Cathode lamp, hollow

Two radiation sources are commonly used in commercial AAS instruments, the hollow cathode lamp (HCL) and the electrodeless discharge lamp (EDL). Both types of lamps are operated to provide as much intensity as possible while avoiding line-broadening problems caused by the collision processes described earlier. 

Such a narrow line would be detectable only under extremely high resolution (i.e., very narrow bandpass), which is not encountered in commercial AAS equipment. 

The HCL emits narrow, intense lines from the element that forms the cathode. Applying a high voltage across the anode and cathode creates this emission spectrum. Atoms of the filler gas become ionized at the anode and are attracted and accelerated toward the cathode. The fast-moving ions strike the surface of the cathode and physically dislodge some of the surface metal atoms (a process called sputtering ). The displaced atoms are excited by collision with electrons and emit the characteristic atomic emission spectrum of the metal used to make the cathode. The process is 

Each hollow cathode emits the spectrum of metal used in the cathode. For this reason, a different HCL must be used for each different element to be determined. This is an inconvenience in practice and is the primary factor that makes AAS a technique for determining only one element at a time. The handicap is more than offset, however, by the advantage of the narrowness of the spectral lines and the specificity that results from these narrow Unes. 

Single-element HCLs cost between 400 and 700 per lamp, while multielanent lamps cost between 500 and 700 each for most elanents. Particularly rare elements like Rh, Ir, and Os may cost 1000- 2000 per lamp. 

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The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

Atomic Fluorescence System - Millennium Excalibur PSA 10.055 -was used in our work. This system consists of the autosampler, the integrated continuous flow vapour generator and the atomic fluorescence spectrometer with the boosted dischar ge hollow cathode lamp and a control computer. 

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. 

Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected resonance line with lines emitted by some other element this arises because in practice a chosen line has in fact a finite bandwidth . Since in fact the line width of an absorption line is about 0.005 nm, only a few cases of spectral overlap between the emitted lines of a hollow cathode lamp and the absorption lines of metal atoms in flames have been reported. Table 21.3 includes some typical examples of spectral interferences which have been observed.47-50 However, most of these data relate to relatively minor resonance lines and the only interferences which occur with preferred resonance lines are with copper where europium at a concentration of about 150mgL 1 would interfere, and mercury where concentrations of cobalt higher than 200 mg L 1 would cause interference. 

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. 

It should have a lamp turret capable of holding at least four hollow cathode lamps with an independent current stabilised supply to each lamp. 

Double-beam AA spectrophotometers are still marketed by instrument manufacturers. A double-beam system compensates for changes in lamp intensity and may require less frequent re-zeroing than a single-beam instrument. These considerations had more merit some years ago when hollow cathode lamps suffered from some instability. It should be noted, however, that the optical... 

Never view the flame or hollow cathode lamps directly protective eye wear should always be worn. Safety spectacles will usually provide adequate protection from ultraviolet light, and will also provide protection for the eyes in the event of the apparatus being shattered by an explosion. 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

FPD Flame photometric detector HC(L) Hollow cathode (lamp)... 

Fig. 14.1 (a) Red shift of Cr emission line peaks as a function of Ar bath gas pressure at 3,230 K. The three curves correspond to the three peaks of the triplet centred on 27,820 cm-1, (b) Corrected MBSL spectra (orange) and Cr emission from a hollow cathode lamp at low pressure (blue). Relative red shifts for each peak are indicated [11] (reprinted with permission from Annual Reviews)... 

By changing excitation source (e.g., hollow cathode lamps in AAS)... 

The most commonly used sharp line source is the hollow cathode lamp. 

Glow discharge lamp (analogous to hollow cathode lamp) in which the sample acts as the cathode. Attached to a standard atomic emission spectrometer. 

Sources emitting radiation characteristic of element of interest (hollow-cathode lamp). Flame or electrically heated furnace or carbon rod. Monochromator, photomultiplier, recorder. 

Ideally, the emission line used should have a half-width less than that of the corresponding absorption line otherwise equation (8.4) will be invalidated. The most suitable and widely used source which fulfils this requirement is the hollow-cathode lamp, although interest has also been shown in microwave-excited electrodeless discharge tubes. Both sources produce emission lines whose halfwidths are considerably less than absorption lines observed in flames because Doppler broadening in the former is less and there is negligible collisional broadening. 

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