Asbestos analysis methods: PLM, SEM-EDS, TEM and XRD

Asbestos analysis methods combine a microscope that makes the fibres visible with a measurement that shows the chemical composition or crystal structure of the mineral. The most common methods are polarised light microscopy (PLM), scanning electron microscopy with elemental analysis (SEM-EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). For samples of building materials, PLM and SEM-EDS are the most widely used. Semlab analyses material samples with a JEOL SEM-EDS electron microscope according to the ISO 22262-1 standard.

This article compares the methods in general terms: what each of them detects, what it is used for and where its limits lie. For an overview of what an asbestos analysis is, see the article What is an asbestos analysis?, and for how Semlab's instrument works, see the page Asbestos sample analysis.

What does the analysis method need to establish?

Identifying asbestos is a chain of three questions. The methods differ in which of these questions they can answer.

1. Does the sample contain fibres? Finding the fibres requires sufficient magnification and resolution.
2. Are the fibres asbestos? Fibrous particles also occur in asbestos-free materials, so shape alone is not enough. Information about composition or crystal structure is needed.
3. Which type of asbestos? The six types of asbestos differ in chemistry and structure. Read more in the article Serpentine and amphibole.

It also makes a difference whether a material sample or an air sample is being examined. A material sample establishes whether the building material contains asbestos. An air sample counts how many fibres are in the air.

Polarised light microscope (PLM)

A polarised light microscope is a light microscope in which the sample is viewed in polarised light. Minerals are identified by their optical properties, such as shape, refractive index, birefringence and extinction. Dispersion staining is also often used: the fibres are placed in a liquid of known properties, and the mineral is identified from the coloured fringes around the fibres.

• Use: the traditional basic method for asbestos analysis of building materials. The main method of the ISO 22262-1 standard is polarised light microscopy with dispersion staining. According to the standard, identification can also be carried out with an SEM or TEM electron microscope together with elemental analysis, and an electron microscope may be needed when the fibres are too thin for the light microscope or the binder adheres too firmly to them.
• Strengths: an established and widespread method that allows the whole sample to be scanned quickly.
• Limits: the resolution of light limits the ability to detect very thin fibres. Bitumen, plastic and other binders can hide the fibres; the sample then has to be pre-treated. Identification is based on optical observations, so the analyst's experience plays a large role.

Phase contrast microscope (PCM)

A phase contrast microscope is a light microscope that has been used to count fibres in air samples. The fibres counted on the filter are those that meet the WHO fibre definition: longer than 5 micrometres, thinner than 3 micrometres and with a length-to-width ratio greater than 3:1.

• Use: fibre counting in air samples.
• Limits: the method does not distinguish asbestos fibres from other fibres and does not detect the thinnest fibres. For this reason it is increasingly being replaced by electron microscopy for counting fibres in air samples.

SEM-EDS: scanning electron microscope and elemental analysis

A scanning electron microscope (SEM) images the sample with a beam of electrons instead of light. The image shows the shape and proportions of the fibre, even when the fibres are too thin for a light microscope. An energy-dispersive spectrometer (EDS) attached to the instrument measures the X-rays generated in the fibre, which gives the elemental composition of the fibre.

• Use: both material samples and air samples.
• Strengths: shape and chemical composition are determined on the same fibre. The composition makes it possible to distinguish asbestos from other fibres and to tell the six types of asbestos apart.

TEM: transmission electron microscope

In a transmission electron microscope (TEM), the electron beam passes through a very thin specimen. Even the very thinnest fibres are visible with the instrument. Electron diffraction shows the crystal structure of the fibre, and an attached EDS shows its composition.

• Use: mainly air samples, dust samples and special investigations.
• Strengths: the highest resolution, and both crystal structure and composition are determined on the same fibre.
• Limits: sample preparation is more demanding than with the other methods, and TEM is less common in routine analysis of building materials.

XRD: X-ray diffraction

In X-ray diffraction (XRD), X-rays are directed at a ground sample, and the crystal structure of the crystalline minerals is identified from their diffraction pattern.

• Use: determining mineral composition, for example when examining aggregates and industrial minerals.
• Limits: XRD does not show the shape of the particles, so it cannot distinguish the fibrous asbestos form of a mineral from its non-fibrous form. Small amounts are difficult to detect among other minerals. XRD is therefore a supplementary method and is not sufficient on its own for asbestos analysis of building materials.

Why does Semlab use a JEOL SEM-EDS electron microscope?

Semlab analyses all material samples with a JEOL SEM-EDS electron microscope. The reasons are practical:

• The instrument also reveals thin fibres that are difficult to see in a light microscope.
• Elemental analysis distinguishes asbestos from other fibres and identifies all six types of asbestos.
• The analysis follows the ISO 22262-1 standard, and the result is emailed in English on the same day the sample arrives at the laboratory.

Semlab is a laboratory in Finland that analyses samples which customers have taken themselves and sent by post. We do not carry out on-site asbestos surveys.

The method cannot make up for a poor sample

Even the most accurate method will not find asbestos that is not in the sample. The sample must represent the material you intend to remove or work on, and all layers must be included.

When taking the sample, wear disposable gloves and a respirator of at least class FFP3, moisten the sampling spot, remove a piece of about 2–5 centimetres through all layers, pack it in two resealable plastic bags and label the bag with the sampling location. Finally, cover the sampling spot with tape and wipe the area with a damp cloth.

More detailed instructions are in the article How to take an asbestos sample, and how many samples you need is explained in the article How many asbestos samples do you need?. How to read the result is explained in the article Understanding your asbestos result.

How much does an asbestos analysis cost?

The analysis costs €49.99 incl. VAT per sample. Postage is not included in the price. You can order the analysis online, pay by card and post the sample to the laboratory in Finland as a letter or small parcel with your national postal service.