Environmental SEMs

Frank has been studying ice fog formation in the Fairbanks, AK area, and trying to see if there are connections with the high levels of wintertime PM2.5 sulfates measured in the area. He located this article featuring an Scanning Electron Microscope (SEM) image of Fairbanks ice fog crystals (left).

Only one, BIG problem - SEMs operate under near-vacuum conditions, and with samples possessing conductive coatings. These ice crystals don't meet these criteria. So, it shouldn't even be possible to obtain this image. How was it done?

Starting in the 1980's, an Australian research group pioneered ways to keep samples at one pressure, while keeping the rest of the electron gun assembly at much lower pressures, and also to remove the necessity of having conductive samples altogether. So, an entire new universe of imaging became reachable with what are now called Environmental SEMs. Quite impressive! I didn't know anything about all this!

The wonders of SCIENCE!

The write-up below regarding Environmental SEMs implies the methods aren't perfect. Still, it's amazing! I am reminded of the (very non-PC) quote by Samuel Johnson, which I adapt:

A[n Environmental SEM] is like a dog's walking on his hinder legs. It is not done well; but you are surprised to find it done at all.

Here is the interesting write-up:

The Environmental Scanning Electron Microscope (ESEM) or 'wet' SEM is the latest version of one of the most powerful analytic tools available to scientists. Like all scanning electron microscopes, the ESEM offers the ability to image specimens at very high spatial resolution; as high as 2 nanometres in some cases. However, conventional SEM's have two important constraints: the specimen chamber must be kept under high vacuum conditions, and the specimen must either be electrically conductive or given a conductive coating. The ESEM, on the other hand, permits a small amount of gas in the specimen chamber, up to 20 torr. Although this gives rise to a number of practical considerations, the main points are that the ESEM can examine uncoated insulating specimens as well as wet materials, including unfixed biological tissues. Thus, the range of specimens that can be examined and the types of investigations that can be performed are vastly increased as compared to what can be done with conventional scanning electron microscopes.

Building such a microscope requires that two main considerations be overcome. First, the electron gun and most of the optical column must be maintained under high vacuum conditions .... If the chamber is to be held at several torr of pressure, a very steep pressure gradient must be sustained. ... Second, conventional secondary electron detectors are also designed to operate under high vacuum conditions. Therefore, a new type of secondary electron detector must be employed....

In the ESEM, the pressure gradient between the specimen chamber and the electron optics is achieved by creating a series of differentially pumped vacuum zones, separated by pressure limiting apertures. These apertures are sufficiently large to allow the electron beam to pass through, but still small enough to severely limit gas flow from one compartment to the next. ... The pressure zones created by the apertures are then pumped by various means with increasing efficiency as the gun is approached. Mechanical roughing pumps are sufficient for the specimen chamber and lower column regions. Most of the electron column is pumped by either diffusion pumps or turbo-molecular pumps, depending on the specific model. For instruments with lanthanum hexaboride filaments or field emitters, the gun chamber is connected to an ion pump sufficient to achieve pressures of 10-6 and 10-9 torr, respectively. Thus, pressure gradients of up to 10 orders of magnitude can be realised over a distance of about 50 cm!

If water vapour is used as the gas in the specimen chamber, wet specimens can be maintained in a hydrated state. In fact, liquid water can be brought into thermodynamic equilibrium with the vapour phase. ... Thus, typical working conditions in the ESEM might be something like 2ºC and 5.3 torr of water vapour. If the specimen temperature is held constant, increasing or decreasing the partial pressure of water vapour results in condensation or dehydration, respectively. This ability to control the hydration state of a specimen has allowed us to explore a large number of application areas, ranging from examining gels and emulsions to the drying of thin film coatings and studying the structure of ice cream!