Atomic spectroscopy hollow-cathode lamp

Line source In atomic spectroscopy, a radiation source that emits sharp atomic lines characteristic of the analyte atoms see hollow-cathode lamp and electrodeless discharge lamp. 

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. 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

The most widely used spectral line source for atomic absorption spectroscopy is the hollow cathode lamp. An illustration of this lamp is shown in Figure 9.5. The internal atoms mentioned above are contained in a cathode, a negative electrode. This cathode is a hollowed cup, pictured with a C shape in the figure. The internal excitation and emission process occurs inside this cup when the lamp is on and the anode (positive electrode) and cathode are connected to a high voltage. The light is emitted as shown. 

EDL stands for electrodeless discharge lamp. It is an alternative to the hollow cathode lamp as a light source in atomic absorption spectroscopy. 

Hollow cathode lamps produce monchromatic radiation for atomic absorption spectroscopy. 

The technique of atomic absorption spectroscopy needs a source of monochromatic radiation such as a hollow cathode lamp BECAUSE... 

The presence and concentration of various metallic elements in petroleum coke are major factors in the suitability of the coke for various uses. In the test method (ASTM D5056), a sample of petroleum coke is ashed (thermally decomposed to leave only the ash of the inorganic constituents) at 525°C (977°F). The ash is fused with lithium tetraborate or lithium metaborate. The melt is then dissolved in dilute hydrochloric acid and the resulting solution is analyzed by atomic absorption spectroscopy to determine the metals in the sample. However, spectral interferences may occur when using wavelengths other than those recommended for analysis or when using multielement hollow cathode lamps. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

Determination with atomic absorption spectroscopy with the use of an acetylene-air flame and hollow-cathode lamp, e.g. 

Hollow cathode discharges are perhaps the most common glow discharges used in analytical chemistry. Most spectroscopists are familiar with these devices as hollow cathode lamps used for atomic absorption spectroscopy. Figure 2.10 contains... 

S. Carol Ed., Improved Hollow Cathode Lamps for Atomic Spectroscopy, Ellis Horwood Ltd, Chichester, England (1985). 

What is the primary advantage of a hollow-cathode lamp used in atomic absorption spectroscopy ... 

If the light comes from a source made from zinc it contains a very high proportion of wavelengths that are absorbed by zinc atoms. In atomic absorption spectroscopy the light source used is a hollow cathode lamp, specially made for each element to be determined. Measurement of absorbance of the light from a zinc hollow cathode lamp gives a very selective method for the measurement of the concentration of zinc in a solution introduced into the flame (Figure 6.1). 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

Hollow-cathode lamps made atomic absorption spectroscopy practical. 

The most useful radiation source for atomic absorption spectroscopy is the hollow-cathode lamp, shown schematically in Figure 28-17. It consists of a tungsten anode and a cylindrical cathode sealed in a glass tube containing an inert gas, such as argon, at a pressure of 1 to 5 torn The cathode either is fabricated from the analyte metal or serves as a support for a coating of that metal. 

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. 

In atomic emission spectroscopy, the radiation source is the sample itself. The energy for excitation of analyte atoms is supplied by a plasma, a flame, an oven, or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. In atomic absorption spectroscopy, the radiation source is usually a line source such as a hollow cathode lamp, and the signal is the absorbance. The latter is calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomized sample. 

Hollow-cathode lamp A source used in atomic absorption spectroscopy that emits sharp lines for a single element or sometimes for several elements. 

Caroli S. (1985) Improved hollow cathode lamps for atomic spectroscopy, Ellis Horwood, Chichester. 

Figure 11.8 Schematic diagram of a hollow-cathode lamp used in atomic absorption spectroscopy. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.

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). 

Line sources for atomic fluorescence spectroscopy can be the hollow cathode lamps or electrodeless discharge lamps discussed previously. The source should have the highest possible output intensity since, as in molecular fluorescence spectroscopy, the intensity of fluorescence is directly proportional to the... 

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