BL01 - MIRAS

TRANSMISSION MODE (%T)

This geometry requires the preparation of thin samples < 5 to 30 µm is a typical thickness range for polymers, biological tissues and geological and organic materials.

If the material under study cannot be measured as a free-standing sample, an infrared transparent substrate material is required. Many materials are available, such as KBr, CaF2, BaF2, ZnS, diamond, Si etc., and the best suited for a particular application will be recommended.

REFLECTANCE MODE (%R)

The reflection geometry is required for thicker samples.

Data interpretation is more tricky depending upon the nature and flatness of the analysed sample. Depending on the nature of the sample and the configuration of the optics, different types of reflectance measurements can be made, including specular reflectance (SR), diffuse reflectance (DR) - not available at MIRAS at the moment -, reflection-absorption spectroscopy or "transflectance", and attenuated total reflectance (ATR).

In FTIR microspectroscopy, one of the most common measurements is transflectance, where the infrared beam penetrates the sample, reflects off a highly reflective substrate, commonly a gold mirror, and passes back through sample to the illumination objective, see Figure. In this case the samples must be very thin, as the beam passes twice through them. There are special IR reflective and visible transmitting slides available also for this type of measurement, allowing one to view the sample in transmission, and measure in transflectance.  Also highly reflective samples, or highly polished samples which cannot be cut by microtome to a thin section (eg. Bone, teeth, etc.) can be studied in the reflectance mode. 

Here, flat and smooth sample surfaces are important requirements to avoid scattering, as irregular surfaces can produce artefacts in the spectral baseline. 

ATTENUATED TOTAL REFLECTANCE (ATR)

Attenuated total reflectance is a technique that probes the near-surface region of materials in contact with the high reflective index materials used for ATR. Many materials can be analysed using the ATR technique, as long as their refractive index is lower than that on the crystal material in the ATR objective.

The sample is placed in contact with the surface of a crystal of high refractive index (such as ZnSe, Ge, or diamond, Si). The IR beam entering the crystal is totally internally reflected within the crystal at the crystal-sample interface, generating an evanescent wave which penetrates orthogonally into the sample in intimate contact with the ATR crystal. Some of this beam is absorbed by the sample and the reflected radiation returns this information to the objective.

GRAZING INCIDENCE REFLECTANCE (GIR)

Grazing incidence angle sampling in FTIR spectroscopy is a useful technique for measurement of thin films on highly reflective substrates. The technique is more often used for sub-micron thick coating substrates. Films that are typically less than 1µm thick can benefit from this technique.

The basis of the technique involves measurements of the reflected beam from a sample surface at a given angle of incidence, using s Schwarzschild objective that contains a beam mask, transmitting only the grazing incidence rays onto the sample.

Typically the objective transmits at angles between (65-80°). The beam mask determines the size of the angle. In this mode, the mask reduces the flux incident on the sample, which in turn reduces the penetration depth of the beam. So due to the low throughput in grazing incidence mode, sample preparation is extremely critical.