Gaseous atoms/ions spectra

In this simple technique, the metal to be determined, in the form of a solution of a suitable compound, is sprayed into a flame. As in atomic absorption, when the solvent evaporates in the flame, the solid obtained is atomised and a gaseous metal ion is excited to a higher electronic level. When this drops to a lower level, a line spectrum is emitted and its intensity is measured. Flame photometers rely on the use of filters to isolate the line emitted, which is detected by a photocell and its output is measured by a calibrated galvanometer. The method is applicable to 16 metals. Reliable results are only obtainable by careful control of the experimental conditions. These depend on temperature (i.e. the type and rate of flow of the flammable gas and the oxidant which is usually air), the rate of flow of the solution to the flame as well as the compound tested and solvent used. A method used to minimise the effects of these variables is to add a known constant amount of an internal standard of a compound of a metal other than the metal to be determined but with similar excitation characteristics. Ihe ratio of the intensities of the standard and the test sample is determined. A calibration plot of the logarithm of the intensity ratio and the logarithm of the concentration of the test element is drawn. The concentration of an unknown is found by interpolation of the calibration plot. Alternatively, the standard additions method as in Sec.2.4.3 is used. In all cases, allowance should be made for any dilution effects. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Figure 21-3 A portion of the emission spectrum of a steel hollow-cathode lamp, showing lines from gaseous Fe, Ni.and Cr atoms and weak lines from Cr and Fe+ ions. The monochromator resolution is 0.001 nm, which is comparable to the true linewidths.

The geometrical structure of gaseous PH2 in its X Ai ground state appears to be similar to that of ground-state PH2 (with an internuclear distance of r=1.42 A and an interbond angle of a = 92° see p. 72). This was inferred from a sharp increase of the photodetachment cross section at threshold, measured by ion cyclotron resonance [2, 3] and from the predominance of the (0, 0, 0)<-(0, 0, 0) transition in the PH2, X Bi PH, X A photoelectron spectrum [4]. r=1.34 0.05 A and a = 92 5 were taken from the isoelectronic H2S molecule (and used to calculate the thermodynamic functions of PH, see p. 109) [5]. r and a have also been theoretically calculated by several ab initio MO methods, i.e., at an MP2 [6, 7], a CEPA (coupled electron pair approximation) [8], and an HF level [9 to 15]. r was also obtained from a united-atom approximation [16] a was also calculated by a semiempirical (CNDO/2) method [17] and estimated by extended Huckel calculations [18]. 

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