Element Lamps

If a laboratory requires equipment to determine a large number of elements by atomic absorption, the expense involved in providing hollow cathode lamps can be substantial. As a result, attempts to construct multiple-element hollow cathode lamps have been made, with some success. One method is to construct the hollow cathode of a metal alloy thus spectra of all the elements in the alloy would be produced. Sintered hollow cathodes 

FIGURE 10-6. High-intensity hollow cathode lamp. [From J. V, Sullivan and A. Walsh, High Intensity Hollow Cathode Lamps, Spectroch/m. Acta, 21, 721 (1965). Used by permission of Pergamon Press.] 

Difficulties occur with multiple-element hollow cathodes and only certain combinations of elements have been successful. The principal source of difficulty is due to different volatilities of elements. The more easily volatilized element is preferentially vaporized and, on cooling, deposits over the entire hollow cathode surface. The intensities of emission of other metals in the hollow cathode thus are seriously reduced. 

Another approach is to introduce two or more hollow cathodes, each of a single element, into one envelope. This technique also is subject to the problem of selective volatilization but at a lower rate than the single hollow cathode design. 

The intensity of emission from multiple-element hollow cathodes is generally lower than single-element tubes, although several successful combinations are possible if proper choice of elements is made, taking into account volatilization rates and noninterfering emission spectra. For example, a multiple-element combination of magnesium, calcium, and aluminum is available, as is a combination of silver, lead, and zinc. 

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As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

Hollow-cathode lamps are currently available for over sixty elements. Several multi-element lamps have been constructed and are useful for routine determinations, but they have proved to be of doubtful performance up to now. More successful with regard to multi-element analysis have been computer controlled automated systems, which enable a programme of sequential measurements to be made with instrumental parameters being adjusted to the optimum for each element to be measured. 

Normally, a different lamp is used for each element. Multi-element lamps (e.g. Ca-Mg, Fe-Mn or Fe-Ni-Cr) are available, but are less satisfactory owing to the differing volatilities of the metals. Demountable (water-cooled) hollow-cathode lamps have also been marketed, but are not widely used. 

EDL or hollow cathode lamps are used to determine Na, K, Mg, and Ca. Single-element lamps are preferred, but multielement lamps may be used. EDLs are more intense than hollow cathode lamps, and are preferred for K and Na. When performing analyses in emission mode, no lamps are needed. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

A major expense is the purchase of the lamps needed for the number of elements to be determined. There are several multi-element lamps available, which helps to reduce this cost. 

Any emitting surface, cold-cathode, hotdevelop vapor pressures from the elements/compounds of their construction when heated. These vapors will also manifest themselves particularly during the initial start-up (breaking in) time of the lamp. Early atomic absorption, multi-element lamps were shown to have this problem. Some of these additives specifically react with residual gases and elements that might have a deleterious effect on lamp life. The electronics industry has used what are called "getter" substances in their tubes to remove some of these substances and improve emission. 

For a number of elements, lamps where the hollow cathode consists of several elements may also be used. The number of elements contained in one lamp is limited because of the risk of spectral interferences. 

A number of "multi-element" lamps are also available commercially. Various metals, in powdered form, are mixed in predetermined ratios, pressed and sintered to produce the cathode material. However, only certain combinations are practicable. The obvious advantages of this format is that fewer lamps are required and the time required to switch from one element to another is minimised, however, the intensity of the emissions are generally lower than from the corresponding single element lamps. 

The thought of building several elements into one cathode occurred to workers in atomic absorption almost immediately after equipment became available. Initial eflForts were defeated mainly by the metallurgical naivete of the experiments. However, at the present time a considerable variety of multi-elements lamps are available, although they vary in usefulness. Depending on the metals involved, the cathodes are made from alloys, intermetallic compounds, or mixtures of powders sintered together. 

The importance and value of multi-element lamps should be appraised realistically and not over-estimated. Many presently available... 

All commercial atomic absorption equipment oflFers easy interchange of lamps, usually in some sort of pre-focused mount. In single-beam instruments, where warm-up time delays are appreciable, a multi-element lamp is attractive because all its elements are ready when one is. In double-beam equipment, lamp warm-up time is not a factor. Here, the charm of multi-element lamps is confined to their lower cost, and the smaller requirement for storage. 

Not all elements can be usefully combined in multi-element lamps. Some combinations are diflBcult or impossible from a metallurgical viewpoint. More important to the user, some combinations, though feasible to manufacture, yield spectral interferences. Here, the emission lines... 

Figure 18, Emission of Perkin-Elmer 6-element lamp around iron resonance line...

A very strong Cu II line is shown only 2.5A. away. This causes problems with iron determination when the six-element lamp is used. The scan was made with the Perkin-Elmer Model 303, equipped with emission accessory. As a result of this scan, the iron-copper combination lamp was modified... 

Flame AA The major consumable supplies used in flame AA are the hollow cathode lamps. Depending on nsage, you should plan to replace three of them every year, at a cost of 400-600 for a good quality, single-element lamp. However, if a continuum source AA systan is being used, there will not be a requirement to replace lamps on a regular basis. Other minor costs are nebulizer tubing and autosampler tubes. These are relatively inexpensive, but should be planned for. The total cost... 

To a hmrted extent, atomic absorption spectrometry can also be used for multielement determinations. Several manufacturers introduced systems with multilamp turrets, where different lamps can be held under pre-heated conditions. Here, rapid switching from one lamp to another enables sequential multi-element determinations to be made by flame atomic absorption, for a maximum of around five elements. Simultaneous determinations are possible with multi-element lamps, however, the number of elements that can be brought together and used as a hollow cathode lamp with a sufficiently stable radiation output and lifetime is rather limited. The use of continuous sources facilitates flexible multi-element determinations for many elements in principle. It is necessary to use high-resolution spectrometers (e.g., echelle spectrometers) with multi-channel detection. CCDs of-... 

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