Hollow cathode lamps design

Source of Radiation As the sensitivity in AFS is directly proportional to the source intensity, intense LSs are needed. Typical HCLs are insufficient to guarantee a high intensity of excitation radiation the previously described EDLs, however, can do so. High-intensity hollow cathode lamps (HI-HCL), first designed by Sullivan and Walsh, are commonly used in modern atomic... 

Modulation can be accomplished by interposing a motor-driven circular chopper between the source and the flame, as shown in Figure 28-18. Segments of the metal chopper have been removed so that radiation passes through the device half the time and is reflected the other half. Rotation of the chopper at a constant speed causes the beam reaching the flame to vary periodically from zero intensity to some maximum intensity and then back to zero. As an alternative, the power supply for the source can be designed to pulse the hollow-cathode lamps in an alternating manner. 

The radiation source in the case of this flame AAS is a line source hollow cathode lamp (HCL). These are the most commonly used sources in AAS. It can be designed for... 

The atomic absorption method for determining the concentration of metallic elements has now gained wide acceptance. Instrumentation is relatively inexpensive and simple to use. Analytical interferences are less prevalent than with most other techniques means of recognizing and combating the interferences that do exist are described. The article discusses the basic principles of atomic absorption and also describes the fundamental design and modern improvements in the major components of instrumentation hollow-cathode lamps, burners, photometers, and monochromators. Atomic absorption is compared with some of its rival techniques, principally flame emission and atomic fluorescence. New methods of sampling and the distinction between sensitivity and detection limit are discussed briefly. Detection limits for 65 elements are tabulated. 

The narrow emission lines which are to be absorbed by the sample are generally provided by a hollow cathode lamp—ue, a source filled with neon or argon at a low pressure, which has a cathode made of the element being sought. Such a lamp emits only the spectrum of the desired element, together with that of the filler gas. The considerations afiFecting the design and choice of lamps will be treated later in some detail. 

An unsought element in the sample absorbs light from the same element present by accident or improper design in the hollow-cathode lamp. This problem limits the number and type of elements that can be combined in one lamp responsible manufacturers do not generally design lamps which may produce spectral interferences. 

Recently (about mid-1967), some important improvements were made in the design of hollow cathode lamps, yielding units with longer life, higher emission, and greater spectral purity than had been available previously. The emission stability has also been enhanced, and the warmup time shortened. Basically, four changes were responsible for these improvements. 

If the resonance detector is well-designed, the vast majority of the magnesium atoms are unexcited. The resonance lines from the magnesium hollow cathode lamp will cause the magnesium atoms in the resonance detector to fluoresce. Some of this fluorescence will fall on a photomultiplier detector placed at right angles to the optical path. The intensity of fluorescence is proportional to the intensity of emission. Non-resonant lines from the lamp or from the flame will have no effect on the resonance detector. Therefore, a system of narrow bandwidth is produced without the requirement of a monochromator. 

As hollow cathode lamps, instruments, and burner designs are improved, more and more elements produce straight working curves over a useful analytical range. Equipment has become available which reads out linearly in concentration, with very much less complexity than was formerly required. In general, a logarithmic conversion is made electronically inside the instrument, and the readout takes place on a meter scale or recorder. Figure 28, for example, shows direct linear measurement of calcium concentration made on a relatively simple atomic absorption instrument. 

The half width of elemental lines is of the order of 0.002 nm when observed by emission spectroscopy with flame or electrothermal atomisation. A number of reasons can cause broadening of the linewidth, of which the most important and best understood are natural, pressure, resonance, and Doppler broadening. If a stable and sensitive detection is to be achieved, the linewidth of the excitation radiation must be narrower than the full width at half maximum (FWHM) of the analyte line. Under these conditions, the entire radiant energy produced by the excitation source will be available for absorption by the analyte. The typical line sources used for atomic absorption are element specific excitation sources such as the hollow cathode lamp or the electrodeless discharge lamp. But even continuum sources can be used with appropriate instrumental designs. 

The radiation sources used in atomic absorption are hollow cathode lamps or electrodeless discharge lamps. Although in most cases a lamp needs to be changed for each element of interest, some are designed to be used for several elements (up to seven). 

Many of the virtues of atomic absorption stem from the fact that the desired element absorbs only at very narrow lines, with a half-width of the order of 0.03A. Instrumental design is therefore greatly simphfled if the emission sources emit lines no wider than this, and narrower if possible. Discharge lamps and hollow cathodes have been found to fill the requirements, and particularly the latter have been developed to a considerable level of sophistication. 

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