Hollow cathode lamps multi-element

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. 

While it is to be expected that the effects of these disadvantages will continue to diminish as more becomes known about electrothermal atomization, currently it can be said that if there is sufficient sample for flame or ICP analysis, and that these techniques offer sufficient sensitivity, then they should be used in preference. Plasma techniques should be used in preference to the flame if more than one analyte is to be determined. Recently a multi-element, simultaneous electrothermal instrument has been developed. These spectrometers still use a suite of hollow cathode lamps as sources. At present, a maximum of six analytes can be determined simultaneously. This area is likely to expand very rapidly, which may lead to a resurgence in the technique. If the sensitivity of a flame or ICP-AES is insufficient, and ICP-MS cannot be afforded, electrothermal atomization comes into its own, and is invaluable when either high sensitivity is required or when only small amounts of sample are available. 

Figure 13.7 Typical model of a hollow cathode lamp. The cathode is a hoUow cylinder whose central axis corresponds to that of the optical axis of the lamp. The fill gas (normally neon) is always chosen so that the spectral output of the cathode is free from interference. Right, in the hox, pictorial representation of atoms of the cathode being excited by the impact of neon ions (Ne+). Hollow Cathode lamps are available as either single element or multi-element depending on the application. This particularity of AAS renders it impractical to perform qualitative work.

The advent of the resonance detector has greatly simplified the potential problem of multi-element analysis. It is in principle possible to line up a number of resonance detectors, and, combining this with a multi-element hollow cathode lamp and a single burner, analyze a sample for a number of elements simultaneously. 

In principle, in AA analysis an individual radiation source is needed for every element to be determined. Multi-element hollow cathode lamps have... 

Hollow Cathode Lamps. The intensities of the hollow cathode lamps normally used in A AS are too low for AFS determinations. However, the intensities may be increased when higher lamp currents are periodically used. Several hollow cathode lamps can be used at the same time for simultaneous multi-element analysis provided that the operations of the lamps and the monochromator or filters are synchronized. Special hollow cathode lamps with high intensities have been developed for AFS determinations. 

Simplex optimisation of the overall response of a simultaneous multi-element flame atomic absorption spectrometer (air to fuel ratio, slit width, height above the burner head, and four hollow cathode lamp currents). Cu, Fe, Mn and Zn were measured. 

In a system for coherent forward scattering, the radiation of a primary source is led through the atom reservoir (a flame or a furnace), across which a magnetic field is applied. When the atom reservoir is placed between crossed polarizers, scattered signals for the atomic species occur on a zero background. When a line source such as a hollow cathode lamp or a laser is used, determinations of the respective elements can be performed. With a continuous source, such as a xenon lamp, and a multi-channel spectrometer, simultaneous multi-element determinations can also be performed. The method is known as coherent forward scattering atomic spectrometry [358, 359]. This approach has become particularly interesting since flexible multi-channel diode array spectrometers have become available. 

Zeeman AAS system. As the spectra contain only some resonance lines, a spectrometer with just a low spectral resolution is required. The detection limits depend considerably on the primary source and on the atom reservoir used. When using a xenon lamp as the primary source, multi-element determinations can be performed, but the power of detection will be low as the spectral radiances are low as compared with those of a hollow cathode lamp. By using high-intensity laser sources, the intensities of the signals and accordingly the power of detection can be considerably improved. Indeed, both Ip k) and Iy k) are proportional to Io(k). When furnaces are used as the atomizers, typical detection limits in the case of a xenon arc are Cd 4, Pb 0.9, T1 1.5, Fe 2.5, and Zn 50 ng [358]. They are considerably higher than in furnace AAS. 

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-... 

Atomic fluorescence is an extremely sensitive technique for determination of elements in samples. We should reiterate that in atomic fluorescence an external Ught source is used to excite the analyte atoms. An ideal light source for AFS must be much more intense than ahoUow cathode lamp to achieve improvements in sensitivity. As a result, pulsed hollow cathode lamps and lasers are frequently used in AFS measurements. Excitation with alight source such as a hollow cathode lamp, which only emits radiation specific for the element of interest, makes AFS virtually completely free from spectral interferences. In addition, AFS is like AES in that a multi-element analysis can be achieved by putting several light sources around the atom cell, as discussed below. 

As primary sources, continuous sources such as a tungsten halide or a deuterium lamp can be used. They have the advantage that multi-element determinations are possible. However, because of the low radiant densities, saturation is not obtained and the power of detection is not fully exploited. With line sources, such as hollow cathode sources and electrodeless discharge lamps, much higher radiances can be obtained. Even ICPs into which a concentrated solution is introduced can be used as a primary source, through which multi-element determinations become possible. 

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