Spectroscopy wavelength selection

See also Optical Spectroscopy Wavelength Selection Devices Detection Devices. Spectrophotometry Instrumentation Derivative Techniques Biochemical Applications Pharmaceutical Applications. 

W.R. Hruschka, Data analysis wavelength selection methods, pp. 35-55 in P.C. Williams and K. Norris, eds. Near-infrared Reflectance Spectroscopy. Am. Cereal Assoc., St. Paul MI, 1987. P. Geladi, D. McDougall and H. Martens, Linearization and scatter-correction for near-infrared reflectance spectra of meat. Appl. Spectrosc., 39 (1985) 491-500. 

Sx, Ti -> Tx). Figures 3.2 and 3.3 illustrate the principle of flash spectroscopy/65 If the second light source is continuous, the change in optical density due to the transient species can be monitored as a function of time at a particular wavelength selected on a monochromator. This type of system is illustrated in Figure 3.4. 

Previous experience in arc and spark emission spectroscopy has revealed numerous spectral overlap problems. Wavelength tables exist that tabulate spectral emission lines and relative intensities for the purpose of facilitating wavelength selection. Although the spectral interference information available from arc and spark spectroscopy is extremely useful, the information is not sufficient to avoid all ICP spectral interferences. ICP spectra differ from arc and spark emission spectra because the line intensities are not directly comparable. As of yet, there is no atlas of ICP emission line intensity data, that would facilitate line selection based upon element concentrations, intensity ratios and spectral band pass. This is indeed unfortunate because the ICP instrumentation is now capable of precise and easily duplicated intensity measurements. 

Angel, S.M., and M.L. Myrick. 1990. Wavelength selection for fiber optic Raman spectroscopy. Applied Optics 29 1350-1352. 

Although there are a variety of wavelength selection methods available, the vast majority of Raman instruments utilize either dispersive or Fourier transform spectrometers. These are shown schematically in Fig. 1.6. The high throughput and spectral resolution obtainable from these instruments make them obvious choices for Raman spectroscopy however, each has specific strengths and drawbacks which make them more suitable in specific applications. 

It should be pointed out here that wavelength selection in atomic absorption spectroscopy is largely accomplished by the choice of the monochromatic sharp line source, possessing the wavelength of a resonance line of the element to be determined, a specificity of selection unobtainable by any other means. Any additional wavelength selection can be considered merely secondary and the methods to this end should be examined with this in mind. 

A blank or additive interference produces an effect that is independent of the analyte concentration. These effects could be reduced or eliminated if a perfect blank could be prepared and analyzed under the same conditions. A spectral interference is an example. In emission spectroscopy, any element other than the analyte that emits radiation within the band-pass of the wavelength selection device or that causes stray light to appear within the band-pass causes a blank interference. 

As in almost all spectroscopic methods, the instrumentation for infrared or Raman spectroscopy consists of a radiation source, a monochromator or wavelength-selection device of some type, a sample holder, and a detector. 

Two-step laser mass spectrometry, especially in combination with jet cooling, offers ways to obtain complete and accurate oligomeric distributions in many cases at a level of detail that is unavailable from conventional techniques. Additional information can often be obtained from the wavelength selectivity in the ionizing step, ultimately allowing for optical spectroscopy in combination with mass spectrometry. Thus the techniques described in this chapter extend the tools of polymer analysis and can provide new insights in polymer properties. 

When used in combination with mass spectrometry, RTPI allows mass- and wavelength-selective spectroscopy, even if the spectral lines of the different species overlap. This is particularly important for molecular isotopes with dense spectra, which overlap for the different isotopes. This is illustrated by Fig. 1.40, where the differences in the line positions in the spectra of Li3 and Li3 are caused partly by the different masses but mainly by the different nuclear spins. Such isotope-selective spectra give detailed information on isotope shifts of vibrational and rotational levels and facilitate the correct assignment of the spectral lines considerably. Furthermore, they yield the relative isotopic abundances. 

The components essential to perform PAS spectroscopy are a source that determines the PAS spectral range, a spectrometer or interferometer for wavelength selection, a method of modulation of the incident beam, and a photoacoustic cell. 

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