SEM for membrane and ion exchange analysis.
SEM is one of the most widely used methods for analysing fouling on membranes and ion exchange resins. Its power is in that it allows you to see exactly what is happening on the membrane surface at the microscopic level and help find solutions. Follow us below to learn more about this powerful technique.
What is a SEM?
A Scanning Electron Microscope (SEM) produces images of a sample by scanning the surface with a focused beam of electrons, instead of light. Electrons interact with atoms in the sample, giving topographic information about the surface and composition of the sample. A SEM reaches much greater magnification (up to 300,000x), resolution, and depth-of field than an optical microscope.
The SEM uses a Tungsten filament as the “light” source, similar to an incandescent light bulb. The filament is heated in a vacuum until it glows and gives off electrons. We then use electromagnetic lenses to focus the electron beam. This electron beam is scanned across the surface and the reflected signal is used to produce an image. In the most common SEM mode, secondary electrons emitted by atoms are detected using a secondary electron detector.
Specimens can be analysed under high or low vacuum, with high vacuum enabling higher magnification images and better elemental analysis. The issue here is surface charge, which affects the signal used for imaging. For a high vacuum analysis the samples must be coated in carbon or gold to ensure conductivity and prevent the build up of surface charge. Most membrane or Ion Exchange resin samples are analysed using low vacuum to avoid interference from the coating. For most of our applications, a low vacuum analysis is more than sufficient.
What is SEM-EDS used for?
SEM-EDS is one of the more common methods used during membrane and ion-exchange resin autopsies or characterisation. With some additional tools, it allows us to do advanced analytical techniques such as:
- Elemental analysis using Energy Dispersive X-Ray Spectroscopy (EDS or EDX)
- Composition mapping to show different elements across a surface
- Identification of physical damage on the membrane surface
- Identification of foulants, particles and other contaminants
It is one of the more powerful and versatile tools at our disposal for understanding a membrane or Ion-exchange problem.
Membrane cross-section
SEM image showing surface contamination, polyamide active layer and porous polysulphone support layer.
Hollow Fibre UF Membrane
SEM image of hollow fibre membrane cross section
Aragonite fouling
SEM image of dendrimers of calcium carbonate (aragonite crystals) on a RO membrane.
Gypsum fouling
SEM image showing Gypsum crystals fouling a RO membrane
What is EDS Microanalysis?
Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). (See Handbook section on SEM.) The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam, which characterises the elemental composition of where the electron beam is striking. Unlike broad methods like XPS, XRF or XRD, the EDS can focus on features as small as 1 µm or less.
X-ray Microanalysis can identify, locate, and quantify the elements that compose a membrane sample.
SEM-EDS is one of our more commonly used analysis techniques. It is a powerful tool for identifying individual particles on foulants on the membrane surface. The EDS spectra gives us individual peaks emitted by specific atoms. For large peaks, there are occasionally secondary peaks associated with this, as can be seen in the below spectra for calcium sulphate (gypsum).
The downside to using SEM-EDS in membrane analysis is that the electron beam tends to penetrate up to 10um into the sample, and as such cannot be used to analyse the active layer of RO and NF membranes. Additionally, at low vacuum, the quantification of light elements like carbon, nitrogen and oxygen can be problematic. Thus, when a more accurate measure of the membrane surface is required, other techniques are employed. For example, when looking for oxidative damage, we use X-ray Photoelectric Spectroscopy (XPS) and for complex scales we use X-ray Diffraction (XRD) to give a positive identification of the crystalline structures.
Want to know more?
If your membrane performance has declined and you think an autopsy will help solve your issue, contact us. We will be able to walk you through the process. If your membranes are still under warranty, contact your supplier as well, as an autopsy may be covered.
If an autopsy is recommended, our experienced staff will help you identify which modules you should remove for testing and identify system information and water quality data needed to get the most out of the process.