Nestetypen ja nesteheliumin vertailu kryogeeneinä kryo-elektronimikroskopiassa käyttäen ultramikrotomisoituja pehmeän aineen näytteitä

For electron beam-sensitive samples, such as soft matter and biological specimens, radiation damage is the biggest limiting factor when it comes to achieving high-quality images in electron microscopy. Liquid nitrogen is widely used as a cryogen to attempt to minimize electron beam damage by cooling the sample to cryogenic temperatures. However, there have been varied results about the efficacy of slowing down the deterioration of the structure of the sample by further decreasing the temperature by using liquid helium as a coolant. The aim of this study is to examine how liquid helium compares to liquid nitrogen as a cryogen in cryogenic transmission electron microscopy when it comes to reducing radiation damage and achieving high-quality images of ultramicrotomed sections of three different specimens: a soft matter specimen of a 1:1 ratio of P4VP and PDP, a block copolymer with a liquid crystal layer and poly(ethylene oxide) (PEO). In this thesis, the extent of radiation damage occurring on the liquid crystal and P4VP(PDP) is studied in liquid nitrogen and liquid helium temperatures by analyzing fading Fast Fourier Transform-patterns from different regions of interest on the specimens. It is determined that liquid helium cooling decelerates radiation damage of P4VP(PDP) and the liquid crystal by factors of 1.6 and 1.5 respectively. Furthermore, the destruction of PEO under liquid helium and liquid nitrogen temperatures is studied qualitatively, by visually studying image sets acquired from different regions of interest. It is determined that liquid helium cooling significantly reduces radiation damage of the PEO specimen, by a factor of 3.5-4.