摘要:

SUMMARYThere are three techniques to measure local water fractions in the cryomicroscope. First, water content may be measured by a direct analysis of oxygen in bulk samples using a windowless detector. Secondly, mass thickness may be estimated in frozen‐hydrated, then frozen‐dried sections. This technique offers unrivalled spatial resolution, especially if the radiation dose in the frozen‐hydrated state is kept low by the use of electron scattering techniques instead of an X‐ray microanalytical background determination. External water content standards can be used instead of frozen‐hydrated sections and the whole analysis can even be performed exclusively on frozen‐dried sections at room temperature. Thirdly, local water fractions can be evaluated from X‐ray microanalytical measurements of element concentrations per mass in the frozen‐hydrated and frozen‐dried state. Corrections necessary for the other techniques cancel out. However, the high radiation dose required for a fully quantitative analysis excludes the use of these methods in thin or u

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1991.tb03079.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

SUMMARYThere are several limitations to the low‐temperature techniques which are currently being used for the preparation, examination and analysis of biological and organic samples by means of high‐energy beam instrumentation. The low thermal conductivity of samples and the inadequacy of rapid cooling techniques means that, with the exception of thin‐film suspensions and the surface of impact‐cooled bulk specimens which may be vitrified, ice crystals of varying sizes will be present in nearly all samples which are quench cooled. Data are presented which indicate the depth to which adequate cryo‐fixation may be achieved for both morphological and analytical studies. Although dynamic processes may be time resolved in the outer parts of quench‐cooled samples, the decreased freezing rate below the surface makes resolution of these processes much less certain. The quality of information which may be obtained from quench‐cooled samples is limited by radiation damage. Low‐dose microscopy of vitrified thin‐film suspensions of macromolecules continues to provide valid structural information at the molecular level. The increased doses needed for X‐ray microanalysis present serious problems with the high spatial resolution analysis of thin frozen‐hydrated sections although much less damage is observed in dried samples. A case is presented for using the outer fracture faces of frozen‐hydrated bulk samples for low‐resolution anal

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1991.tb03080.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

SUMMARYElectron‐induced radiation damage can cause errors in interpreting electron micrographs. Radiation damage is distinguished from contamination (polymerization of hydrocarbons) and etching (radiolysis in the presence of water), both of which can be controlled by a proper specimen environment in the microscope. While temperature has little effect on the primary interactions of fast electrons with matter, most secondary radiation‐damage processes are temperature dependent. Because damage mechanisms differ so greatly among materials, there is no simple factor by which specimen stability is improved as a function of temperature (some cases improve fivefold, others improve 100‐fold). While some specimens are stable to almost arbitrarily high doses, some tests reveal damage at 1 e/nm2. This paper surveys damage rates and temperature dependencies of various materials as a guide for future electron microscopic studies of organic spec

ISSN:0022-2720    

DOI:10.1111/j.1365-2818.1991.tb03081.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY