22 Jul 2019

Diffuse and Specular reflectance techniques

There are different ways that FTIR spectra can be collected and this article describes the Diffuse and Specular Reflectance techniques for their application of use.

drifts reflectance accessory

A 'MiniDiff' diffuse reflectance FTIR accessory

External Reflection Techniques

In general, FTIR spectral information can be gathered using either a transmission or reflection technique. The reflection technique can be further broadly split into ATR, Diffuse and Specular reflection classifications.

From the comparison of transmission with an ATR technique for applications of use, ATR is a reflection technique, whereby the analysis beam of light radiation is brought to a sample for interaction by means of an internal reflection event. When considering both diffuse and specular reflection techniques, the light radiation interacts with a sample from an external source.  Radiant light reaching the sample does not rely on a reflection event from an IRE crystal/sample interface to measure the sample itself. The radiant light strikes the sample surface directly.

The nature of the sample type itself determines how an external radiant light source is reflected from the sample surface. It may be scattered in different directions known as diffuse reflection and/or reflected directly which is known as specular reflection. Both diffuse and specular reflection methods of measurement for collection of an FTIR spectrum are non-destructive sampling techniques.

Diffuse Reflection

Diffuse reflection infrared spectroscopy can commonly be referred to as DRIFTS. The image shown represents a radiant light beam striking a “rough” surface sample such as a powder mixture consisting of different size particles for a typical scattered, reflected light beam path from the surface. The reflected light can travel in many directions from the surface of an individual particle within the sample to have further interactions with neighbouring particles etc. Reflected light that re-emerges from the sample overall may be collected for spectral analysis using appropriate spectroscopic accessory equipment, such as Specac’s Minidiff Plus or Selector Diffuse reflection accessories.

Where a sample has a rough surface such as a powder, any specular reflected light component is a minor contributor to the overall reflected light signals that can be collected. Rough surface samples that may be measured directly using the diffuse reflection technique include powders, fibres or matt surfaced samples such as textiles. Specac diffuse reflection accessories such as the Selector which has an off-axis optical design, are optimised for the collection of diffusely reflected light with elimination of any specular reflected light, as the specular component may adversely affect the FTIR spectrum to be collected for the sample analysis. 

Specular Reflection

The image shown for the specular reflection technique represents a radiant light beam striking a “smooth” surface sample such as a coated metal surface, for a typical reflected beam path from the surface. The external radiant light beam source is reflected from the sample surface for a focal point at the same angle of incidence to the perpendicular and in the same plane of orientation. The reflected light beam path is not a result for scattering of the light beam from the incident angle of light radiation interacting with the sample.

The amount of reflected light depends upon the angle of incidence of the external light radiation source, the refractive index, surface roughness and absorption properties of the sample. Specular reflection measurements are used typically to measure coating thicknesses etc on smooth reflective surfaces. Strictly speaking, such a spectral measurement is representative of the transmission technique for the coating thickness as the light radiation travels through the sample coating layer, rather than a specular reflection measurement per se from the front surface. Typical applications include the study of surface treated, painted or polymer coated metals.

If the coating thickness is very small (e.g. unit Angstrom levels of thickness – 1 x 10-8 m), then an increased pathlength through the thin coating can be achieved by increasing the angle of incidence away from the perpendicular. The maximum sensitivity of measurement is achieved using the Specac Monolayer Grazing Angle accessory at angles of incidence between 80° to 85°. For micrometre range thickness coating layers, the angle of incidence is reduced to typically 30°. 

Both solid and liquid sample types (e.g. a thin film oil surface layer on an aqueous solution) can be studied using the Monolayer Grazing Angle accessory for molecular vibrational interactions of surface species. For any data processing, a Kramers-Kronig transformation, mathematical calculation can be applied to any specular reflectance spectra to eliminate the effect of variations in the refractive index of the sample.  A K-spectrum and an N-spectrum result, which are a plot of a true absorbance spectrum and of the refractive index vs the wavenumber position respectively. 

Conclusion 

The transmission and ATR techniques for a spectral sample measurement have been used with FTIR spectroscopy for many years. They can both be quick, qualitative and quantitative confirmatory techniques to employ, to establish a sample for its chemical classification and molecular structure.

However, certain solids, semi-solids and liquid sample types by their nature may lend themselves to a different reflection technique of measurement offered by diffuse or specular reflection. Similarly, in comparison to the ATR technique, there may be minimal or no sample preparation stages necessary for the sample types that can be analysed using the diffuse or specular reflection technique. The sample can simply be introduced as is into the sample measurement environment for direct interaction with an external FTIR radiant light source.