Internal thickness band, absorbance

If the spectra obtained are in the linear optical range, a reference spectrum can be subtracted from the sample spectrum. The absorbances can be scaled to correct for differences in the amount of sample in the beam if an internal thickness band for the sample can be selected. An internal thickness band is a band whose intensity is a function of only the amount of sample in the beam. Generally, isolated bond stretching modes such as the C—H modes have this important property. The absorbance of the internal thickness band for the reference sample can be written... 

Fig. 4.38. Absorbance of the poly(butylene terephthalate) (PBT) methylene absorption of bands at 1460 ( ) (relaxed form) and 1485 (o) cm (stressed form) as a function of reversible stress and strain. The absorbances have been corrected for changes in sample thickness by using the aromatic ring band at 1510 cm as a reference internal-thickness band. (Reproduced with permission from Ref. [37] 1979 Steinkopff Verlag Darmstadt.)...

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

Infrared Spectroscopy. An inspection of the infrared spectra of dry or hydrated pure Nafion in the sulfonic add or various cationic salt forms reveals a multiplidty of bands (28, 30, 31) some of which are inconveniently located in close proximity to the aforementioned peaks characteristic of silicon oxide strudures. The Nafion contribution to the composite spectra was subtracted in each case using the 2860 cm band (combination 1140 + 1720 cm, both CF,CF,) as an internal thickness standard. While this band appears weak and may not be an ideal internal standard (Membranes were not available to test absorbance vs. thickness linearity.), it is backbone-related, lies in a region of peak noninterference and the resultant subtractions do appear effective. 

Attenuated total reflection (ATR) FTIR is one of the most useful tools for characterising the chemical composition and physical characteristics of polymer surfaces [53]. One useful application is the measurement of molecular orientation using polarised infrared ATR spectroscopy [54,55]. The polarised infrared ATR spectra normally include three-dimensional (e.g., machine, transverse, and thickness direction) orientational information in contrast to the polarised transmission infrared linear dichroism. In addition, band absorbance of less than 0.7 au is easily achieved, even with the strong absorption bands, because the penetration depth of ATR from sample surfaces can be adjusted to a few micrometers by changing the internal reflection element and/or the angle of incidence. If successful combination of the dynamic infrared spectroscopy and the ATR methods can be achieved, more useful dynamic orientational information can be obtained. 

IR spectroscopy has been widely used in the elucidation of PP stereostructure ever since Natta and co-workers [26-29] first reported the spectrum of the crystalline polymer. Most studies have concentrated on identifying suitable bands to measure tacticity and to correlate with other indices of stereoregularity. The bands apparently associated with isotactic helices absorb at 8.19, 8.56,10.02,11.11,11.89, and 12.36 pm and of these the bands at 11.89 and 10.02 pm have been principally used for the derivation of calibration curves (Figure 8.4). Since it is difficult to prepare films of a standard thickness, it is customary to use an internal reference band and the absorptions at 6.85, 8.56, and 10.28 pm have been generally used for this purpose. It is perhaps worth noting that the origin of the reference band at 10.28 pm, which is observable both in the melt of isotactic samples [19, 30] as well as in purely atactic material is disputed [30]. Thus, the band, which has been attributed [30, 31] specifically to the PP head-to-tail sequence of repeating units, has also been associated with the presence of short isotactic helices apparently still present in the melt or atactic material [25]. 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

Colors and impurities obviously filter out certain radiation bands. Increasing the thickness of a container will obviously increase the absorption of any given absorbed band. One of the most difficult items to measure is the effect of the container geometry on the exposure. Depending on the physical shape of the container and the angle(s) of incidence of the radiation (cosine law effect), radiation can be reflected or refracted by flat surfaces and possibly focused internally by rounded corners. 

The calibration curve is obtained by plotting the ratio of the absorbance of the analyte to that of the internal standard, against the concentration of the analyte. The absorbance of the internal standard varies linearly with the sample thickness and thus compensates, for it.. The discs or mulls must be made under exactly the same conditions in order to avoid any intensity changes or shifts in band positions. 

Simple solid mixtures may also be quantitatively analysed. These are more susceptible to errors because of the scattering of radiation. Such analyses are usually carried out with KBr discs or in mulls. The problem here is the difficulty in measuring the pathiength. However, this measurement becomes unnecessary when an internal standard is used. When using this approach, addition of a constant known amount of an internal standard is made to aU samples and calibration standards. The calibration curve is then obtained by plotting the ratio of the absorbance of the analyte to that of the internal standard, against the concentration of the analyte. The absorbance of the internal standard varies linearly with the sample thickness and thus compensates for this parameter. The discs or mulls must be prepared under exactly the same conditions to avoid intensity changes or shifts in band positions. 

It is extremely difficult to produce a film thin enough to observe the bands in the 1200 cm region. An internal reflection spectrum should be considered for very strong absorbers such as this sample because the effective thickness determined by the physics of internal reflection is only a fraction of a wavelength. 

Diffuse reflectance results from the incident radiation, which has traveled through some finite thickness of a material and is scattered or reflected from internal surfaces. Diffuse reflection spectra show a reduction in reflectance at frequencies where the absorption bands occur as a result of the radiation penetrating the sample to a distance comparable to its wavelength and being partially absorbed. The optical diagram for the diffuse reflectance experiment is shown in Fig. 3.14. 

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