Forensic Instruments : ATR-FTIR (part 2)

Technique when handling sample

1.The sample must be in direct contact with the ATR crystal

Evanescent wave or bubble only extends beyond the crystal 0.5µ – 5µ

2. The refractive index of crystal must be significantly greater than that of the sample or else internal reflectance will not occur.

The light will be transmitted rather than internally reflected in the crystal.

3.Typically , ATR crystals have refractive index between 2.38 and 4.01 at 2000 cm-1.

most of the sample have much lower refractive indices.

ATR accessories

The traditional ATR design in which a thin sample was clamped against the vertical face of the crystal has been now been replaced by a horizontal design.
In horizontal ATR (HATR) units, the crystal is a parallel-sided plate, typically about 5 cm by 1 cm, with the upper surface exposed.
The number of reflections at each surface of the crystal is usually between five and ten, depending on the length and thickness of the crystal and the angle of incidence.
When measuring solids by ATR, it is essential to ensure good optical contact between the sample and the crystal.
The accessories have devices that clamp the sample to the crystal surface and apply pressure. This works well with elastomers and other deformable materials, and also with fine powders but many solids give very weak spectra because the contact is confined to small areas.
The effects of poor contact are greatest at shorter wavelengths where the depth of penetration is lowest.
The issue of solid sample/crystal contact has been overcome to a great extent by the introduction of ATR accessories with very small crystals, typically about 2 mm across.
The most frequently used small crystal ATR material is diamond because it has the best durability and chemical inertness.
These small area ATR crystal top-plates generally provide only a single reflection but this is sufficient, given the very low noise levels of PerkinElmer’s modern FT-IR spectrometers.
Much higher pressure with limited force can now be generated onto these small areas.
A much smaller area of contact is now required in comparison to the HATR units.
As a result, spectra can be obtained from a wide variety of solid materials including minerals.