Water absorption bands

Finally, although temperature had a large effect on both the position (wavelength) and the intensity of the water absorption bands in the emulsion NIR spectra, careful experimentation demonstrated that the 1618mn vinyl C-H band used in the calibration model did not shift in either position or intensity with... 

The Earth s atmosphere is composed primarily of non-polar molecules like N2 and O2, especially at greater altitudes where the H2O concentrations are small. One would therefore expect collision-induced contributions to the absorption of the Earth s atmosphere from N2-N2, N2-O2 and O2-O2 pairs. The induced rototranslational absorption of nitrogen has not been detected in the Earth s atmosphere, presumably because of strong interference by water absorption bands, but absorption in the various induced vibrational bands is well established (Tipping 1985). Titan (the large moon of Saturn) has a nitrogen atmosphere, somewhat like the Earth methane is also present. Collision-induced absorption by N2-N2 and N2-CH4 is important in the far infrared. 

FTIR Spectra. After reaction, the mineral samples were subjected to ultrasonic treatment in acetone for periods of up to 1 hour. Transmission IR spectra of the released product fines from reactions of 60-mesh labradorite and microcline with 10 2 mol dm 3 CsCl in distilled water at 150 C for 47 days are presented in Figure 3. The spectrum obtained from of the labradorite reaction product (Figure 3(a)) is identical with that of a sample of natural pollucite (Figure 3(c)). The spectrum obtained from the microcline reaction product (Figure 3(b)) contains bands in the 500 cm""1 to 800 cm"1 region, due to unreacted microcline, in addition to the major bands of pollucite. Heating the samples overnight at 105 C resulted in the disappearance of the water absorption bands at about 3500 cm 1 and 1680 cm 1. 

Another critical spectral region is 1630 cm-1 where water absorption bands have been reported (41) In this study this band develops from chemical change and not from absorbed water. This is supported by the spectral changes in 3360 snd 1630 cm—1 which do not procede on the same time frame, consequently the chemical changes in these groups are not due to the same chemical species. The assignment of the 1630 cm-1 absorption as carbonyl is consistent with the spectra of -glucose in its aldohexose form. (41)... 

The MSC has been shown to work well in several empirical studies [9, 10], which showed an improvement in the performance of multivariate calibrations and a reduction in the number of factors in PCA. For example, NIR reflectance spectra of 20 powder samples of microcrystalline cellulose are shown in Figure 4.8a. Due to differences in particles size from sample to sample, there are significantly different baseline offsets. The same spectra are shown in Figure 4.8b after multiplicative scatter correction. The different baseline offsets observed in Figure 4.8a are so large that they mask important differences in the water content of these samples. These differences are revealed in the water absorption band at 1940 nm after the baseline offsets have been removed by MSC. 

All of these microtechniques are successful in transferring the sample to the potassium bromide micropellet, but losses of sample are inherent for each of them. These losses can be very great without attention to detail. Elimination of water absorption bands is also quite diflBcult to accomplish. A further disadvantage of this procedure is the difficulty in recovery of the sample for evaluation by other means. But the excellent sensitivity achievable does recommend this sampling procedure for micro quantities of material. 

NIR spectroscopy has become a popular choice for water determination. Water absorption bands exist in the NIR region, which provide good selectivity and sensitivity. NIR is a quick and nondestructive technique. However, it is a secondary method of analysis for water. This means that a drug substance reference material of known water content must be used to validate an NIR method for that particular drug substance. 

Si-OR groups in the film. The C-H absorption is reduced to nearly zero as a result of the 400 C cure. However, the silanol and water absorption bands remain. The 900°C spectrum shows no absorption band due to sSi-OH, =Si-OR, or H2O Indicating that the film is highly denslfled and has little propensity for absorbing H2O. 

Figure 11 FTIR spectra of a 10% solution of sucrose in water A) and of water B) and the difference spectrum obtained by spectral subtraction (A-B). The features in the difference spectrum are the absorption bands of sucrose. The region between 3500 and 3150 cm-i is obscured by digitisation noise due to the intense water absorption band in this region.

Here the temperature of a chemical system (any chemical system that is in equilibrium) is quickly raised by 10-30 °C, typically now by irradiating, for example, a protein solution by a pulse of near-IR laser light tuned to weak water absorption bands. As the chemical system evolves to a new equilibrium point deflned by the new temperature, structural changes are monitored by spectroscopic probes. 

Your instructor will provide you with an infrared spectrum for each unknown sample and will either post known spectra for comparison or will urge you to use appropriate sources in the chemical literature. Particularly useful literature sources for matching spectra are Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, 1978, especially pages 109, 127, 129, 134, 135, and 142, and also Sadtler Standard Infrared Spectra that provide actual spectra for many compounds. You can even determine that a particular salt is at least partially hydrated (contains water in its crystal lattice) by noting the presence of water absorption bands in the infrared spectrum of the salt. 

Absorption Bands. Success of remote NIR spectroscopic sensing depends on the identification and isolation of unique spectral bands for the targeted analytes. These analyte bands must be distinguishable from those of water and other analytes in the sample. Water has large absorption bands with peak maxima at 6876, 5267, and 3800 cm in the NIR (12). The spectral region of 5000-4000 cm corresponds to an optical window between two large water absorption bands with a minimum around 4500 cm ... 

IR spectroscopy was found to be very useful for studying DND because the surface of DND particles is usually saturated by various functional groups, which are covalently bound to them to form a dense molecular shell around a nanoparticle. Typical features of DND IR spectra can be found in Refs. 37,102-108. Most IR spectra of DND in the literature reveal the presence of water absorption bands, which consist of a broad composite band close to 3600 cm- and a somewhat weaker band, near 1620 cm. The first of them derives from the symmetric and asymmetric stretch vibrations of water molecules, and the other, from the OH bend vibrations. The disagreement with 1645 cm the value usually quoted in the literature, is usually assigned to adsorption effects. 

Until the potential of the electrode reaches 0.25 V, corresponding to the begiiming of glycine electrooxidation or 0.09V for a-alanine, in FTlR-speetra the water absorption bands (1650 and 3300 cm ), deprotonated carboxyl group (1370 cm ), as well as the bands. 

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