Barium, resonance line

Adding an excess of a more easily ionized element to all standards and samples eliminates ionization interference. This addition creates a large number of free electrons in the flame. The free electrons are captured by the analyte ions, converting them back to atoms. The result is to suppress the ionization of the analyte. Elements often added as ionization suppressants are potassium, rubidium, and cesium. For example, in the AAS determination of sodium, it is common to add a large excess of potassium to all samples and standards. Potassium is more easily ionized than sodium. The potassium ionizes preferentially and the free electrons from the ionization of potassium suppress the ionization of sodium. The detection limit of the sodium determination thereby decreases. The ionization suppression agent, also called an ionization buffer, must be added to all samples, standards, and blanks at the same concentration for accurate results. An example of the use of ionization suppression is shown in Fig. 6.20. Absorbance at a barium resonance line (atomic absorption) and absorbance at a barium ion line (by barium ions in the flame) are plotted as a function of potassium added to the solution. As the potassium concentration increases, barium ionization is suppressed the barium stays as barium atoms. This results in increased atomic absorption at the resonance line and a corresponding decrease in absorbance at the ion line. The trends in absorbance at the atom and ion lines very clearly show that barium ion formation is suppressed by the addition of 1000 ppm of the more easily ionized potassium. 

Barium is a test of instrumental excellence. It is sensitive from the wall of the furnace but it requires a high atomization temperature which, at the long wavelength of the Ba resonance line, often produces a noisy background. 

In most laser studies of optical isotope shifts the crossed, laser-atomic beam method is used to avoid Doppler-broadening. In some cases short-lived radioactive isotopes are detected directly on-line, in others, by irradiating a target which is subsequently heated to form the source of an atomic beam. Figure 7 shows a spectrum obtained some years ago in Oxford on the resonance line of barium at 554 nm S] ... 

Fig. 8.16. Experimental spectrum of doubly-excited resonances in the barium spectrum obtained by three-photon spectroscopy. The horizontal arrows in the figure indicate lines which are not spectral features but frequency markers. Because of the mode of excitation, the lines tend to be more symmetrical than in some of the other spectra, but nevertheless exhibit a clear q reversal as the main feature is traversed. A theoretical fit by MQDT is also shown (after F. Gounand et al. [421]).

A source matrix which shows no line broadening due to unresolved quadrupole splitting and which gives a large recoil-free fraction at room temperature is barium stannate, BaSnOa [18], and this is rapidly becoming the most popular source for tin MQssbauer spectroscopy. The /-fraction is 0-6 at 293 K and 0-46 at 690 K. The source linewidth is close to the natural width [19]. A method of preparation has been detailed [19], and the material can also be used with high eflSciency in a resonant counter [18]. 

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