## Specimen holder The specimen holder itself looks like a long metal bar, with one, two or more rubber ring for airtight, and in the end a circular holder to store the copper grid for TEM samples. Inside the TEM, [[Vacuum system|high vacuum/ultra high vacuum]] is maintained, to avoid scattering of electrons. Different parts requires different levels of vacuum, so valves are applied to control the levels among these chambers. When we load the sample, we first transfer the sample holder into an intermediate airtight chamber, vent this small chamber, then open the door to take the specimen holder out. The detail procedures depends on machines, and for UHV systems, when we put the sample back, it could take long time (>10 min) to regenerate the required vacuum level. It is always reasonable to double check the vacuum of different parts inside the TEM, before performing any critical operations. In some occasions, especially for those operated in UHV, before inserting the sample holder, one should check whether there are **small fibers** on the inner part of holder and the rubber rings under optical microscopes, to avoid contamination and ensure the system is airtight.