Carbon dioxide background spectrum

Figure 1. FT-IRRAS double modulation spectrum of six monolayers of Cadmium Arachidate on silver. The inset shows the background water vapor and carbon dioxide present in the measurement. IBM Instruments IR/90 series.

Figure 1. Background (no sample) spectrum at different scale expansions illustrating system wavelength response, absorption attributable to typical amounts of residual water and carbon dioxide, and high signal-to-noise. The spectrum required 12 minutes of data acquisition at 8-cm optical retardation.

Figure 3 is the absorbance spectrum of a sample of the ambient laboratory air drawn into the cell. Here, in accord with the usual procedure, the initially determined spectrum was first corrected for radiation that had reached the detector without having passed through the sample (room temperature background radiation entering the optical path via imperfect optical components and nonoverlap of the source and detector pupils and fields), ratioed against a zero-sample spectrum, and converted to absorbance. Trace (A) shows the spectrum from 3600-600 cm l. The massive absorbances seen here truncated at 1% transmission are due to water vapor and to carbon dioxide. 

Figure 4b shows the %T IR spectrum for the peak at 24.09 minutes. Because of the density programming, this spectrum was collected at a higher CO2 density than that of the background scans obtained at the beginning of the run. As a result, the dominant spectral features are those of the carbon dioxide. 

Figure 4.14 A background spectrum of air, showing the absorption bands due to water vapor and carbon dioxide. Collected on a Paragon 1000 FTIR spectrometer, PerkinElmer Instruments, Shelton, CT (www.perkinelmer.com).

An infrared spectrum may be defined as a sample-dependent change induced on the intensity distribution of infrared radiation emitted by a source over the entire infrared region. The intensity distribution (spectrum) of the mid-infrared radiation emitted by a source is shown in Figure 3.2a. This spectrum does not have absorptions by any sample (except for absorptions of atmospheric water vapor and carbon dioxide), and it is denoted as the reference spectrum (sometimes called the single-beam background spectrum) B v). ( ) is deter-... 

The as-measured infrared spectrum of a sample usually contains, in addition to the genuine spectrum of the sample, what is not required for spectral analysis for example, noise signals arising from the detector and electronic circuits of the spectrometer, a tilted background arising from a coarse surface of the sample, bands due to water vapor and carbon dioxide existing inside the spectrometer, and so on. If these unnecessary features can be removed from the as-measured spectrum, the spectral features intrinsic to the sample will be seen clearly and they will become more useful for their intended purpose. 

Computer-interfaced FT-IR instruments operate in a single-beam mode. To obtain a spectrum of a compound, the chemist first obtains an interferogram of the background, which consists of the infrared-active atmospheric gases, carbon dioxide and water vapor (oxygen and nitrogen are not infrared active). The interferogram is subjected to a Fourier transform, which yields the spectrum of... 

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