Infrared spectra percent transmittance

The absorbance, A, is measured as the peak height or peak area in a spectrum. Percent transmittance spectra (%T) should not be used for quantitative analysis since the relationship between %T and concentration is complex and nonlinear. The concentration, c, in Beer s Law is the concentration of analyte in the sample. The analyte must be an absorbing species that is, it must have at least one peak in the infrared spectrum to be analyzable. The concentration units used in Beer s Law can be whatever is convenient for the analyst such as moles/liter, parts per million (ppm), weight percent, etc. The predicted concentrations in the unknown sample will be in the same units as the standards used to obtain the calibration. The pathlength in Equation 5.1 is the thickness of sample seen by the infrared beam as seen in Figure 5.1. For solids and liquids analyzed in the mid-infrared, pathlengths on the order of microns are typical. 

An example of an absorption spectrum—that of ethanol exposed to infrared radiation—is shown in Figure 12.12. The horizontal axis records the wavelength, and the vertical axis records the intensity of the various energy absorptions in percent transmittance. The baseline corresponding to 0% absorption (or 100% transmittance) runs along the top of the chart, so a downward spike means that energy absorption has occurred at that wavelength. 

Fig. 5.5. Illustration of the method to determine the percent transmittance of a peak in an infrared spectrum.

Clear films in thin widths, for example 0.025 mm, are transparent in the visible light, near-UV and near-infrared spectrum with a transmittance of about 90% and haze of a few percent. Refractive indices are roughly 1.46. 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

All infrared spectrometers generate data that are contained in the infrared spectrum (see Fig. 10.1). The spectrum represents the ratio of transmitted intensities with and without sample at each wavelength. This intensity ratio is called transmittance (7 ) can be replaced by percent transmission (%7 ) or by absorbance A = log(l/T). If the experiment is conducted using reflected or diffuse light, pseudo-absorbance units are used (cf. 10.10.2). Finally, it is common to report wavelengths in terms of wave number v (cm-1 or kaysers) knowing that ... 

Nowadays, most instruments use a FT-infrared (FT-IR) system, a mathematical operation used to translate a complex curve into its component curves. In an FT-IR instrument, the complex curve is an interferogram, or the sum of the constructive and destructive interferences generated by overlapping light waves, and the component curves are the IR spectrum. The standard IR spectrum is calculated from the Fourier-transformed interferogram, giving a spectrum in percent transmittance (%T) versus light frequency (cm ). 

The infrared spectrum of hexane is shown in Figure 14.15. There are two horizontal scales one in reciprocal centimeters (the fi equency scale, v) and the other in x (the wavelength scale in microns or micrometers, which is 1(H m). There are also two vertical scales absorbance (A) and percent transmittance (% T). If all of the infi ared light is absorbed by the molecule, the value of A = 100%, which means that no light is transmitted, and % T = 0. If no light is absorbed by the sample, there is 100% transmittance (% T = 100) and 0% A. A larger value of A is associated with a stronger peak and a smaller value of A with a weaker peak. 

If the cell windows are sufficiently flat and parallel the cell thickness of a fixed cell may be measured by observing the interference fringes or percent transmittance undulations which result when the spectrum of the empty cell is taken. Interference fringes result because the transmitted beam finds itself accompanied by the infrared beam which has been twice reflected inside the cell and is retarded by twice the cell thickness relative to the unreflected beam. The thickness t in cm may be calculated by... 

The y-axis of an infrared spectrum can also be plotted in units called percent transmittance (%T), which measures the percentage of light transmitted by a sample. %T spectra are calculated as follows ... 

FIGURE 1.4 The infrared spectrum of polystyrene plotted in percent transmittance (%T). 

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