Infrared spectra—continued

If the x-data of an object are time-series or digitized data from a continuous spectrum (infrared, IR near infrared, NIR) then smoothing and/or transformation to first or second derivative may be appropriate preprocessing techniques. Smoothing tries to reduce random noise and thus removes narrow spikes in a spectrum. Differentiation extracts relevant information (but increases noise). In the first derivative an additive baseline is removed and therefore spectra that are shifted in parallel to other... 

Infrared radiation is produced principally by the emission of solid and liquid materials as a result of thermal excitation and by the emission of molecules of gases. Thermal emission from solids is contained in a continuous spectrum, whose wavelength distribution is described by... 

Figure 2.2-2 The visible, near, middle and far infrared region of the spectrum drawn in a scale linear in wavenumbers. The infrared (IR) and far-infrared (FIR) spectrum is recorded by absorption of light from a continuous spectrum in the range of A = 2.5. .. 100 pm = i) = 4000. .. 100 cm and A = 100. .. 1000 pm = 100. .. 10 cm". Raman spectra can be excited by monochromatic radiation, emitted by different lasers in the visible (VIS) or near-infrared range (NIR). Molecules emit Raman lines with a frequency difference AF to that of the exciting frequency >o between 0 and -f 4000 or - 4000 cm. Usually only the Raman spectrum which is shifted to. smaller wavenumbers, the Stokes Raman spectrum, is recorded. Its range is indicated by bars for different exciting lines Ar" " laser at 488 and 515 nm, HeNe laser at 623 nm, GaAs laser at 780 nm, and Nd YAG laser at 1064 nm.

Many investigators employ xenon lamps because they have a continuous spectrum and cover the UV, VIS, and short infrared (IR) bands similar to solar irradiance. The use of an IR dichroic mirror in combination with a UV filter for fhe UV band with these lamps produces a very close simulation of the daylight irradiance (Fig. 1). 

Like many other gases in the atmosphere, carbon monoxide was discovered as a terrestrial absorption feature in the infrared solar spectrum (Migeotte, 1949). A variety of sporadic measurements until 1968, reviewed by Pressman and Warneck (1970), established a CO mixing ratio of the order of 0.1 ppmv. Robinson and Robbins (1968a) and Seiler and Junge (1970) then developed a continuous registration technique for CO in air based on the reduction of hot mercury oxide to Hg and its detection by atomic absorption spectrometry. In addition, gas chromatographic techniques have been utilized. 

In early work (8) we used infrared spectroscopy coupled with attenuated total reflection optics. This work was done before the availability of infrared equipment based on Fourier transform methods. Due to their relative speed these methods now permit in situ, real time measurements with a resolution of 1 sec or less (9), and continue to yield valuable data, particularly in the hands of the Battelle group in a series of studies dating from 1979 (10). In our early infrared work we had to be content to rinse and dry the surface before obtaining the infrared reflection spectrum Nevertheless the values of surface concentration were remarkably close to those determined more recently. Infrared studies of proteins suffer generally from the fact that the main features of protein spectra are similar for all proteins and therefore it is difficult to distinguish one from another. 

The infrared reflectance spectrum in the FAR and MID ranges is presented in Fig. 6. The almost continuous decrease of the reflectivity as a function of the wave number shows that this spectrum is dominated by a conduction mechanism (Gosnet et al., 2008). Moreover, first... 

The emission spectrum for hydrogen atoms in the visible region is shown in Figure 2.36. This series of lines is called the Balmer series, after Johann Balmer, who first observed these lines. Similar sets of lines are observed in the ultraviolet (Lyman series) and infrared regions of the electromagnetic spectrum (Figure 2.37). An emission or line spectrum (Figure 2.38) differs from a continuous spectrum in two important ways ... 

Atoms and simple molecules absorb electromagnetic radiation only at certain discrete frequencies, rather than as continuous functions of frequency. For example, Eigure 11.1 shows the infrared absorption spectrum of hydrogen bromide. Discrete absorption frequencies imply discrete energy spacings. This is the basis for quantum mechanics. 

Currently used instruments are so-called Fourier transform infrared (FTIR) spectrometers using a Michelson interferometer. The light source is a mercury lamp giving a continuous spectrum. From the interferogram (l x)), the frequency-domain spectrum (G(u)) is obtained by a mathematical procedure (Fourier transform) expressed in the following equation ... 

A(v) is an expression of an infrared absorption spectrum as a function of wavenumber. Actually, however, a spectrum obtained from a Fourier transform infrared (FT-IR) spectrometer is not a continuous function but a collection of absorbance values at M discrete... 

Figure 9.10 is a plot of this function for different values of x, treating / as a continuous variable. Notice that a curve drawn through the tops of the transition lines in Figiue 9.7 would have the form of a curve in Figure 9.10. Indeed, at low resolution, where the individual lines overlap or are not resolved, the branches of an infrared absorption spectrum may resemble the curves in Figure 9.10.

I. A source of radiation with a continuous spectrum over a wide range of infrared wavelengths... 

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