‘Cool ESEM’ - Imaging Ice-Containing Systems at Freezer Temperatures

Abstract

<jats:p>Conventional scanning electron microscopy permits the use of cryogenic techniques, which has led to the examination of ice-containing samples (eg ice cream) at temperatures of -80°C and below. At these temperatures moderate etching of the ice crystals occurs which helps to identify the crystals in the surrounding glassy matrix. However, imaging at higher temperatures - equivalent to those at which most ice-containing systems are actually utilised - is impossible because of rapid sublimation. We have set out to develop a system which will enable us to examine ice crystals at temperatures of -20°C and below, corresponding much more closely to realistic conditions of use.</jats:p><jats:p>In the environmental scanning electron microscope (ESEM), a gas can be maintained around the sample. Not only can this be used to maintain the sample in its native state, but the gas is actively involved in the imaging process through its role in signal amplification.</jats:p>

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: “Subzero ESEM”

“…Both static and dynamic experiments in low-temperature microscopy were compared in high-vacuum and low-vacuum conditions (Stokes et al, 2004) in which the imaging gas was nitrogen, suggesting the need for the addition of a small partial pressure of water vapor in the specimen chamber to enable observation at temperatures of >−80°C. The idea of mixing two gases in ESEM, with one being water vapor to prevent ice sublimation and the second being a signal amplifying gas, had been presented earlier (Fletcher et al, 1998). Later work demonstrated an experimental set-up and preliminary results using nitrogen gas and water vapor, aimed at observing frozen-hydrated samples in the −85 to −35°C thermal range (Waller et al, 2008).…”