Emission background continuum

The use of graphite electrodes is responsible for the observed characteristic emission lines of radicals and organic functional groups (e.g. cyanogen CN bands between 320 and 400 nm) and also for the background continuum they generate. 

In atomic absorption the background continuum is usually negligible and the resonance line intense. To give the maximum discrimination against stray radiation, and hence the lowest detection limit, the slit width should be small. In atomic emission and fluorescence the analytical signal is smaller and the background due to scattered light and con-... 

Depending on the pressure in the lamp, both line and continuum emission may be observed. Low-pressure lamps operated at low current densities and temperatures produce sharp atomic lines with little or no continuous background. Increasing pressure and temperature cause the lines to broaden and increase the intensity of the background continuum. 

Analysis of the photons emitted by the plasma This step is shared by other atomic emission spectrometries. However, the high electron temperature of laser-induced plasmas requires the use of temporal resolution to improve the detection sensitivity. The electron density of the plasma remains very high during the time the laser pulse interacts with the solid and the vapour. The emission spectrum obtained in this situation is a continuum due to ion-electron recombination. Ionic and atomic lines are superimposed on the emission background. Once the laser pulse has extinguished, the system rapidly loses energy and many atoms are excited. Observation of the plume at this time allows species to be much more easily detected. 

At complete ionization of the hydrogen (e.g. when added to a plasma with another gas as the main constituent) ne = p/(2 x k x Te) has a maximum at a wavelength of X — (7.2 x 107)/Te or at a fixed wavelength, the maximum intensity is found at a temperature Te = (5.76 x 107)/2. Thus, the electron temperature can be determined from the wavelength dependence of the continuum intensity. As Te is the electron temperature, absolute measurements of the background continuum emission in a plasma, e.g. for the case of hydrogen, allow determination of the electron temperature in a plasma, irrespective of whether it is in local thermal equilibrium or not. Similar methods also make use of the recombination continuum and of the ratio of the Bremsstrahlung and the recombination continuum. 

Spectral interference has been well studied and are probably best understood in atomic emission spectroscopy. The usual remedy to alleviate a spectral interference is to either increase the spectral resolution of the spectrometer (which often is not possible with a given type of instrument) or to select an alternative emission line. Three types of spectral interference can be discriminated 1. Direct wavelength coincidence with another emission line, 2. partial overlap of the hne under study with an interfering line in close proximity, 3. a linear or non-linear increase or decrease in background continuum (see Fig. 12.33). 

Flame emission spectra are relatively simple, and small spectrometers were thus considered adequate. The more versatile and larger modern instruments have greater resolving power so that more spectral lines are resolved and background continuum is made weaker by their greater dispersion. 

In assessing the overall fluorescence emission intensity, account must also be taken of the spectral output of the excitation source. As an illustration of this point, the emission characteristics of a typical X-ray tube are plotted in Figure 5. The tube output comprises two components characteristic fluorescence lines derived from the tube anode and a broad background continuum component. The excitation efficiency is maximized for elements that have absorption edges just below the energy of the characteristic tube lines, where the maximum tube output intensity is observed. 

Medium pressure 1-10 bar) Hg lamps. These lamps emit several spectral lines in the UV and in the visible, over a very weak continuum emission background. The main Unes are listed in Table 4.1, together with their relative intensities, expressed as power (watt) or as number of photons. Small differences in the... 

The atomic emission from inductively coupled plasma frequently resides on a substantial background emission. The majority of background continuum emission is due to bremsstrahlung radiation (deceleration of fast-moving electrons) or an electron-ion recombination process. Another cause of the background radiation is the molecular band emission from the OH species originating from the aqueous samples (Eig. 8C). As shown in Fig. 8D, the... 

Typical elemental detection limits are listed in Table 1. The detection limit is the concentration that produces the smallest signal that can be distinguished from background emission fluctuations. The continuum background is produced via radiative recombination of electrons and ions e — M+ hv or M + e + e — ... 

Deuterium arc background correction. This system uses two lamps, a high-intensity deuterium arc lamp producing an emission continuum over a wide wavelength range and the hollow cathode lamp of the element to be determined. 

Background emission by the flame (Figure 8.23) includes contributions from molecular species and continuum radiation from incandescent particles and depends upon the combination of fuel and support gases used. The sample solvent and matrix will further augment background radiation. 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

Electron probe microanalysis functions by direct examination of the primary X-rays produced when the specimen is used as a target for an electron beam. Focused electron beams allow a spot analysis of a 1 pm3 section of the specimen. One current development employs the electron beam within a scanning electron microscope to provide both a visual picture of the surface of the sample and an elemental analysis of the section being viewed. Spectra obtained from primary X-rays always have the characteristic emission peaks superimposed on a continuum of background radiation (Figure 8.32). This feature limits the precision, sensitivity and resolution of electron probe microanalysis. 

Spectroscopic interferences can also manifest themselves, either as an increase in the continuum background emission or as line overlap, especially if samples with a complex matrix, or organic solvents, are analysed. An increase in the continuum background emission can be easily compensated for by subtraction of the background adjacent to the analytical line. For a sloping background then measurements must be made on both sides of the line and usually the mean value is subtracted. These options are summarized in Fig. 4.18. Line overlap is a particular problem when an element, present in large excess in the matrix, has an emission line close to. 

Q. How would you correct for an increase in the continuum background emission caused by aluminium ... 

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