Slits, atomic spectroscopy

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

Temperature Profiles. Figure 9 3 shows a temperature profile of a typical flame for atomic spectroscopy. The maximum temperature is located in the flame about 2.. i cm above the primary combustion zone, it is important — particularly tor emission methods (Section lOC-I)—to focus the same part of the llanie on the entrance slit for all calibrations and analytical measurements. 

M HNO3. The concentration of Cu and Zn in the diluted supernatant is determined by atomic absorption spectroscopy using an air-acetylene flame and external standards. Copper is analyzed at a wavelength of 324.8 nm with a slit width of 0.5 nm, and zinc is analyzed at 213.9 nm with a slit width of 1.0 nm. Background correction is used for zinc. Results are reported as micrograms of Cu or Zn per gram of FFDT. 

An atomic site in a rare-gas matrix is, of course, of microscopic size. However, atoms in a beam can be excited by laser spectroscopy to extremely high n values if such a beam is pointed at a metallic grating made up of micrometre-size slits, then a cut-off will be observed in the maximum n value of the atoms which can fly through the apertures unimpeded. Such an experiment has been reported by Fabre [37] and coworkers in Paris. While it is experimentally very challenging to realise, the principle of this... 

Inconspicuous instrumental, environmental, or chemical effects often cause a loss of instrument response. In atomic emission spectroscopy, for example, sensitivity is affected by such instrumental factors as flame temperature, aspiration rate, and slit width. In amperometric measurements, diffusion currents vary with temperature, and a significant loss in sensitivity may occur with a drop in sample temperature. In ion-selective electrode measurements, sensitivity may be affected by chemical effects, such as changes in ionic strength or pH. 

Multielement atomic absorption spectroscopy is complicated by the need for a multielement emission source. Some multielement hollow cathode lamps are available and continuum sources are possibilities. Another method is to place several hollow cathode lamps along the focal plane of a spectrometer, at positions corresponding to the desired wavelengths, thus permitting all the radiation to emerge from a single slit. In this approach the... 

The behavior of silver, which is not easily oxidized, is quite different as shown in Figure 9-4, a continuous increase in the number of atoms, and thus the absorbance, is observed from the base to the periphery of the flame. By contrast, chromium, which forms very stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip this observation suggests that oxide formation predominates from the start. The.se observations suggest that a different portion of the flame should be used for the determination of each of these elements. The more sophisticated instruments for flame spectroscopy are eqnipped with monochromators that. sample the radiation from a relatively small region of the flame, and so a critical step in the optimization of signal output is the adjustment of the position of the flame with respect to the entrance slit. 

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