Atomic absorption spectroscopy cathode lamp used

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

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. 

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. 

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... 

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. 

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. 

Photometers At a minimum, an instrument for atomic absorption spectroscopy must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for a measurement from other lines that may interfere with or diminish the sensitivity of the method. A photometer equipped with a hollow-cathode source and filters is satisfactory for measuring concentrations of the alkali metals, which have only a few widely spaced resonance lines in the visible region. A more versatile photometer is sold with readily interchangeable interference filters and lamps. A separate fdter and lamp are used for each element. Satisfactory results for the determination of 22 metals are claimed. 

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

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). 

Lithium metabolism and transport cannot be studied directly, because the lack of useful radioisotopes has limited the metabolic information available. Lithium has five isotopes, three of which have extremely short half lives (0.8,0.2, 10 s). Lithium occurs naturally as a mixture of the two stable isotopes Li (95.58%) and Li (7.42%), which may be determined using Atomic Absorption Spectroscopy, Nuclear Magnetic Resonance Spectroscopy, or Neutron Activation analysis. Under normal circumstances it is impossible to identify isotopes by using AAS, because the spectral resolution of the spectrometer is inadequate. We have previously reported the use of ISAAS in the determination of lithium pharmacokinetics. Briefly, the shift in the spectrum from Li to Li is 0.015 nm which is identical to the separation of the two lines of the spectrum. Thus, the spectrum of natural lithium is a triplet. By measuring the light absorbed from hollow cathode lamps of each lithium isotope, a series of calibration curves is constructed, and the proportion of each isotope in the sample is determined by solution of the appropriate exponential equation. By using a dual-channel atomic absorption spectrometer, the two isotopes may be determined simultaneously. - ... 

Iron in iron ores can, of course, also be analyzed by the classical redox titration with standard dichromate solution using a diphenylamine sulfonate indicator. Trace manganese in ores can also be determined using colorimetric methods or atomic absorption spectroscopy. An atomic absorption spectrophotometer, however, will cost a minimum of about 4500 and requires the periodic replacement of expensive hollow-cathode lamps. The point is that one usually has some choice of analytical methods, each with its particular advantages and disadvantages for the problem at hand. 

The energy source most frequently used in atomic absorption spectroscopy is the hollow-cathode lamp. 

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. 

In the original paper by Walsh describing the atomic absorption process he advocated use of hollow cathode sources since this source seemed to meet most of the criteria for a good source as described above. Such lamps had been developed prior to the advent of atomic absorption spectroscopy and were known to produce sharp, intense line spectra. 

Atomic absorption spectroscopy involves beaming light of an appropriate wavelength into a flame into which the sample has been sprayed, and thus contains an atomic vapour of the metal. The diminution in the intensity of the radiation is correlated with the concentration of the element. The majority of atoms in the flame remain in the ground state, so the technique is potentially more sensitive than flame photometry. The first application of atomic absorption in quantitative analysis was the determination of mercury by Hewlett in 1930, but it was not until after A. Walsh introduced the hollow cathode lamp as the light source in 1955 that the method came into general use. Today, it is the most widely used method of estimating metals in solution, but is likely to be overtaken in the future by inductively coupled plasma spectroscopy. 

Sources that emit a few discrete lines find wide use in atomic absorption spectroscopy, atomic and molecular fluorescence spectroscopy, and Raman spectroscopy (refractometry and polarimetry also use line sources). The familiar mercury and sodium vapor lamps provide a relatively few sharp lines in the ultraviolet and visible regions and are used in several spectroscopic instruments, Hollow-cathode lamps and electrodeless discharge lamps are the most important line sources for atomic absorption and fluorescence methods. Discussion of such sources is deferred to Section 9B-1. 

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