Absorbance subtraction

Convert all percent transmittance readings to absorbance. Subtract the absorbance at the baseline from the absorbance at the tip of the 817 cm"1 peak in each spectrum. 

Data Processing Techniques Using Digitized Infrared Spectra 4.1 Absorbance Subtraction... 

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

There are two fundamental assumptions in the use of absorbance subtraction. First, the absorbance and shape of a band does not change with the optical thickness. This assumption is tested with every subtraction and if the residual absorbance after a subtraction has a different shape than the original absorbance bands, then this assumption is violated and the procedure or samples should be reexamined for the appearance of effects producing nonlinear results. A number of these effects will be discussed below. Secondly, one also assumes that the absorbance of a mixture is the stun of the absorbances of the components, that is, that the components do not interact with each other differently at different relative concentrations, such interactions will lead to frequency shifts and band shape changes 7S>, but rarely are such effects observed in solids. Care must be exercised, however, since wedging and orientation can produce nonlinear effects in solids. 

The applications of absorbance subtraction to the study of polymers are numerous as later sections of this review will reveal. The uses of absorbance subtraction for identification80,811, quality control80 821 and forensic applications831 have all been reported. 

The data manipulating capability of a computerized infrared spectrometer allows the spectroscopist to delve more deeply into the structural origin of the infrared absorptions by using data processing techniques to purify, manipulate, and correlate the spectra. If one can systematically vary the relative amounts of various structural contributions, absorbance subtraction can be used to isolate the spectral contributions of the structural components. 

Poly(vinylidene fluoride) PVDF2, has been studied by absorbance subtraction in order to isolate the spectral features of the different phases in particular, the difference spectra were used to interpret the structure of phase III212). The spectrum of the unoriented phase-III sample before annealing is shown in Fig. 15. The spectrum after annealing at 160 °C for 20 hr is also shown with the difference... 

Infrared spectroscopy has often been used to measure energy differences between conformational isomers. With FT-IR one can systematically study the differences introduced by temperature by doing absorbance subtraction. Studies were made by examining the difference spectra of PVC recorded at elevated temperatures in the range of 80 to 180 °C 201). From the intensities, Van t Hoff plots were made and energy barriers determined. These results further confirmed the band assignments to the various conformational sequences. Studies have also been carried out on PVC which has been plasticized 203). In these studies the contributions of the plasticizer were substracted out to reveal the changes in the PVC conformations. 

The intense absorption of water over most of the infrared spectrum restricts the regions where aqueous solutions of carbohydrates can be usefully studied. Absorbance subtraction makes it possible to eliminate water absorbance and magnify the remaining spectral features to the limit of the signal-to-noise ratio. Many other data-processing techniques, such as the ratio method,4 the least-squares refinement,5 and factor analysis,6 should be of benefit in the study of carbohydrate mixtures. 

Figure 5. Spectrum of single crystal of diundecanoyl peroxide (UP) before and after photolysis (4000-400 cm 1) (a) UP at approx. 20 K before photolysis (b) after photolysis (c) difference. Note that spectral windows exist near 3700, 2350, 2280, and 660 cm 1, and that C02 bands are easily visible in absorbance subtraction. The scale in (a) and (b) is 5/2 that of (c).

Figure 10. C02 asymmetric stretching region in FTIR absorbance subtraction of lightly photolyzed crystals of UP. (a) Species A, after photolysis at 20K ( 0.05% conversion) (b), (c), and (d) are representative of species formed after warming through temperatures where EPR spectra show transitions. 

Figure 12. Absorbance subtraction of v3 to show intermediates on pathway from A to B, as well as extent of reaction. The numbers indicate the fraction of A and B subtracted. 

Because most FT—IR spectrometers have a computerized data system as an integral component of the instrument, it is a relatively simple matter for programs to be written that operate on spectra as soon as they have been computed from the interferogram. One of the most important of these programs is the scaled absorbance subtraction routine. This program can be used to achieve the same result as placing a sample in the reference beam of the spectrometer, but the thickness of the reference cell or sample does not need to be controlled as accurately... 

The intensity of the incident radiation (/q) and the intensity of the transmitted radiation (/) can be measured by the base-line method illustrated in Fig. 10. In the FTIR spectrometer, the computer is capable of converting the transmittance into absorbance, subtracting the background or the solvent interference from the sample spectrum, generating a calibration curve, and calculating the unknown concentration. 

Another common treatment consists of background spectral correction by subtracting the baseline spectrum before the peak appearance. Drifts in the baseline of electropherograms can be circumvented by absorbance subtraction. Beyond these simple corrections, more sophisticated treatments for detrending and noise hltering using wavelets, artihcial neural networks, and so on can be used. 

Applications of the Spectral Subtraction Technique. Based on the advantage of precision wavenumber measurement provided by computerized FT-IR Instrumentation, the absorbance-subtraction technique has become a practical method in analysis of multicomponent mixtures [ ]. It was also found in this research that difference photoacoustio spectroscopy can be used to distinguish small differences between two samples. By comparing the PAS spectra of treated and untreated materials, the common spectral features can be cancelled out. The remaining bands can be interpreted in terms of the near-surface chemical species due to the treatment. 

For the accentuation of these small differences in the spectra of the stressed and unstressed polymer the absorbance subtraction technique has proved particularly useful. In Fig. 3 this is illustrated with reference to the 972.5 cm absorption band of the v(0—CH2) skeletal vibration of polyethylene terephthalate. Fig. 3 a shows the shape of this absorption band for the unstressed and stressed (300MN/m ) polymer. In the difference spectrum (see Fig. 3 b) the shift of the peak maximum toward lower wavenumbers and the low-frequency tailing are reflected by a pronounced asymmetrical dispersion-shaped profile. 

Fig. 16a and b. FTIR spectra recorded during uniaxial elongation of a hi -doisity polyethylene film at 300 K with unpolarized radiation (a) and absorbance subtraction of successively recorded spectra (b) (a 83% strain, b 50% strain, a-b difference spectrum)... 

Fig. 36a-e. Accentuation of the strain-induced crystallinity bands in the spectrum of sulfur-crosslinked natural rubber by absorbance subtraction a structural absorbance spectrum of 500% elongated sample b structural absorbance spectrum of unstretched material c difference spectrum (a)-(b)... 

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