Scanning electron microscopy suitable for asbestos testing

Friday, 05 July, 2024

Scanning electron microscopy suitable for asbestos testing

Researchers at the US National Institute of Standards and Technology (NIST) have verified that scanning electron microscopy (SEM) is a highly accurate way to test air samples for the presence of asbestos, offering a less expensive, easier-to-use alternative to transmission electron microscopy (TEM) and phase contrast microscopy. Their work has been published in the journal Analytical Methods.

In 1989, the US Environmental Protection Agency (EPA) required most schools undergoing asbestos abatement to use TEM to test for the presence of asbestos fibres in air samples before reopening. Several states still require or recommend using TEM for testing as part of asbestos removal in commercial buildings — but TEM must be carried out in a specialised lab by highly trained staff and can be expensive. Another approach, phase contrast microscopy, is easier and cheaper but less precise.

Now, NIST researchers Jason Holm and Elisabeth Mansfield have claimed that SEM offers “a viable alternative to the current regulatory methods for asbestos identification and classification” and can achieve results roughly comparable to TEM. Since SEM is, in many cases, cheaper and more convenient than TEM, its use could potentially speed up and reduce the expense of asbestos remediation in the United States, which costs an estimated US$3 billion every year.

In both TEM and SEM, technicians focus electron beams on a microscopic amount of material. Electrons interact with the material to produce highly detailed information on the material’s composition, structure and shape.

With TEM, the electrons pass through the sample, whereas with conventional SEM, they are reflected off the surface. This enables TEM to produce more detailed images and probe the surface’s interior. TEM also offers much better spatial resolution — the ability to distinguish between objects very close together — than SEM.

But in recent years, SEM manufacturers have improved the technology’s imaging power and other capabilities. Several companies now produce tabletop SEMs, making it possible to use the technology in the field, while TEM must still be done in a lab. Holm said training to use and operate SEM equipment can be completed in several months, while “expertise in TEM can take years to establish”.

To test SEM on asbestos, Holm and Mansfield used NIST Standard Reference Material (SRM) 1866, a sample of asbestos fibres the agency produces for labs to benchmark their equipment and testing procedures. The SRM comes with extensive data characterising the properties of the material.

Using SEM, the researchers analysed SRM 1866. Their results closely agreed with those listed in the SRM’s documentation, indicating the method’s accuracy.

“There are some capabilities TEM has which SEM doesn’t, but we think SEM is good enough [for asbestos abatement],” Holm said. He and Mansfield summarised SEM’s potential advantages by writing that it could result in “lower equipment cost, less stringent operator training requirements, increased sample throughput and greater field of view compared to TEM”.

Image credit: Emelyanov

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