COMPARISON WITH SEM/PCM/PLM

WHAT IS A TRANSMISSION ELECTRON MICROSCOPY (TEM)?

The concept of the TEM is to produce electron at the top of the microscope within the electron gun by heating a tungsten filament which will send, through a vacuum column, electrons through the sample. The sample is disposed at the middle of the vacuum column by a specific sample holder.  
Comparing to SEM where electrons will reflect on the sample, here electrons will transmit the sample. All electrons will then hit the fluorescent screen enable the image recording system to produce the image you are looking for.

grid
grid hole
specimen holder
1. The source of illumination
Compared to Light Microscopy (LM), the Transmission Electron Microscopy (TEM) is built at the opposite. The source of illumination is at the top of the microscope and the screen at the bottom.   The source of illumination comes from the electron gun which by heating a tungsten or crystal filament generates electrons transmitting the sample all way through the microscope within a high speed.  Electrons are accelerated in the gun section within a voltage between 80kV and 200 kV. The thicker is your sample, the higher voltage you need to penetrate the sample.
2. The vacuum system
The TEM is equipped with various vacuum systems enabling the machine to create vacuum in the column essential for electron microscopy. The vacuum is needed to allow the voltage isolation between the electromagnetic lens working as a cathode and the electron without generating an arc. The vacuum is also needed to reduce collision frequency of electrons with gas atoms.
3. The specimen holder
The sample/specimen holder allows the insertion of samples into the vacuum with a minimal loss on the column thanks to an O-ring that separates the atmospheric pressure from the vacuum. A vacuum pump evacuates the air on the pre-chamber before inserting the sample in the column. The sample preparation is put on a metal grid about 3 mm of diameter. We usually use a copper grid. The metal grid gets various mesh size ranging from a few to 100 microns. The TEM stage allows the movement of the sample in the XY plan to locate the region of interest.
4. The apertures
We distinguish 3 kind of apertures within the TEM. One CONDENSER aperture where you chose the size of the beam (small spot size of the beam, loose intensity but increase the accuracy); one OBJECTIVE aperture which you can select out and filter out the electrons that deflected in the sample to increase contrast; and one INTERMEDIARY aperture which enable for the electron diffraction to select the region of interest on the sample.
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SAMPLE JOURNEY

Hereafter a short video that shows the process of preparing and analysing a sample using the TEM.

WHAT TO LOOK FOR?

Tremolite

Crocidolite

Chrysotile

Anthophillyte

Amosite

Actinolite

Find the exact materials

The analyst makes his decision depending on three final criteria:

  1. Morphology: The analyst looks deep into the sample and investigates all fibrous shapes with the length to width ratio of 3 to 1.
  2. Crystal structure: Defined with convergent beam electron diffraction technique.
  3. Chemical composition: An Energy-Dispersive X-ray Spectroscopy (EDS) is used for element characterisation. Comparison of the specimen’s spectrum with the spectra of asbestos known compositions produces qualitative results (example below with an amosite)

The analyse report results by asbestos presence or absences.

amosite edx + periodic table
EDXS

MICROSCOPY TECHNOLOGY COMPARISON

TEMvsPLM
TEMvsPCM
TEMvsSEM

EXAMPLES OF ANALYSIS

SAMPLE: Vinyl Tile

Magnification
x19.0k
Acceleration Voltage
100kV
Emission Current
10uA

RESULTS:

PLM: no asbestos detected

TEM: “carpet” of chrysotile fibres (mostly Thin Asbestos Fibres and Short Asbestos Fibres that are undetectable through PLM)

SAMPLE: Vinyl Tile/Vinyl Sheet

Magnification
x19.0k
Acceleration Voltage
100kV
Emission Current
10uA

RESULTS:

PLM: no asbestos detected

TEM: trace of tremolite asbestos (World Health Organisation fibres characterisation)

SAMPLE: Crayons

Magnification
x19.0k
Acceleration Voltage
100kV
Emission Current
10uA

RESULTS:

PLM: no asbestos detected

TEM: trace of tremolite asbestos (World Health Organisation fibres characterisation)

SAMPLE: Isolated Fibres

Magnification
x19.0k
Acceleration Voltage
100kV
Emission Current
10uA

RESULTS:

PLM: unidentified Mineral Fibres

TEM: identification of the variety (Actinolite, Anthophyllite, Tremolite : World Health Organisation fibres characterisation)