Infrared, near, ’combination bands

Near-infrared spectroscopy is quickly becoming a preferred technique for the quantitative identification of an active component within a formulated tablet. In addition, the same spectroscopic measurement can be used to determine water content since the combination band of water displays a fairly large absorption band in the near-IR. In one such study [41] the concentration of ceftazidime pentahydrate and water content in physical mixtures has been determined. Due to the ease of sample preparation, near-IR spectra were collected on 20 samples, and subsequent calibration curves were constructed for active ingredient and water content. An interesting aspect of this study was the determination that the calibration samples must be representative of the production process. When calibration curves were constructed from laboratory samples only, significant prediction errors were noted. When, however, calibration curves were constructed from laboratory and production samples, realistic prediction values were determined ( 5%). 

Wirzing 216a, 216b) found that the combination bands in the near infrared were particularly well-suited for the distinction of silanol groups and water. Bands were observed at 4545-4365 cm and at... 

Another complication arises in the interpretation of absorption spectra. If a molecule vibrates with pure harmonic motion and the dipole moment is a linear function of the displacement, then the absorption spectrum will consist of fundamental transitions only. If either of these conditions is not met, as is usually the case, the spectrum will contain overtones (multiples of the fundamental) and combination bands (sums and differences). Most of these overtones and combination bands occur in the near-infrared (0.8-2.0/un). 

Near-infrared reflectance analysis is particularly well suited to chemical sensing because it operates on "as is" samples and yet has chemical specificity. The absorptions observed originate from vibrations of a relatively few chemical groups whose overtones and combination bands appear in the near infrared region. 

Although the Fourier compression method can be effective for reducing data into frequency components, it cannot effectively handle situations where the dominant frequency components vary as a function of position in the spectrum. For example, in Fourier transform near-infrared (FTNIR) spectroscopy, where wavenumber (cm-1) is used as the x-axis, the bandwidths of the combination bands at the lower wavenumbers can be much smaller than the bandwidths of the overtone bands at the higher wavenumbers.31,32 In any such case where relevant spectral information can exist at different frequencies for different positions, it can be advantageous to use a compression technique that compresses based on frequency but still preserves some position information. The Wavelet transform is one such technique.33... 

In addition to providing data on the compositions of coexisting phases, infrared data is also of potential value in providing structural information about phases. The mid-IR spectral region, particularly the CH-stretching and -bending regions, has been extensively used for this purpose (17-18). The near-IR bands ve observe between 4500 and 4000 cur"1" are the CH bend-stretch combination bands. It seemed likely that their band positions and intensities could also be sensitive to phase structure. 

A further development in the characterization resulted from a study of the near infrared region, by Washburn and Mahoney who showed that the first overtone of the cyclopropyl C-H stretching frequency absorbs at 6097 cm with a combination band also present at ca. 4465 cm . The overtone band is sharp though weak and separated from the first overtone of the saturated aliphatics. Any possible ambiguity with terminal methylene groups can be removed in infrared spectroscopic terms by analysis of the fundamental of the methylene. 

Near-infrared (NIR) absorption spectroscopy is another technique of importance to the context of the development of analytical Raman spectroscopy. The method is generally referred to as NIR, despite the unfortunate confusion with NIR-Raman. NIR absorption is based on overtone and combination bands of mid-IR transitions, as shown in Figure 1.1. Such transitions are quantum mechanically forbidden and significantly weaker than mid-IR fundamentals. However, the higher energy photons involved in NIR absorption are transmitted by fiber optics and common optical materials, and the method has... 

FIGURE 4 Near-infrared spectra of a-lactose monohydrate (solid trace) and /Mactose anhydrate (dashed trace), highlighting the strong water combination band at 1940 nm. The figure was adapted from data presented in reference 80. 

Another prominent feature of the near-infrared spectrum is the large number of combination bands. In addition to the ability of a band to be produced at twice or three times the frequency of the fundamental, there is a tendency for two or more vibrations to combine (via addition or subtraction of the energies) to give a single band. 

Hydroxyaryl)-3-hydroxyindolones Inductive effects 102 Infrared polarization spectroscopy 368 Infrared spectroscopy 367-386 density functional theory and 368 hydrogen bonding and 368, 370, 371, 375-386 in analysis 997 hnear dichroism (LD) 368 near-, overtone and combination bands in 371... 

From this point of view the water molecule has been particularly well studied. To a first approximation the band which appears around 5154 cm (1940 nm, see Figure 10.24) resuits from the combination of the asymmetrical vibration situated at 3500 cm and the deformation vibration (scissoring, see Section 10.6) at 1645 cm (Figure 10.16). These simpie caicuiations (3500 + 1645 = 5145) are an imperfect approach, which conceais a certain number of factors of the physical state of the compounds to which infrared is sensitive. Among the approximations are those for the combination bands of the bonds C-H, N-H and C-0 in the near-IR. 

Post JL, Noble PN (1993) The near-infrared combination band frequencies of dioctahedral smectites, micas, and illites. Clays Clay Minerals 41 639-644... 

Fig.27. Near-infrared spectra of H-ZSM-5 with combination bands, v(OH)-t6(OH) on increasing activation temperatures (adopted from [557])...

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