polymers.

Could you briefly explain the basic technique behind Inverse Gas Chromatography?

Inverse gas chromatography is a powerful way to characterize particulate and fibrous materials, especially their surface properties, using gas or vapor adsorption.

Inverse Gas Chromatography (IGC) reverts the traditional roles of the stationary and mobile phases of analytical chromatography.  In IGC, a column is packed with powder, particulate, fiber, or film.  Then, IGC measures how long it takes a pulse of gas or vapor to travel through the column.  This retention time allows access to a wide range of physicochemical properties of the solids in the column, including surface free energies, works of adhesion/cohesion, heats of sorption, glass transition temperatures, solubility parameters, crosslink densities, and diffusion coefficients.  

Inverse Gas Chromatography. Image Credit: Surface Measurement Systems

How does the iGC-SEA technique differ from traditional inverse gas chromatography?

In traditional gas chromatography, the material to be characterized is an unknown gas or a liquid, which typically you want to identify and or quantify and do so by passing this through a reference GC column. In iGC-SEA, we want to study an unknown particulate material, and do this by packing a small glass column with say 25 mg of powder. We then elute known vapors through this packed column and use the vapor adsorption retention behavior to characterize the surface properties of the powder of interest.

The iGC-SEA instrument developed by Surface Measurement Systems uses a unique injection system to allow experiments to be performed over a wide range of vapor/gas concentrations.  Traditional IGC instruments used fixed injection amounts or offer a limited range in injection concentration.  The iGC-SEA precisely controls the amount of vapour/gas injected in order to perform experiments at precise surface/adsorbate ratios.  This allows the unique capability to quickly and accurately determine surface energy distributions to measure the energetic heterogeneity of materials.  Traditional IGC instruments typically only produce a single surface energy value, despite the fact that most materials are energetically heterogeneous.  

How does inverse gas chromatography compare to contact angle and atomic force microscopy?

All three techniques share the same conceptual basis- using a molecular probe such as a liquid, AFM tip, or adsorbing vapor molecule, to characterize a solid-state material. AFM and contact angles work well for flat substrates like films but are very poor for particulates. IGC, on the other hand, is ideal for particulate materials and can provide a wealth of surface chemical data, much of which cannot be measured using contact angles or AFM.

The contact angle is a liquid-based technique, while IGC is a gas/vapor phase technique.  In practical terms, it is very difficult to obtain reliable contact angles on particles, powders, and fibers.  In particular, it is nearly impossible to obtain reliable contact angles on porous surfaces or when the contact angle liquids swell/dissolve into the solid.  Since IGC is a gas phase technique, it is not constrained by the above limitations.  Therefore, IGC is often the only method to obtain reliable surface energy values on powders, particulates, and fibers.  In AFM, the forces are measured on only very small areas of the sample in contact with the AFM tip.  This typically requires hundreds, if not thousands of measurements to obtain surface energy values representative of the entire surface.  In contrast, the injected gases/vapors in IGC can have access to the entire surface in the column and surface energy values are more representative.  

What are the primary applications of inverse Gas Chromatography?

Surface properties, especially surface energy and surface energy heterogeneity are two major applications.

The primary application of IGC is determining surface energy values.  Materials surface energy is directly related to the thermodynamic work of adhesion and cohesion.  Therefore, surface energy values are important for powder flow, agglomeration, and interfacial adhesion.  In addition, surface energy values are used to investigate the effects of different surface treatments, modifications, and coatings.  Also, the IGC can measure very subtle changes in surface energy, enabling applications such as batch-to-batch variability, process-induced disorder, and surface aging/degradation.  Other important IGC applications include determining Henry’s constants, solubility parameters (both 1-D Hildebrand and 3-D Hansen), and heats of sorption.

Are there any particularly interesting case studies you would like to discuss?

Our recent Application Note 226 is a good example of both the new types of materials that can be studied using IGC as well as the unique information that can be obtained. This Note reports on carbon nanotubes and shows how different surface treatments result in differing carbon nanotube surface energies. It also shows how IGC can be used to measure the distribution of surface energies and gives us a surface energy heterogeneity profile for these carbon nanotubes.

We have recently used iGC-SEA to investigate the surface properties of biomaterials.  In particular, we investigated the surface energy and surface wettability of protein samples incorporated into an amorphous solid dispersion.  The iGC-SEA allows the unique capability of studying surface energy values as a function of relative humidity.  The surface energy values of the protein dispersion were measured between 0 and 80% RH and 25°C.  These variable RH experiments provide information on the humidity where the surface is saturated; thus yield information on binding sites at the surface of enzymes, and an optimum water concentration at which the substrate displaces water molecules to interact with the protein.

Please click here if you would like more information on the instrument used in this article or a quote

Can you tell me a little about the upcoming Inverse Gas Chromatography Symposium in May?

We have assembled a list of key speakers who will present their most up to date work on IGC in the USA this May. This includes international academic and industrial researchers.

The IGC Symposium will gather world-leading experts in IGC methodology and applications.  The first day will focus on a broad range of applications/materials including minerals, composites, polymers, and nanomaterials.  The second day will focus on pharmaceutical applications ranging from powder flow to formulation development.  There will also be a poster session and several networking opportunities to interact with speakers and attendees.

Who should attend the symposium and how will attending benefit them?

Professionals who work with powders, fibers, and nanomaterials, across virtually all industrial sectors, will find this an interesting meeting. Especially those who are interested in powder surface properties, flow, formulation, and performance.

Anyone currently performing IGC research or considering IGC as a technique to use would benefit greatly from this event.  This event will expose attendees to cutting-edge IGC research over a broad range of applications, such that novice to expert IGC users will gain valuable information.  Further to this, any scientist whose research involves measuring, altering, or controlling surface properties would find the meeting beneficial.  Several presenters will discuss how surface properties can affect bulk material properties and performance.

What are the most noteworthy events at the symposium?

As well as meeting with the expanding community of IGC researchers, I am especially interested to hear about the characterization of carbon nanofibers by the researchers at the Naval Research Laboratory as well as the use of IGC by the pharmaceutical researchers from Abbvie and Boehringer Engelheim.

The IGC Symposium will gather world experts in IGC technology and applications. This event will provide a unique opportunity to network with leading academic and industrial IGC researchers over a broad range of industries.  

Surface Measurement Systems' Inverse Gas Chromatography Symposium takes place in Newark, New Jersey from May 4th - 5th 2015. 

About Dr. Daryl Williams

Dr. Daryl R. Williams graduated with a B.Sc. (Hons) in Physical Chemistry from the University of Melbourne, Australia, and an M.Sc. in Polymer Science from Lehigh University, USA before coming to Imperial College London complete his Ph.D.  He is currently the Director of the Discovery Space and a Reader in Particle Science in the Department of Chemical Engineering.

DRW has published over 80 papers in refereed journals and been granted international 5 patents. His research interests include the surface and bulk characterization of complex organic solids, including especially biopharmaceutics, foods, pharmaceuticals, and polymers as well as their manufacture using spray drying, crystallization, freeze-drying, milling, and granulation.  

About Dr. Dan Burnett

Dan Burnett is the Director of Science Strategy for Surface Measurement Systems, Ltd. He is located at SMS’s North American headquarters in Allentown, PA.  He received his bachelor’s degree in Professional Chemistry from Eastern Michigan University in 1997.  He received his Ph.D. degree in Chemical Engineering from the University of Michigan in 2001.  Since joining SMS in 2001, he has continued his interests in sorption science and surface chemistry on a range of materials including pharmaceutical powders, natural and man-made fibers, polymers, films, and food/flavor systems.  Dr. Burnett has authored or co-authored over 25 papers in peer-reviewed journals and presented at numerous national and international conferences.