Dispersion devices, spectroscopy

Basic instrumentation for both UV and IR spectroscopy consists of an energy source, a sample cell, a dispersing device (prism or grating) and a detector, arranged as schematically shown in Figure 2.1. 

Sources. The ultimate source for spectroscopic studies is one that is intense and monochromatic but tunable, so that no dispersion device is needed. Microwave sonrces such as klystrons and Gnnn diodes meet these requirements for rotational spectroscopy, and lasers can be similarly nsed for selected regions in the infrared and for much of the visible-ultraviolet regions. In the 500 to 4000 cm infrared region, solid-state diode and F-center lasers allow scans over 50 to 300 cm regions at very high resolution (<0.001 cm ), but these sources are still quite expensive and nontrival to operate. This is less trne... 

Nowadays, many analytical laboratories are equipped with an infrared (IR) and a Raman spectrometer, be it a dispersive device or a Fourier transform (FT) instrument. Raman and IR spectra provide images of molecular vibrations that complement each other and thus both spectroscopic techniques together are also called vibrational spectroscopy. The concerted evaluation of both spectra gives more information about the molecular structure than when they are evaluated separately. 

This overview on analytical atomic spectrometry touches on the basics of three dominant methods of conducting optical spectroscopy for the purposes of qualitative and quantitative elemental analysis. There are a number of variations in sources, atom cells, dispersive devices, etc. that have not been discussed. As an example, laser-induced breakdown spectroscopy employs a high-intensity laser to ablate samples where the extreme radiant energy also produces a plasma that ultimately produces electronic excitation of the ablated material. Similarly, there are a number of nonoptical approaches that represent variations of some of these schemes that have... 

Detectors for IR radiation fall into two classes thermal detectors and photon-sensitive detectors. Thermal detectors include thermocouples, bolometers, thermistors, and pyroelectric devices. Thermal detectors tend to be slower in response than photon-sensitive saniconduc-tors. The most common types of detectors used in dispersive IR spectroscopy were bolometers, thermocouples, and thermistors, but faster detectors are required for FTIR. FTIR relies on pyroelectric and photon-sensitive semiconducting detectors. Table 4.5 summarizes the wavenumber ranges covered by commonly used detectors. 

The extent of reaction ( is determined by mass balancing the open system in steady state. The concentrations of carbon monoxide and carbon dioxide are measured by non-dispersive infrared spectroscopy (Binos, Rosemount), oxygen is determined by use of a magnetic device (Magnos 3, Hartmann fz Braun). 

Although other detector technologies exist, the current detector of choice for virtually all types of dispersive Raman spectroscopy is the silicon CCD (charge-coupled device) array. The CCD array meets more of the desired detector characteristics for Raman spectroscopy than any other currently available detector technology. These characteristics include the following ... 

This application was developed at Monsanto in order to characterize the manufacture of phosphorus trichloride (PCI3) [18-20]. It is probably one of the most extensively documented Raman applications and is included here as a model for future applications. In addition, it is of historical interest, as it was one of the few application where a FT-Raman spectrometer was used for on-line analysis. Most recently, dispersive Raman spectroscopy based on charge-coupled device (CCD) spectrometers have displaced the original FT-Raman analyzer [21] for several reasons, as follows. 

Thus light of a particular frequency can simultaneously induce a dipole moment in a molecule and then couple with the dipole components to result in light absorption Raman spectra are observed within the spectram of light scattered from an intense source. Induced vibrational transitions are observed with a dispersive device (monochrometer) and some sort of electronic detection (in the visible range) at 9(f from the light source (laser) beam. Remarkably, C. V. Raman first observed this effect with a handheld spectroscope in 1928 for which he received the Nobel Prize in 1930. Thus we can examine the symmetry properties of second-order combinations of the Cartesian coordinates (in column 2 ) and use them to indicate a yes/no answer as to whether a given molecular vibration will occur in Raman spectroscopy. 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

The spectroscopy system uses a dispersive element and a detector which is either a charge-coupled device (CCD) or a diode array. A computer is required for instrument control and for intensive data processing. 

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