Atomic fluorescence hollow cathode lamps

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. 

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

Official methods have been published for the determination of nitric-perchloric acid-soluble lead [111] and ammonium pyrrolidine dithiocarbamate-extractable lead [ 112] in soil. Atomic absorption spectrometric evaluations of the digest or extract is conducted at the 217 nm emission line from a lead hollow cathode lamp. Rigin and Rigina [122] determined lead in soil by flameless atomic fluorescence using electrolytic preconcentration. The limit of detection is 15 pg lead and the standard deviation is not greater than 0.04. 

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

Radiation absorbed by atoms under conditions used in atomic absorption spectrometry may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have absorbed the primary radiation and is emitted 1n all directions. It may be monitored in any direction other than in a direct line with radiation from the hollow-cathode lamp which ensures that tha detector will not respond to the primury absorption process nor to unabsorbed radiation from the lamp. The intensity of fluorescent emission is directly proportional to the concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other species within the flame, a process known as quenching. Nitrogen and hydrocarbons enhance quenching, and flames incorporating either should be avoided or their effect modified by dilution with argon. 

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. 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

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... 

In the early work on atomic fluorescence, conventional hollow-cathode lamps often served as escitalion sources. To enhance the output inicnsity without destroying the lamp, it was necessary to operate the lamp with short pulses of current that were greater than the lamp could tolerate for continuous operation. The detector was gated to observe the fluorescence signal only during pulses of source radiation. 

Perhaps the most widely used sources for atomic fluorescence have been the EDLs (Section 9B-1), which usually produce radiant jnlensiljes greater than those of hollow-cathode lamps by an order of magnitude or two. EDLs have been operated in both the continuous and pulsed modes. Unfortunately, this type of lamp is not available for many elements. 

In theory, no monochromator or lilter should be necessary for alomic fluorescence measurements when an LDI. or hollow-cathode lamp serves as the excitation source becau.se the emitted radiation is, in principle, that of a single element and will thus excite onlv atoms of that element. A nondispersive system then could be... 

The atomic absorption method for determining the concentration of metallic elements has now gained wide acceptance. Instrumentation is relatively inexpensive and simple to use. Analytical interferences are less prevalent than with most other techniques means of recognizing and combating the interferences that do exist are described. The article discusses the basic principles of atomic absorption and also describes the fundamental design and modern improvements in the major components of instrumentation hollow-cathode lamps, burners, photometers, and monochromators. Atomic absorption is compared with some of its rival techniques, principally flame emission and atomic fluorescence. New methods of sampling and the distinction between sensitivity and detection limit are discussed briefly. Detection limits for 65 elements are tabulated. 

If the resonance detector is well-designed, the vast majority of the magnesium atoms are unexcited. The resonance lines from the magnesium hollow cathode lamp will cause the magnesium atoms in the resonance detector to fluoresce. Some of this fluorescence will fall on a photomultiplier detector placed at right angles to the optical path. The intensity of fluorescence is proportional to the intensity of emission. Non-resonant lines from the lamp or from the flame will have no effect on the resonance detector. Therefore, a system of narrow bandwidth is produced without the requirement of a monochromator. 

The first commercial plasma atomic fluorescence spectrometer was developed by Demers and Allemand. Hollow cathode lamps are used as radiation sources and an inductively coupled plasma torch as an atomizer. Detection limits are reported for more than 30 elements. The linear dynamic range is normally 10 to 10. ... 

Spectrophotometric techniques have been the basis of many coal analysis methods. One of the most widely used techniques for analysis of trace elements is atomic absorption spectrometry, in which the standards and samples are aspirated into a flame. A hollow cathode lamp provides a source of radiation that is characteristic of the element of interest and the absorption of characteristic energy by the atoms of a particular element. X-ray fluorescence is also employed as a quantitative technique for trace element determination and depends on election of orbital electrons from atoms of the element when the sample is irradiated by an x-ray source. 

Hollow cathode lamps of the type commonly used in atomic absorption do not have sufficient intensity to be useful for atomic fluorescence. Increasing the current to a hollow cathode lamp is not sufficient for atomic fluorescence since very high currents may actually result in decreased emission intensity due to a high degree of self-absorption. The lifetime of the lamp also is reduced when high currents are used. 

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. 

In many instrumental analysis methods the instrument response is proportional to the analyte concentration over substantial concentration ranges. The simplified calculations that result encourage analysts to take significant experimental precautions to achieve such linearity. Examples of such precautions include the control of the emission line width of a hollow-cathode lamp in atomic absorption spectrometry, and the size and positioning of the sample cell to minimize inner filter artefacts in molecular fluorescence spectrometry. However, many analytical methods (e.g. immunoassays and similar competitive binding assays) produce calibration plots that are intrinsically curved. Particularly common is the situation where the calibration plot is linear (or approximately so) at low analyte concentrations, but becomes curved at higher analyte levels. When curved calibration plots are obtained we still need answers to the questions listed in Section 5.2, but those questions will pose rather more formidable statistical problems than occur in linear calibration experiments. 

In atomic fluorescence spectrometry (AFS), the analyte is introduced into an atomizer (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (hollow cathode lamp, electrodeless discharge lamp, or tuned laser). Subsequently, the fluorescence radiation, which may be of the same wavelength (resonance fluorescence) or of longer wavelength (nonresonance fluorescence), is measured. 

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. 

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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