Mid-infrared measurements

De Braekeleer, K. De Juan, A. Sanchez, F. etal., Determination of the end point of a chemical synthesis process using on-line measured mid-infrared spectra Appl. Spectrosc. 2000, 54, 601-607. 

What technique should be employed for measuring infrared spectra from thick samples for which a transmission measurement does not work Examples of target samples in this category are crystals and polymers (including rubber) having flat surfaces. To analyze such samples, reflection measurements should be considered. To record infrared reflection spectra from such samples, two representative techniques are available, namely, specular reflection (reflection at normal or near-normal incidence) and attenuated total reflection (ATR). This chapter deals with external reflection at normal incidence, which has been used for a long time for measuring mid-infrared spectra from optically thick materials with flat surfaces. ATR will be discussed in Chapter 13. 

Today, ATR-based methods are very commonly used procedures (particularly within industrial laboratories) for measuring mid-infrared spectra from a wide variety and diverse... 

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

Principles and Characteristics Both mid-IR (2.5-50 p.m) and near-IR (0.8-2.5 p.m) may be used in combination to TLC, but both with lower sensitivity than UV/VIS measurements. The infrared region of the spectrum was largely ignored when the only spectrometers available were the dispersive types. Fourier-transform instruments have changed all that. Combination of TLC and FTIR is commonly approached in two modes ... 

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

Figure 7 Hydroperoxide index (HI) determined from mid-infrared spectroscopy (ratio of the integrated intensity of the 3,552 cm 1 band to the integrated intensity of the band at 2,010 cm-1) as a function of total hydroperoxide content measured by iodiometric titration.

The atoms in molecules can be regarded as behaving like two spheres connected by a spring, with a natural vibration frequency. It so happens that the vibrations of atoms tend to correspond to an energy equivalent to a wavelength in the mid-infrared. It would appear that mid-infrared wavelengths would be useful for measuring the quantity of substances in systems like flour. In practice that is not so. Infrared peaks have a lot of... 

However, any vibrating system not only has a natural vibration frequency but will also vibrate at twice that frequency, which is known as the first overtone. The first overtone of the vibrations of molecules like water, proteins and fats correspond to a frequency in the near-infrared. Because these frequencies are overtones all of the spectroscopic problems that preclude making quantitative measurements in the mid-infrared are not present in the near-infrared. 

McCarty GW, Reeves JB III, Reeves VB, Follett RF, Kimble JM. Mid-infrared and near-infrared diffuse reflectance spectroscopy for soil carbon measurement. Soil Sci. Soc. Am. J. 2002 66 640-646. 

Infrared spectroscopy has proven to be a very informative and powerful technique for the characterization of zeolitic materials. Most infrared spectrometers measure the absorption of radiation in the mid-infrared region of the electromagnetic spectrum (4000-400 cm or 2.5-25 xm). In this region of the spectrum, absorption is due to various vibrational modes in the sample. Analysis of these vibrational absorption bands provides information about the chemical species present. This includes information about the structure of the zeolite as well as other functional... 

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

A dispersive phenomenon occurring when the wavelength of scattered electromagnetic radiation in the mid-infrared spectral region is shifted relative to that of the incident beam of exciting radiation. Spectral excitation is typically measured at a nonabsorbing wavelength, and the Raman effect occurs when the polarizabihty of a bond varies with the internuclear distance, as specified by the equation ... 

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