Single-beam spectrum

In Fig. 2.12 characteristic IR reflectance spectra of concentrated (3 M) methanol on platinum at potentials between 0.45 V and 1.3 V are shown. The single beam spectrum at 0 mV was taken as background. The following characteristic bands allow the identification of bulk products ... 

Spectra of a spent bauxite-based desulfurization catalyst pellet ( 7 x 13 mm, examined in air) are shown in Fig. 9. The outside of the pellet was black and the single-beam spectrum S showed some of the continuum absorption found with chars. The compensated spectrum S/So, however, showed appreciable spectral structure. The broad band near 750 cm is probably due to the bauxite, and the absorptions near 3000, 1320 and 1000 cm-- - to a mixture of hydrocarbons and thio species formed during the reaction. The feature near 1640 cm l is probably caused by an olefinnic species. 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

Figure 1.2 Interferogram recorded by a d.c.-coupled detector in which the signal counts can vary from 0 to 16384 (top). Fourier transformation of the recorded interferogram profile yields a single-beam spectrum (middle). Single-beam spectra from a sample can be ratioed point-by-point in the spectral domain to single-beam spectra acquired without a sample in the beam path, yielding absorbance spectra (bottom). The absorbance features in a spectrum can be correlated to the molecular properties of the sample (dark profile), while a featureless spectrum (light profile) denotes the lack of sample in the beam path.

Figure 6. Single-beam spectrum from a DIT cell containing carbon tetrachloride.

Figure 2. Interferogram (top) and resulting single beam spectrum after Fourier...

Fig. 4.1.2. Important elements of a FTIR spectrospcopy. 1 Interferogram. 2 Single-beam spectrum. 3 100% line...

The encoded spectral information in the interferogram (I (<5)) is stored in a computer and transformed into the more familiar form of a single-beam spectrum (Fig. 3) by means of a fast Fourier transform. 

So, the function representing the calculated single-beam spectrum is ... 

Fig. 3. Single-beam spectrum resulting from the Fourier transformation of the interferogram in Fig. 2 b.

It is interesting to compare the single-beam spectra [35] (Figure 5) recorded on an nm-Pt/GC electrode at 0.0 and 0.70 V, respectively. In normal cases, such as CO adsorption on a massive Pt electrode, IR bands of CO adsorbates often cannot be observed in single-beam spectrum since the IR absorption of COad is usually too small and is buried in the strong background, only the CO2 band that is derived from COad oxidation and is in the thin layer may appear due to the strong IR absorption... 

I is the intensity measured in a single beam spectrum of sample and I0 is the intensity measured in the background spectrum. Transmittance is often expressed as %T, which forms the scale of the vertical axis in Figure 9.20c. The spectrum can also be presented as absorbance (A) versus wavenumber as shown in Figure 9.21. The absorbance is calculated from the transmittance. 

The spectrum was recorded with the use of an ATR sampling accessory and has been ratioed against a single-beam spectrum of water. 

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