Thermal background/emission

Dispersive Instruments. In dispersive instruments monochromators are employed for selection of the wavelength. When a line-like radiation source is employed, a monochromator of low resolution is adequate, but for a continuum radiation source a high resolution monochromator is required. In dispersive equipment the exit slit width is narrower than that in non-dispersive equipment. In this way, thermal background emission and stray light originating from the atomizer can be considerably decreased, but at the same time the optical transmission also decreases. The schematic construction of a dispersive AFS instrument is shown in Figure 144. 

All telescopes suffer from this thermal background, depending on the temperature of the telescope and its optics. In practice, telescopes with clean and freshly applied mirror coatings (such as silver) have emissivities >1% per surface at wavelengths beyond 1/rm. Of course as the optics degrade with time, dirt, etc. theemissivity will grow. 

Almost all of the presented solid sorbents show a kind of background emission after a certain period of time. This is due to thermal or photochemical degradation of the sorbent material. Moreover, many sorbents can react with certain adsorbed compounds or reactive gases like 02, 03 or NOx and reaction products will show up in the analysis (Hanson et al., 1981 Clausen and Wolkoff, 1997 Kleno et al., 2002). 

The cosmic ray electrons which are responsible for the radio-halo and relic synchrotron emission inevitably Compton scatter the CMB (as well as other local background) photons which will then gain energy and emit at higher energy E ss 2.7 keV (E/GeV)2. Electrons with E E a few GeV produce emission in the HXR range, while electrons with E < 400 MeV produce soft X-rays and UV emission. There is actually evidence for an excess of emission w.r.t. the thermal bremsstrahlung emission by the hot IC gas in about 20 nearby clusters... 

A serious problem of using an Nd YAG laser to excite FT-Raman is the difficulty of attempting to study samples at temperatures > 150°C. The thermal blackbody emission from the sample becomes more intense (broad background) than the Raman signal. The S/N ratio is lowered, and the detector becomes saturated. 

Flame AFS combines features of both AAS and FES. The excitation of atoms is by the absorption of light. When individual element spectral line sources are used, the spectral selectivity should be as high as that in AAS, although scatter may be more of a problem in AFS. Quantification is by comparison of the intensity of fluorescence emitted by samples with that emitted by standards of known concentration. At low determinant concentrations, it is necessary to discriminate between small fluorescence emission signals and the background light levels associated with thermally excited emission from the flame. Therefore in AFS, as in FES, it is desirable to have low flame background emission. This is discussed further in Chapter 2, where instrumental aspects of flame spectrometric techniques are discussed. 

With 1064 nm infrared excitation, generally used in FT-Raman studies, the effect of sample heating is noted first in emission at higher wavenumber shifts (> 2000 cm ), but the problem is exacerbated when the sample itself is being thermally investigated. For this reason, the background emission of infrared radiation at higher wavenumbers with 1064 nm excitation varies with sample temperature until near 200 °C it becomes effectively unworkable, e.g. the Raman spectrum of Sg at elevated temperatures. 

A complete analysis of the thermal emission of surface species and their measurements has been reported [71]. A problem of extending the emission technique to surface analysis is that strong background emission can be superimposed on the weaker emission from the surface. Thus the ideal sample for emission studies is a very thin surface layer supported on a perfect reflector. An experimental emission sampling system designed specifically for the study of surface species has been described [72]. 

The fact that our eyes are so exclusively tuned to the Sun has thus blinded us to almost all forms of radiation. This includes radiation from media at very different temperatures, such as the relic cosmological background that filters down to us from the beginning of time, and the great majority of non-thermal emissions, such as the signals from pulsars and supernova remnants. 

The emitting species for sulfur compounds is excited S2. The lambda maximum for emission of excited S2 is approximately 394 nm. The emitter for phosphorus compounds in the flame is excited HPO with a lambda maximum equal to doublet 510-526 nm. In order to detect one or the other family of compounds selectively as it elutes from the GC column, the suitable band-pass filter should be placed between the flame and the photomultiplier tube to isolate the appropriate emission band. In addition, a thermal infrared filter is mounted between the flame and the photomultiplier tube to isolate only the visible and UV radiation emitted by the flame. Without this filter, the large amounts of infrared radiation emitted by the combustion reaction of the flame would heat up the photomultiplier tube, thus increasing its background signal. 

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