Because the test is sensitive, it may be influenced by a variety of factors. Monitoring the environment for airborne asbestos requires the use of sensitive sampling and analysis procedures. Quality Control/Quality Assurance Procedures (Data Quality Indicators) For instruments that incorporate a eucentric tilting specimen stage, all specimens and the cross grating replica must be placed at the eucentric position. Before the TEM calibration is performed, the analyst must ensure that the cross grating replica is placed at the same distance from the objective lens as the specimens are. After any maintenance of the microscope that involved adjustment of the power supplied to the lenses or the high-voltage system or the mechanical disassembly of the electron optical column apart from filament exchange, the magnification must be recalibrated. The frequency of calibration depends on the past history of the particular microscope. A logbook must be maintained, and the dates of calibration and the values obtained must be recorded. The field of view must be measurable or previously inscribed with a scale or concentric circles (all scales should be metric). Define a field of view on the fluorescent screen either by markings or physical boundaries. This is performed with a cross grating replica (e.g., one containing 2,160 lines/mm). The TEM must be calibrated at the grid opening magnification (if used) and also at the magnification used for fiber counting. The magnification calibration must be done at the fluorescent screen. The camera constant is one-half the diameter of the rings times the interplanar spacing of the ring being measured.ģ. An average camera constant using multiple gold rings can be determined. Since the unknown d-spacings of most interest in asbestos analysis are those which lie closest to the transmitted beam, multiple gold rings are unnecessary on zone-axis ED patterns. Thicker gold film would normally give multiple gold rings, but it will tend to mask weaker diffraction spots from the unknown fibrous particulate. In practice, it is desirable to optimize the thickness of the gold film so that only one or two sharp rings are obtained on the superimposed ED pattern. A thin film of gold is evaporated on the specimen TEM grid to obtain zone-axis ED patterns superimposed with a ring pattern from the polycrystalline gold film. This can be achieved by using a carbon-coated grid on which a thin film of gold has been sputtered or evaporated. The camera length of the TEM in ED operating mode must be calibrated before ED patterns on unknown samples are observed. Determination of Camera Constant and ED Pattern Analysis.
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