High-intensity hollow cathode lamps

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... 

Lowe, R.M. 1971. High-intensity hollow-cathode lamp for atomic fluorescence. Spectrochim. Acta B 26 201-205. 

Since the foundations of atomic absorption spectroscopy were laid by Walsh a number of improvements in instrumentation and techniques have been made. Russell, Shelton, and Walsh modulated the hollow cathode signal and used an amplifier tuned to the modulating frequency so measurements could be made without interference from flame emission. Sullivan and Walsh developed very high-intensity hollow cathode lamps that led to lower detection limits. Willis proposed the use of nitrous oxide-acetylene flame as a means of overcoming certain interferences and produce a higher population of free atoms in the flame. 

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.]... 

Some of the more recently developed high intensity hollow cathode lamps are useful. Sullivan and Walsh developed such lamps but they require two power supplies since two sets of independently controlled electrodes are required. One set of electrodes controls the sputtering action and the second set controls the excitation process. These lamps have been used to a limited extent in atomic fluorescence. 

Hollow cathode lamps A HCL is composed of a silica envelope that contains l-5Torr of argon or neon and two metal electrodes. HCLs are almost ideal line sources for AAS because of their high stability and narrow linewidth (0.002 nm), but their relatively low intensity is a disadvantage for AES. High-intensity hollow cathode lamps (HI-HCLs) provide increased intensity by use of an additional electrode to separate the atomization and excitation processes. The irradiance of the HI-HCLs is a factor of 20-100 times greater than that of conventional HCLs, and provides better sensitivity for AES. 

We have seen the relationship between absorption spectrophotometry and spectrofluorometry. A similar relationship exists between atomic absorption spectrophotometry and atomic fluorescence spectrophotometry. In atomic fluorescence, the flame retains its role as a source of atoms these atoms, however, are excited by an intense source of radiation and their fluorescent emission is assayed at an angle of 90° in a manner similar to that of spectrofluorimetry. Lack of sufficiently intense source for many elements has been the limitation of this technique, however, with time instrumental developments are overcoming this problem. High intensity hollow-cathode lamps, or xenon or mercury discharge lamps are used. 

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. 

Deuterium arc background correction. This system uses two lamps, a high-intensity deuterium arc lamp producing an emission continuum over a wide wavelength range and the hollow cathode lamp of the element to be determined. 

High intensity is not a source requirement in AAS and therefore electrodeless discharge lamps will not replace hollow-cathode lamps. However, for those elements that produce poor hollow-cathode lamps (notably arsenic... 

By far the most common lamps used in AAS emit narrow-line spectra of the element of interest. They are the hollow-cathode lamp (HCL) and the electrodeless discharge lamp (EDL). The HCL is a bright and stable line emission source commercially available for most elements. However, for some volatile elements such as As, Hg and Se, where low emission intensity and short lamp lifetimes are commonplace, EDLs are used. Boosted HCLs aimed at increasing the output from the HCL are also commercially available. Emerging alternative sources, such as diode lasers [1] or the combination of a high-intensity source emitting a continuum (a xenon short-arc lamp) and a high-resolution spectrometer with a multichannel detector [2], are also of interest. 

Brief mention should also be made here of high intensity (also known as boosted output ) hollow cathode lamps.7 In these lamps an auxiliary current of around 200-400 mA is applied to the dilute cloud of atoms sputtered outside the central zone of the normal hollow cathode. The atoms are thus excited and emit intense radiation which may be used in AAS or AFS. Once again an auxiliary power supply is required, and the lamps themselves are more complex and correspondingly more expensive. Such lamps have had a rather chequered history, finding great favour in some environmental analytical laboratories but never being widely used on any routine basis. 

While conventional or high-intensity (boosted output)4 hollow cathode lamps are usually simply operated at room temperature, electrodeless discharge lamps are sometimes cooled with a regulated flow of air maintained at a constant temperature,5 and this flow too must be optimized with respect to signal-to-noise ratio. Sometimes these sources are operated in a vacuum jacket to enhance sensitivity and/or to improve stability.6... 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

The use of hollow cathode lamps is sufficient for the majority of elements. The low intensity and short service life of these lamps when used for volatile elements such as As, Te, Se, Bi, etc. may, however, be a problem when determining traces of these elements. Electrodcless lamps or high intensity lamps (with auxiliary hot cathode) can be used in these cases to enhance the quality of analysis of these elements. 

The stability of the wavelength setting of a monochromator can be a problem in high resolution spectrometry. This difficulty has been overcome by the use of the resonance monochromator (S24), consisting of a hollow cathode lamp modified to produce only an atomic vapor. The vapor is irradiated with the light to be analyzed and fluorescence occurs at the resonant wavelength of the cathode element. The intensity of the fluorescence is proportional to the component of that wavelength in the primary radiation. 

A widely used photometer used as a high-pressure liquid chromatographic (HPLC) detector uses the intense 254-nm resonance line produced by a mercury arc lamp (see Chapter 6). Others employ a miniature hollow cathode lamp as a very-narrow-wavelength intense source. For example, a zinc hollow cathode lamp gives a line at 214nm that is adequately close to the maximum wavelength of peptide bond absorption (206 nm) so that it can be used to measure peptides and proteins. Details on the hollow cathode lamp are found in the section on Atomic Absorption Spectrophotometry. The hollow cathode lamp also has a long, useful Hfetime if a lower-current, nonpulsed power supply is used. j... 

Hollow-cathode lamps for about 70 elements are available from commercial sources. For certain elements, high-intensity lamps are available. These provide an intensity that is about an order of magnitude higher than that of normal lamps. Some hollow-cathode lamps are fitted with a cathode containing more than one element such lamps provide spectral lines for the determination of several species. The development of the hollow-cathode lamp is widely regarded as the single most important event in the evolution of atomic absorption spectroscopy. 

Coherent forward scattering (CFS) atomic spectrometry is a multielement method. The instrumentation required is simple and consists of the same components as a 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, multielement 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, T11.5, Fe 2.5 and Zn 50 ng [309]. They are considerably higher than in furnace AAS. 

Excitation sources used in atomic absorption spectroscopy are usually hollow cathode lamps or electrodeless discharge tubes, both of which produce high-intensity line excitation. Continuum sources, which emit a continuous level of energy over a large spectral region, are also used, though less frequently, The choice of the spectral source will affect the sensitivity and linearity of the analysis (5,30). 

---