Wavelength Spectrometer

Infrared Space Observatory Short-Wavelength Spectrometer 300... 

A modified version of the spectroscopy exercise could be conducted for physical chemistry laboratory courses, and could even be simplified to be used for honors freshman chemistry labs. In order to obtain frequencies that are accessible with radio telescopes, measurements have to be conducted at microwave or millimeter wavelengths. Spectrometers that operate in this range are not readily available to physical chemistry teachers however, a simple direct... 

One IR spectroscopic system often used for industrial monitoring is the selective-wavelength spectrometer... 

These effects have been used by investigators to reduce the background radiation levels in x-ray spectroscopy. For Bremsstrahlung radiation, the maximum polarization vector is parallel to the electron path in the x-ray tube. In many conventional-wavelength spectrometer designs, the specimen and crystal surfaces will become parallel when 26 equals the takeoff angle 2 ( 45°). In addition, the x-ray tube axis and the rotational axes of the crystal and detector are parallel. This so-called parallel optics cannot take advantage of the polarization of the white spectrum. If, however, the surface plane of the crystal and specimen remain perpendicular... 

Champion and Wittem [11] used this effect and redesigned a conventional-wavelength spectrometer with a resulting maximum background reduction of nearly 5 1, under ideal circumstances. 

For both Eqs. (11.37) and (11.39), it can be shown that the percent standard deviation in the concentration at the detection threshold Cmdl is greater than 60.8% and approaches 60.8% as the background approaches infinity. To achieve a 10% standard deviation in the analyzed concentration, a concentration at least six times the Cmdl value is required. This result is identical to that given in Sec. 11.2.1 for the wavelength spectrometer. The general comments in Sec. 11.2.1 regarding the calculation and interpretation of minimum detection limits should be reviewed and applied to energy-dispersive systems at this point. 

Beyond the x-ray tube and specimen chamber, the x-ray spectrometer itself can contribute to the background. With the wavelength spectrometer, the most important contribution is due to second- and higher-order diffraction [1,2]. For example, consider examining a water specimen for trace amounts of iron. The spectrometer... 

The other type of X-ray analysis system used with SEMs today is the wavelength spectrometer, illustrated in Fig. 5. Here the selective diffraction properties of a crystal in conjunction with a proportional counter detector is used to sort out the radiation according to wavelength. With suitable crystals, this system allows the analysis of all elements from Be-U. In addition to allowing the detection of light element radiation, the WDX spectrometer provides superior elemental discrimination compared to an EDX system. 

The silicon concentrations were determined by X-ray fluorescence using a Phillips Automated X-ray Wavelength Spectrometer. 

The X-ray spectrometer itself contributes to the background. With the wavelength spectrometer, the most important contribution is due to the second and higher order diffraction. 

The spectrometer is sometimes used alone as a concentration detector but it is more commonly used as a secondary detector for composition analysis, especially with copolymers [50-52]. Obviously, solutes must absorb at a particular wavelength to which the spectrometer has to be set. The solvent must be transparent at this wavelength, which may cause problems of selection with regard to polymer solubility. An ultraviolet (UV) spectrometer is the most commonly used, and it is very concentration-sensitive when the sample has a good absorption coefficient. Classical UV detectors operate at 254 nm but many multi-wavelength spectrometers are available. Other spectrometers like infrared... 

Figure 3 Far-infrared spectrum of the ejecta from the star IRC 10216 obtained with the Long-Wavelength Spectrometer on board the Infrared Space Observatory at a resolution of about 100. Most of the lines In this spectrum can be Identified with pure rotational transitions In CO, HCN and their Isotopomers (top arrows). Rotational transitions associated with the lowest vibrational states of HCN are also present (bottom arrows). Reproduced with permission from Cernicharo J, Barlow MJ, Gonzalez-Alfonso et al (1996) Astronomy and Astrophysics 315 L201.

Figure 4 The 2-20 xm spectrum of the protostar W33A obtained at a spectral resolution of 250 using the Short-Wavelength Spectrometer on board the Infrared Space Observatory. Except for the 10 xm silicate band, all features are due to simple molecules in ice mantles.

Figure 5 Absorption spectra of a number of protostars in the stretching region of CO and CO2 obtained by the Short-Wavelength Spectrometer on board the Infrared Space Observatory. The broad band at about 4.25 pm is due to the stretching mode of solid CO2. The narrow features in the 4.5-4.9 pm range are the rovibrational transitions of gaseous CO. The broader feature in the Q-band gap of this linear molecule is the stretching mode of solid CO. The narrow emission feature at about 4.66 pm in the spectrum of NGC 7538 IRS 1 is a hydrogen recombination line. These different sources are characterized by different temperatures of the absorbing gas and ices. The relative amounts of the rather-volatile molecule CO in the gas phase and the ice reflect this rather strikingly. Reproduced with permission of the Royal Society of Chemistry from van Dishoeck EF (1998) Faraday Discussions 109 31.

Figure 6 Absorption spectra of the bending mode of CO2 ice towards a number of protostars embedded in their natal molecular clouds obtained by the Short-Wavelength Spectrometer on board the Infrared Space Observatory at a spectral resolution of about 2000. The observed profiles show weak features at about 15.1 and 15.25 im as well as a shoulder at about 15.4 pm. The solid lines are laboratory spectra of the H20 CH30H C02 = 1 1 1 mixture deposited at 10 K and then warmed to temperatures in the range of 114 to 118 K. The sharp structure near 14.97 pm in some sources is due to the rovibrational transitions in the Q branch of gaseous CO2 which at this resolution pile up in one spectral resolution element. The individual P and R branch lines are too weak to be observable at this resolution.

Much higher spectral resolution ( 0.5 cm ) was obtained by the Infrared Space Observatory (ISO) in 1997 (see Coustenis etal., 1997 Coustenis Taylor, 1999). A disk-average spectrum of Titan by the Short-Wavelength Spectrometer (SWS) of... 

Clegg, P. E. (1992). The long-wavelength spectrometer in ISO. In Infrared Astronomy with ISO, 87-102, ed. Th. Encrenaz M. F. Kessler. New Yoik Nova Science Publishers, Inc. 

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