Contamination analysis of the residual gas composition in transmission electron microscopy

Transmission electron microscopy (TEM) is a fundamental tool for many research fields like catalysis or material research. Many experiments require measurements under environmental TEM (ETEM) conditions, in which an atmosphere of a selected gas is in the vicinity of the sample, resulting in a higher pressure in the octagon of the microscope. However, the comparably high pressure in the octagon also influences how long volatile organic components (VOCs), which are often part of the sample itself, remain in the sample environment and can result in carbon contamination and undesired reactions during (E)TEM experiments. We have analyzed contamination in an ETEM octagon by investigating the impact of different organic solvents used in the sample synthesis and preparation, but also how tedious the removal of VOCs is once these contaminants are present inside a microscope. With proton-transfer-reaction mass spectrometry, we have quantified the outgassing contamination of samples consisting of THF, toluene or mesitylene depending on their drying time. We found that high amounts of contaminants adsorb on the inner surfaces of the octagon and we measured how well these VOCs desorb when, after removing the sample, pumping the octagon to typical TEM pressures. For persistent solvents or samples that were not sufficiently dried before insertion, contaminant removal is an extensive process that can be accelerated by plasma cleaning. Our investigation of residual solvents suggests that changes in the sample preparation (i.e. solvent choice and drying time ) can improve the contamination effects in both TEM and ETEM experiments.