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 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 pathlength.
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). Typical sample types include:
- Soft powders and powder mixtures
- Rigid polymers
The DRIFTS technique can also be used with silicon carbide paper for the analysis of large intractable surfaces. Silicon carbide paper can be used to rub off a small amount of a variety of samples for analysis. This technique is a viable alternative to traditional sampling techniques for:
- Paint and varnish surfaces
- Rigid polymers
- Little to no sample preparation – just place in the sample cup
- Fast and easy cleanup – dump the cup and blow or rinse clean
- No need for pressed KBr pellets or messy mulls – samples can be run neat or diluted with KBr powder
Dr. Michael Bradley received his BS degree in chemistry from the University of South Carolina and his PhD in physical chemistry from the University of Illinois, and also completed his MBA in management. He taught graduate and undergraduate chemistry for 15 years, prior to becoming a field applications scientist with Thermo Nicolet, subsequently Thermo Fisher Scientific, in 2002. He is the Global Training Manager for FTIR products.
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