Microscopes in an electron microscope laboratory are built to be within specific tolerances in relation to resolution and magnification. The final specification from the manufacturer should include detail of magnification accuracy, drift rate, contamination rate and resolution, On delivery, the onus is on the installation engineer to confirm that the microscope meets specification. However, the final responsibility for microscope acceptance to specifications is the responsibility of the lab management.. Energy dispersive instrumentation is also specified as to specific resolution; and should also be confirmed on installation.

In some cases, those with final responsibility for equipment performance might not resolve with the manufacturer's representee, at installation, that the instrumentation meets all specifications.

Three commercially available vacuum samplers (also known as microvacuum samplers or microvacs) were examined for compliance with the Standard ASTM Methods for asbestos in settled dust. The American Society for Testing and Materials (ASTM) Standard Test Method (D5755-95) for Microvacuum Sampling and Indirect Analysis of Dust by Transmission Electron Microscopy for Asbestos Structure Number Concentration and the similar Method (D5756-95) for mass require that a sample be collected by vacuuming a known surface area with a standard 25 or 37 mm air sampling cassette using a plastic tube that is attached to the inlet orifice which acts as a nozzle.

Low vacuum and environmental SEMs have become commonplace in many labs for many applications. With the rise in popularity of these instruments comes the need to understand their capabilities and limitations. An article I wrote in this publication in August 1997 summarized the issue of beam scatter in the low vacuum SEM, In this first of a series of three articles, I will go into more detail and give more examples on the phenomena of beam scatter in the low vacuum SEM.

The problem of beam scatter in the low vacuum SEM chamber is well known. Electrons from the primary beam interact with gas molecules in the SEM chamber, scatter, and hit the sample surface some distance away from the primary beam. The result is a “skirt” of electrons much wider than the primary beam. The amount of scatter depends on four variables: chamber pressure, working distance, accelerating voltage, and gas type. I will discuss the first three in this paper. The effect of gas type will be addressed in the next issue.