Mid-infrared spectra

Figure 10.1—Mid infrared spectrum of afilm ofpolystyrene. Typical representation of an infrared spectrum with a linear scale abscissa in cm 1 and % transmittance as the ordinate (in order to better visualise the right-hand portion of the spectrum, it is customary to change scales around 2 000 cm 1 see Fig. 10.12). The abscissa in cm-1 (or kaysers) is linear in energy (E — hc/X), that decreases from left to right (from high to low energies).

The determination of elements by atomic absorption in drinking water at the mg/l level or the recording of a mid-infrared spectrum of a pure organic compound are situations rarely encountered. In these cases, the sample is easily prepared. However, these conditions and similarly those under which analyses made by students in a teaching laboratory are not representative of the difficulties encountered when preparing a real sample for analysis. 

The relative rates of the consecutive hydrolysis reactions and the rate of condensation to form siloxane bonds depend on the structure of the silane molecule. Data such as those presented here provide the knowledge necessary to permit one to tailor the composition of species of alkoxysilane compounds in solution to obtain the desired rate and degree of hydrolysis of alkoxy groups, and the extent of siloxane bond formation. This is accomplished by the use of several easily observed bands in the mid-infrared spectrum. 

The mid-infrared spectrum of the cesium aluminosilicate unequivocally identified it as pollucite. Moreover, the SAM results indicate that the material is uncontaminated by foreign alkali metal ions, such as K+ or Na+ from the feldspars, or the aqueous media. 

Traditionally, data was a single numerical result from a procedure or assay for example, the concentration of the active component in a tablet. However, with modem analytical equipment, these results are more often a spectrum, such as a mid-infrared spectrum for example, and so the use of multivariate calibration models has flourished. This has led to more complex statistical treatments because the result from a calibration needs to be validated rather than just a single value recorded. The quality of calibration models needs to be tested, as does the robustness, all adding to the complexity of the data analysis. In the same way that the spectroscopist relies on the spectra obtained from an instrument, the analyst must rely on the results obtained from the calibration model (which may be based on spectral data) therefore, the rigor of testing must be at the same high standard as that of the instrument... 

Figure 20.2. Mid-infrared spectrum of a cheese recorded in the 3000-2800 cm"1 region.

Fig. 2.3. Combination and difference band progressions involving the ring puckering vibration and a CH2 scissoring mode in the mid-infrared spectrum of cyclobutane.

A rapid FTIR method for the direct determination of the casein/whey ratio in milk has also been developed [26]. This method is unique because it does not require any physical separation of the casein and whey fractions, but rather makes use of the information contained in the whole spectrum to differentiate between these proteins. Proteins exhibit three characteristic absorption bands in the mid-infrared spectrum, designated as the amide I (1695-1600 cm-i), amide II (1560-1520 cm-i) and amide III (1300-1230 cm >) bands, and the positions of these bands are sensitive to protein secondary structure. From a structural viewpoint, caseins and whey proteins differ substantially, as the whey proteins are globular proteins whereas the caseins have little secondary structure. These structural differences make it possible to differentiate these proteins by FTIR spectroscopy. In addition to their different conformations, other differences between caseins and whey proteins, such as their differences in amino acid compositions and the presence of phosphate ester linkages in caseins but not whey proteins, are also reflected in their FTIR spectra. These spectroscopic differences are illustrated in Figure 15, which shows the so-called fingerprint region in the FTIR spectra of sodium caseinate and whey protein concentrate. Thus, FTIR spectroscopy can provide a means for quantitative determination of casein and whey proteins in the presence of each other. 

Measurement by atomic absorption at the mg/L level of elements in drinking water or the scanning of a mid-infrared spectrum of a pure organic compound, as often performed in a teaching laboratory, corresponds to simple situations for which the sample does not need a pretreatment. These conditions are not representative of the difficulties encountered when working on real samples to be analysed. A pretreatment becomes essential. 

In this chapter we have examined the various regions of the mid-infrared spectrum, highlighting and assigning particular molecular vibrations to the bands that may be observed. The spectrum may be divided into the following four regions ... 

Several bands in the mid-infrared spectrum usually disappeared. 

The vast majority of molecules have infrared bands in the spectral range between 400 and 4000 cm . Most of the intense features in any mid-infrared spectrum can be assigned to fundamental transitions. 

To demonstrate calibration linearity and transferability using a fiber-optic probe and simple peak-area methods, the solution spectra of four solutions of phenylisocyanate in acetone were used. The mid-infrared spectrum of most isocyanates are characterized by a sharp band, sometimes exhibiting shoulders, at 2250-2285 cm which is assigned to the asymmetric stretch of the -N C=0 group. In the spectra of acetone solutions of phenylisocyanate, this band appears at 2261 cm with a substantial shoulder at 2283 cm. Figure 1 shows an expanded view of the band, displayed in a screen shot from the GRAMS peak-fitting software with two Lorentzian peaks fitted at 2261 and 2283 cm. ... 

There are a few general rules that can be used when using a mid-infrared spectrum... 

The mid-infrared spectrum of the clay kaolinite in KBr is illustrated below in Figure 5.9. Assign the infrared bands for kaolinite. What information does this spectrum provide about the structure of this clay ... 

Lately, the single-reflection mid-infrared ATR accessories have proven to be a very attractive tool within an industrial enviromnent they provide a simple to use means of quickly recording a mid-infrared spectrum from almost any solid material. With the acceptance of these and the increased number of installations of FT-IR microscope systems, there has been a general decline in the use of the diffuse reflection and photoacoustic techniques in the mid-infrared region, and emission measurements are now very rarely used. 

Tunable diode laser spectroscopy (TDLS) systems are tuned to access specific regions ofthe mid-infrared spectrum where most gases of interest such as UFe have strong absorption while common gases, such as oxygen and nitrogen, do not. TDLS systems have the potential to determine enrichment in UFs gas and to indicate the presence of HF gas, a by-product of enrichment activities. 

Various aspects differentiate a mid-infrared spectrum from the corresponding near-infrared spectrum (Table 6). In the mid-infrared region fundamental molecular vibrations always occur between 4000 and 200 cm , while in the near-infrared region overtones and combination bands of these fundamentals are observed between 12 8(X) and 40(X)cm". Near-infrared bands are due primarily to hydrogenic stretches of C-H. N-H, and... 

A large tenuous cloud surrounds the object and is seen as a reflection nebula illuminated by the starlight that escapes above and below the ring of dust. The discovery of this object has provided dramatic evidence supporting earlier hypotheses that circum-stellar envelopes of infrared stars must be flattened. The large optical depth of the toroid produces a featureless, mid-infrared spectrum (Forrest eit 1976) but the chemical nature of the cloud has been deduced from optical spectroscopy of the reflection nebula (Crampton, Cowley and Humphreys, 1975) and by detection of a molecular cloud association with the source (Lo and Bechis, 1976 and Zuckermann ad 1976). These observations show that the 0.1 M cloud is carbon-rich, and, in fact has led to the suggestion that the source may be the progenitor of a planetary nebula. 

The nature and the extent of the interaction between R(III) cations and the triflate anion has been studied in anhydrous acetonitrile by the same FT-IR technique described for the perchlorate interaction (Di Bernardo et al. 1993). Solubility problems were encountered for the lighter lanthanide ions (La-Eu, < 10 M) so that only the heavier lanthanide ions were investigated. Conductimetric measurements on a Yb(III) triflate solution indicate a 1 1 electrolyte. The 1000-1300 cm region of the mid-infrared spectrum contains the main triflate vibrations and the Vas(S03) mode is particularly well suited for a quantitative study. The number of uncoordinated triflate ions per R(III) ion, ntnf, was determined with the help of a calibration curve, after curve resolution of the spectra (fig. 5). It is always... 

If the vibrational modes were strictly harmonic, no transitions involving changes in u, by more than 1 would be allowed. The effect of anharmonicity is to relax this selection rule (i.e., to allow bands caused by Au, > 1 to become allowed). Thus, overtone (Au, = 2,3,...) and combination (Au, = 1 Ay, = 1, where j represents a different mode) bands commonly appear weakly in the mid-infrared spectrum of organic compounds along with bands due to fundamental transitions (Au, = 1). 

For most pure compounds, a sample thickness of only about 10 pm is needed to yield a mid-infrared spectrum for which the bands are neither saturated (maximum transmittance less than 1 %) nor so weak that they require ordinate expansion. It is often inconvenient and sometimes impossible to prepare such thin samples. In these... 

Because bands in near-infrared spectra are overtones or combinations of fundamentals, the widths of these bands are typically greater than the widths of bands from which they are derived. For example, the FWHH of the first overtone of a C H stretching band is to a first approximation twice that of the corresponding fundamental. As a result, most NIR spectra of liquids are measured at significantly lower resolution than is the corresponding mid-infrared spectrum. 

To achieve a SNR of 10 in the mid-infrared spectrum (Vn x = 4000 cm ), the positional error must be less than 10 cm (1 A) (i.e., about one atomic diameter ). It is a testimony to the power of laser referencing that this specification is easily met by all commercial FT-IR spectrometers. 

No single technique will compensate completely for the effects of fluctuation noise. The best way to minimize this effect is to use a scan speed that causes the infrared wavelengths to be modulated at frequencies outside the frequency range of the perturbations. Atmospheric scintillation is rarely observed at frequencies much above 500 Hz, so these effects are rarely seen in the mid-infrared spectrum when the mirror velocity is greater than about 1 cm s On the other hand, fluctuation noise can be a major problem when the scan speed is very low, as it is with FT-Raman spectrometry (see Chapter 18) and step-scanning interferometers (see Section 5.5). The possible causes of fluctuation noise in step-scan FT-IR spectrometry have been discussed in some detail by Manning and Griffiths [6]. 

To measure such spectra with the same SNR, the dispersive spectrum must be measured M times longer than the FT spectrum. This can be a significant improvement. For example, for a mid-infrared spectrum measured at a resolution of 4 cm , M = 900. Assuming that it takes 15 minutes to measure this spectrum on a grating spectrometer, it would only take 1 second to measure the same spectrum on an FT-IR spectrometer. 

If, on the other hand, n-hexane is to be used as a solvent, the cell thickness must be reduced to about 100 pm. Even though CCLi, CS2, and n-hexane are all nonpolar, the increased structural complexity of n-hexane and the fact that it contains lighter atoms than CCLt and CS2 mean that its fundamental absorption bands cover a greater region of the mid-infrared spectrum than those of CCLt or CS2. Thus, the window regions, where useful information on the solute can be found, are much shorter. 

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