There are several different sample handling techniques used with Fourier Transform Infrared Spectroscopy (FT-IR): Transmission, Diffuse Reflectance, Attenuated Total Reflection (ATR), and True Specular Reflectance/ Reflection-Absorption.
In this article we will discuss Diffuse Reflectance, referred to as DRIFTS. DRIFTS is commonly used for the analysis of both organic and inorganic samples that can be ground into a fine powder (less than 10 microns) and mixed in a powder matrix such as potassium bromide (KBr). Hard polymers and rigid polymers are typically analyzed using the DRIFTS method.
How does DRIFTS work?
When an IR beam is focused onto a fine particulate material, the incident beam can interact with the particle in one of several ways. First, radiation can be reflected off the top surface of the particle without penetrating the particle. Second, the light can undergo multiple reflections off particle surfaces without penetrating into the particle. True diffuse reflectance results from the penetration of the incident radiation into one or more sample particles and subsequent scatter from the sample matrix.
A DRIFTS accessory aids in the reflection and operates by directing the IR energy into a sample cup filled with a mixture of the sample and an IR transparent matrix (such as KBr). The IR radiation interacts with the particles and then reflects off their surfaces, causing the light to diffuse, or scatter, as it moves throughout the sample. The output mirror then directs this scattered energy to the detector in the spectrometer. The detector records the altered IR beam as an interferogram signal, which can then be used to generate a spectrum. Typically, a background is collected with the DRIFTS accessory in place and the cup filled with just the IR matrix. Excellent quantitative and qualitative data can be collected with proper sample preparation. However, transmission and ATR techniques are preferable to diffuse reflectance for quantitative data due to path length.
What are the advantages of diffuse reflectance?
With DRIFTS, little to no sample preparation is needed; just place your sample in the cup. Cleanup is fast and easy; just dump the cup and blow or rinse clean. Another advantage is that there is no need for pressed Potassium bromide (KBr) pellets or messy mulls; samples can be run neat or diluted with KBr powder.
For more information about these different techniques, read the Introduction to FT-IR Sample Handling. It includes an index of sample types as well as a ratings chart for the various techniques that can be used with different samples. To get some basics about Fourier Transform Infrared Spectroscopy, download the Introduction to FT-IR.