Percent transmittance spectrum

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.

Reading the percent transmittance from the recorded IR spectrum for quantitative analysis can be a challenge. In the first place, there can be no interference from a nearby peak due to the solvent or other component. One must choose a peak to read that is at least nearly, if not completely, isolated. 

Secondly, the baseline for the peak must be well defined. The baseline for the entire spectrum is the portion of the trace where there are no peaks. It does not usually correspond to the 100% T line. Thus, two percent transmittance readings must be taken, one for the baseline (corresponding to where the baseline would be if the peak were absent—a blank reading) and one for the minimum percent transmittance, the... 

Obtain a liquid sampling cell from your instructor and fill it with the 20% solution. Place this cell in the FTIR instrument, and obtain the transmittance spectrum according to the instructions specific to your instrument. Record the percent transmittance value at the tip of the 817 cm-1 peak and at the baseline. Repeat with the other three standards, the unknown, and the control sample. 

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. 

A transmission spectrum is a plot of transmittance or percent transmittance vs. wavelength. An emission spectrum is a plot of emission intensity vs. wavelength. 

The preparation of the KBr pellet is standardized to use about 0.20 g of warm, dry, IR-spectroscopic grade KBr and about 2 mg of sample which will yield a pellet approximately 0.5 mm thick and 13 mm in diameter. The typical percent transmittance values of the 1200- to 500-cm "1 region of the spectrum range from a 70-85% base line to 5-30% for the peaks. This set of characteristics adequately allows the resolution of those peaks that can be resolved. The spectra given herein were taken on a Nicolet DX FTIR System. The resolution was 2.0 cm", the gain was set to 1, the interferometer velocity was 20, and 30 scans were taken. 

Percent transmittance (%T) is plotted along the y-axis with 100% T at the top. Absorption bands come down from the top of the spectrum. 

Apparatus Use a suitable variable-wavelength spectrophotometer capable of measuring percent transmittance throughout the visible spectrum and designed to permit the use of sample and reference cells with pathlengths of 2 to 4 cm. The transmittance of all paired cells should agree within 0.5%. 

The infra-red spectrum of a compound represents the variation of absorption, or transmittance, with changing wavenumber. It is customary to present the ordinate of an infra-red spectrum as percent transmittance and most collections of spectra made in the past have been recorded in this way. However, logarithmic conversion to absorbance is now readily achieved and modern instruments usually give a choice for presentation of spectra. 

For quantitative analysis of a single component, a strong absorption band that is relatively free of overlapping bands or interference is selected from the IR spectrum. The intensity of the band is measured either in units of percent transmittance or absorbance. 

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. 

An absorption spectrum is a plot that measures the percent transmittance as a function of frequency. 

Percent transmittance—that is, the percent of radiation that passes through a sample—is plotted on the vertical axis. When a molecule absorbs radiation of a given frequency or wavenumber, this is seen in the spectrum as an inverted spike (peak) at that wavenumber. Certain structural features of molecules appear as absorption peaks in definite... 

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... 

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