FTIR Sampling Method Descriptions
Sampling methods are listed according to the amount of sample preparation required for analysis, i.e. samples for ATR require little to no preparation while those for transmission measurements may require extensive manipulation.
Attenuated Total Reflectance (ATR)
In Attenuated Total Reflectance (ATR) spectroscopy all that is required for analysis is that the sample of interest be brought into contact with the ATR crystal. The infrared beam is passed into the ATR element such that its angle of incidence exceeds the “critical” angle. Under this condition total internal reflection of the beam occurs and a standing evanescent wave is established at the ATR crystal/sample interface. The amplitude of this wave decays rapidly with increasing distance from the reflecting interface thus sample concentration and thickness are not a concern for these measurements. Minimal to no sample preparation is required for this technique and a wide variety of solids and some liquids (dependent upon crystal material) can be analyzed using ATR. This technique is available using the Hyperion microscope and the Nexus spectrometer.
Specular Reflectance (SR)
Specular reflectance typically occurs from bulk samples with a glossy surface such as crystal faces, glasses, and monolithic polymers. These spectra have a derivative like appearance as they represent the wavenumber dependent dispersion of the sample refractive index. Absorbance spectra may be obtained from specular reflection data using the Kramers-Kronig transform if the sample is homogeneous, optically thick, and if spectra are collected at a nearly normal incidence. This technique is available using the Hyperion microscope and the Nexus spectrometer.
Reflection-Absorption (RA)
Reflection absorption occurs when thin films are present on a reflective substrate. The resultant spectra look qualitatively similar to transmission spectra. Samples such as: thin polymer films, residues, paints, and surface adsorbates are suitable for reflection-absorption measurements if they are present as an overlayer on a reflective substrate. This technique is available using the Hyperion microscope and the Nexus spectrometer.
Diffuse Reflectance (DR)
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) involves numerous light-sample interactions. Spectra may exhibit features associated with the transmission and/or reflection (external and/or internal) of infrared radiation. This technique is most often employed in the analysis of powders and rough surfaces. Prior to analysis it is recommended that samples be ground to a particle size of between 2 to 5 microns to reduce the amount of specularly reflected light. Additionally, samples which are highly absorbing or possess a high refractive index are diluted with a diffusely scattering matrix, e.g. KBr. To further enhance DRIFTS capabilities the MCL owns a Si-Carb™ Sampler. This tool is useful for analyzing intractable solids by scratching the sample surface with an abrasive paper and then measuring the spectra of the particles adhering to the paper. This technique is available using the IFS 66/S spectrometer.
Transmission (TR)
Transmission spectroscopy involves passing infrared radiation completely through a sample and measuring the extent of absorption. Consequently, significant sample preparation may be required as concentration, thickness, homogeneity, and particle size must all be considered. This technique is suitable for sampling gases, liquids, and solids (fibers, microtome cuts, thin films, pressed pellets, and mulls). The MCL has predominantly solid sampling capabilities, but a limited number of other sample holders are available upon request. This technique is available using the Hyperion microscope and both spectrometers.
Photoacoustic (PA)
Photoacoustic spectroscopy (PAS) can be quite complex and difficult to perform. The photoacoustic signal is generated when the infrared radiation absorbed by a sample is converted to heat within the sample. This heat diffuses to the sample surface and into the adjacent gas atmosphere. The thermal expansion of this gas produces the photoacoustic signal. Assuming a suitable signal level is attainable, the PAS spectrum of almost any sample can be obtained without preparation. It is also possible to depth profile a sample by varying the modulation frequency of the incident radiation. This technique is available using the IFS 66/S spectrometer.