Did you miss out on our January webinar – Visualizing Hydration and Dehydration of Pharmaceuticals, Foodstuffs, and Other Materials Using Wet Scanning Electron Microscopy? Don’t worry about it, because you can learn about everything that you missed right here or even watch the recorded webinar on demand here: http://analyticalanswersinc.com/about-us/webinars/
Edward Norton, Technical Director at Analytical Answers, presented this session. It focused on the abilities of wet scanning electron microscopy (SEM) in solving challenging problems across many different industries from pharmaceuticals to the food industry.
To begin the webinar, the basics of scanning electron microscopy were reviewed. So, how does it work? An electron source running in a high vacuum produces an electron beam. The beam passes down the column towards the sample. On its way down, coils and lenses scan, deflect, and focus the beam onto the sample. The beam then interacts with the sample and produces secondary and backscattered electrons and x-rays. Scanning electron microscopes are a very useful tool in giving us high resolution, high magnification images.
A wet SEM, or an environmental SEM, is a SEM in which you introduce gas and water vapor and control the humidity. Technologically, a wet SEM differs from a normal SEM by:
• A specialized pumping system, which allows the chamber to be at higher pressures than the electron source
• A cold stage, which allows for wet conditions
• A water source, and
• Specialized detectors, which work at higher pressures than in normal SEM conditions and allow for the greatest surface resolution.
Samples in a wet SEM can start wet or can start dry and become wet. In the first example you are viewing the sample in its natural state. In the latter example, you can observe the sample changing phases from dry to wet. To best capture, visualize, and understand the changes happening to the sample when going from wet to dry, it is best to do cycles. This is when the sample is reproducibly exposed multiple times in succession, which allows us to look at multiple exposure times during specific humidity conditions.
Wet scanning electron microscopy is used for three main reasons:
• The first is charge neutralization. In normal SEM samples must be conductive or made conductive for the technique to work. In wet SEM the gas and water molecules in the chamber perform this function for us.
• The second reason is to prevent dehydration. This is accomplished by examining wet samples and reducing and/or controlling the rate at which they dry out.
• The last reason is to observe phase and material changes. After a dry sample is exposed to humidity, the changes of the sample’s texture and structure are recorded. This can also be used to observe dry samples at higher and higher temperatures to look at deformation as a function of temperature over time.
Although wet SEM is a very useful tool, it does have some limitations. Pressure is significantly below atmospheric pressure, so if the material of the sample is very porous, you may get some gassing/bubbling. Low temperatures are required to achieve condensed (wet) conditions, so reaction kinetics are reduced. Therefore, you must keep the temperature at a constant for a longer period of time, if you are trying to gauge reaction kinetics. Another limitation is the video speed; the fastest video that can be captured is about 1 frame per second due to the gas scattering time. The last limitation is that small sample sizes are required, due to the small stage area.
Analyses are performed by either: 1. Controlling the pressure and altering the temperature or by 2. Controlling the temperature and altering the pressure. Constant temperature is better when monitoring reaction kinetics, but constant pressure is better for video analyses.
An example of a case in which wet SEM is very useful is with a pharmaceutical product – a capsule shell. A capsule shell is the outside casing of a medication. How fast the capsule dissolves needs to be regulated by producers in order to control the rate at which the drug inside the capsule is released and then absorbed by the body. By using wet SEM, different formulations of capsules can be compared to find the perfect dissolve rate.
Another example of a case in which wet SEM is useful is with food products. In wet SEM you can visualize the morphological and textural changes that occur to a food sample at different humidity levels. This can help food producers to be more knowledgeable about the necessary conditions that food products need to be stored in.
Wet SEM has a wide range of applications in pharmaceuticals, food science, and other industries where analyzing wet samples is critical to quality control. It allows for testing of products and substances in ways that simulate real-world conditions, providing insights into performance.