Atomic absorption spectrophotometry hollow cathode lamp

One often unsuspected source of error can arise from interference by the substances originating in the sample which are present in addition to the analyte, and which are collectively termed the matrix. The matrix components could enhance, diminish or have no effect on the measured reading, when present within the normal range of concentrations. Atomic absorption spectrophotometry is particularly susceptible to this type of interference, especially with electrothermal atomization. Flame AAS may also be affected by the flame emission or absorption spectrum, even using ac modulated hollow cathode lamp emission and detection (Faithfull, 1971b, 1975). 

Because each element absorbs at very discrete wavelengths, the lamp used for analysis of a particular metal emits light only at the desired wavelengths and is specific for that element. The two kinds of lamps used in atomic absorption spectrophotometry are the hollow cathode lamp (HCL) and the electrodeless discharge... 

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... 

The determination of technetium by atomic absorption spectrophotometry was studied with a Tc hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution was 3-10 g/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet. Cationic interferences were eliminated by adding aluminum to the sample solutions. The applicability of atomic absorption spectrophotometry to the determination of technetium in uranium and a uranium alloy was demonstrated [42]. A detection limit of 6 10 g w as achieved for measuring technetium by graphite furnace atomic absorption spectrometry. In using the same doublet and both argon and neon as fill gases for the lamp, 6-10 to 3 10 g of technetium was found to be the range of applicability [43]. 

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. 

In this technique, a liquid sample is nebulized in a cloud chamber and passed into a flame, where the element is dissociated by the heat from its chemical bonds and placed in an unexcited or ground state. Narrow wavelength light from a hollow cathode lamp is passed through the flame and some of the ground state atoms are excited by the radiation. This results in a net decrease in the intensity of the beam and this can be measured by a photoelectric detector. The process is therefore analagous to absorption spectrophotometry for the measurement of molecules. 

Light containing only a narrow part of the spectrum. It can be produced from a hollow cathode lamp as in atomic absorption spectrophotometry or by the use of diffraction gratings, prisms and filters to isolate a specific spectral region from a tungsten, hydrogen or other lamp. 

In atomic absorption spectrophotometry, a hollow cathode lamp is used which emits the characteristic line spectrum of the cathode metal. The light from the lamp passes through an atomised mist of the gaseous element and a line of the emitted spectrum (die resonance line) is absorbed. A monochromator then allows this line alone to reach the detector and the narrow absorption band is recorded and/or displayed. Because atoms have no rotational or vibrational levels, transitions from one electronic level to another produces narrow absorption or emission lines. Doppler and pressure broadening vary from 0.01-0.00 Inm. 

A basic set-up for atomic emission, as well as absorption, spectrophotometry is shown in Fig. 6.59. In the absorption mode it is necessary to modulate the output of the lamp, either by modulating the discharge current or by using a mechanical chopper. The signal at the PMT will then consist of a dc contribution due to the thermally excited atoms emitting the line studied and an ac contribution due to the transmitted hollow-cathode... 

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