Self-absorption plasmas

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

Q. Why do some plasmas exhibit less self-absorption ... 

Self-absorption, which has a critical influence on AES sensitivity, could not be accurately quantified by using an equation relating the emission intensity of a resonance line affected by this phenomenon to the sputtering rate of the analyte concentration in the matrix [196]. Subsequent studies revealed that, for Cu and V, ionic lines are more sensitive than atomic lines — the latter proved independent of self-absorption but were strongly influenced by the type of plasma used (Ar or Ne) [197]. 

Proper selection of the observation height in the plasma is crucial with a view to obtaining the best operating conditions (particularly the highest possible signal-to-back-ground ratio for analytical atomic lines in the absence of self-absorption and line broadening). 

Plasma analysis is known to be subject to spectroscopic disturbances including plasma line instability, self-absorption, line broadening, slit problems and grating imperfections. 

The plasma sources can achieve temperatures in the range of 5000 to 10 000 K which are advantageous over flame emission that can only achieve temperature ranges of 1500 to 2500K. The flame in AAS lends itself to self absorption, spectral, chemical and ionization interferences which gives rise to noisy background. These interferences including ionisation are not very severe in plasmas because the extra electrons released by EIEs have little effect on the ionisation equilibrium of other elements and the extra electrons form a small portion of the total electron concentration in the plasmas. 

The temperature of the arc depends upon the composition of the plasma and varies with the nature of the sample. If the sample is made of material with low ionization energy, the temperature of the plasma will be low if the ionization energy of the material is high, the temperature will be high. In addition, the temperature is not uniform in either the axial or radial directions. This results in matrix effects and self-absorption. Arc temperatures are on the order of 4500 K with a range of 3000-8000 K. Emission spectra from arc sources contain primarily atomic lines with few ion lines. The DC arc can excite more than 70 elements. 

Due to the high excitation power of the ICP, the spectrum obtained is very rich with lines caused by atoms and ions, which makes it possible to correct the interference due to the light scatter using a double beam technique. When using a plasma source for excitation, interference caused by the scatter may be corrected by a method based on self-absorption. In this technique the slope of the fluorescence graph is compared to that of the plasma emission graph. 

In flame emission spectroscopy, the concentration of electronically excited atoms in the cooler, outer part of the flame is lower than in the warmer, central part of the flame. Emission from the central region is absorbed in the outer region. This selfabsorption increases with increasing concentration of analyte and gives nonlinear calibration curves. In a plasma, the temperature is more uniform, and self-absorption is not nearly so important. Table 20-4 states that plasma emission calibration curves are linear over five orders of magnitude compared with just two orders of magnitude for flames and furnaces. 

Several other advantages are associated with the plasma source. First, atomization occurs in a chemically inert environment, which tends to enhance the lifetime of the analyte by preventing oxide formation. In addition, and in contrast to arcs, sparks, and flames, the temperature cross section of the plasma is relatively uniform as a consequence, self-absorption and selfreversal effects do not occur as often. Thus, calibration curves are usually linear over several orders of magnitude of concentration. Finally, the plasma produces significant ionization, which makes it an excellent source for ICPMS. 

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