Hollow cathode lamps spectra from

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. 

Radiation is derived from a sealed quartz tube containing a few milligrams of an element or a volatile compound and neon or argon at low pressure. The discharge is produced by a microwave source via a waveguide cavity or using RF induction. The emission spectrum of the element concerned contains only the most prominent resonance lines and with intensities up to one hundred times those derived from a hollow-cathode lamp. However, the reliability of such sources has been questioned and the only ones which are currently considered successful are those for arsenic, antimony, bismuth, selenium and tellurium using RF excitation. Fortunately, these are the elements for which hollow-cathode lamps are the least successful. 

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). 

Figure 14.12 —Schematic of an instrument showing deuterium lamp background correction. Perkin Elmer, model 3300 with a Littrow-type monochromator. This double beam assembly includes a deuterium lamp whose continuum spectrum is superimposed, with the aid of semitransparent mirrors, on the lines emitted by the hollow cathode lamp. One beam path goes through the flame while the other is a reference path. The instrument measures the ratio of transmitted intensities from both beams. The correction domain is limited to the spectral range of the deuterium lamp, which is from 200-350 nm. (Reproduced by permission of Perkin Elmer.)...

Figure 21-3 A portion of the emission spectrum of a steel hollow-cathode lamp, showing lines from gaseous Fe, Ni.and Cr atoms and weak lines from Cr and Fe+ ions. The monochromator resolution is 0.001 nm, which is comparable to the true linewidths.

Figure 7. (a) Emission spectrum from Li hollow cathode lamp [entrance slit 29] (b) Emission spectrum from multielement hollow cathode lamp containing Cr and Mn [entrance slit 10] (c) Composite spectrum obtained by plugging fiber-optic strands into entrance slits 10 and 29 simultaneously. 

Wavelength accuracy. In order to evaluate the ability of each system to locate spectral lines, a preliminary wavelength calibration was carred out with the emission spectrum of a mercury pen lamp and then the peak maxima of several atomic lines from an iron hollow cathode lamp were located. The root mean square (RMS) prediction error, which is the difference between the predicted and the observed location of a line, for the vidicon detector system was 1.4 DAC steps. Because it is known from system calibration data that one DAC increment corresponds to 0.0125 mm, the absolute error in position prediction is 0.018 mm. For the image dissector, the RMS prediction error was 7.6 DAC steps, and because one DAC step for this system corresponds to 0.0055 mm, the absolute error in the predicted coordinate is 0.042 mm. The data in Table II represent a comparison of the wavelength position prediction errors for the two detectors. 

As explained in Chapter 1, section 7, unless a very high resolution monochromator, e.g. an echelle monochromator, is used to isolate a very narrow (< ca. 0.005 nm) band of light from a continuum spectrum prior to absorbance measurement, the sensitivity will be very poor.1,2 Although there are occasional reports of analysis by flame AAS using continuum sources such as xenon arc lamps, these are invariably from research laboratories. The vast majority of reported applications use single element line sources, and more than 99% of these applications use hollow cathode lamps. 

A hollow cathode lamp emits an intense line spectrum of the cathode element, of any other element present in the cathode, and of the filler gas (neon or argon). It is therefore necessary to be able to isolate the lines of the determinant element from any other emitted lines. If we do not, the difference between 7t and /0 will be greatly reduced, and the sensitivity unacceptably poor. Moreover, not all lines of the determinant element give equal sensitivity, and it is therefore also desirable to isolate the determinant line at the wavelength which gives the most useful sensitivity from all other lines. This is done with a grating monochromator. Figure 6 illustrates a typical optical layout in the monochromator of an atomic absorption spectrometer. 

The methods range from simple, inexpensive absorption spectroscopy to sophisticated tunable-laser-excited fluorescence and ionization spectroscopies. AAS has been used routinely for uranium and thorium determinations (see for example Pollard et al., 1986). The technique is based on the measurement of absorption of light by the sample. The incident light is normally the emission spectrum of the element of interest, generated in a hollow-cathode lamp. For isotopes with a shorter half life than and Th, this requires construction of a hollow-cathode lamp with significant quantities of radioactive material. Measurement of technetium has been demonstrated in this way by Pollard et al. (1986). Lawrenz and Niemax (1989) have demonstrated that tunable lasers can be used to replace hollow-cathode lamps. This avoids the safety problems involved in the construction and use of active hollow-cathode lamps. Tunable semiconductor lasers were used as these are low-cost devices. They do not, however, provide complete coverage of the spectral range useful for AAS and the method has, so far, only been demonstrated for a few elements, none of which were radionuclides. 

Figure 1 is a sketch of the atomic absorption process. In lA, the emission spectrum of a hollow-cathode lamp is shown, with emission lines whose half-width is typically about 0.02 A. For most practical purposes, the desired element in the sample can be considered as being able to absorb only the "resonance lines, whose wavelengths correspond to transitions from the minimum energy state to some higher level. In IB, the sample is shown to absorb an amount "x which corresponds to the concentration of the element of interest. As seen in Figure 1C, after the flame, the resonance line is reduced while the others are unaflFected. In order to screen out the undesired emission, the radiation is now passed through a filter or monochromator (ID) which is tuned to pass the line... 

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. - ... 

The ET AAS technique (see Fig. 5.2) is based on fast evaporation of samples to be analysed in a miniature tube furnace (6-8 mm in diameter and 20-30 mm in length) made of graphite [5]. A light beam from the source of a line spectrum (usually a hollow cathode lamp) passes through this tube and the value of the light absorption by free atoms of analyte is measured. A grating monochromator is used to separate the most sensitive resonance line from the atomic spectrum of the element emitted by the light source. 

The principle function of a continuum source background corrector is depicted in Figure 76. The exit slit of the monochromator separates the resonance line of the analyte (half-width about 0.002 nm) from the emission spectrum of the line-like radiation source, and a band of radiation from the continuum spectrum of the deuterium lamp equivalent to the bandpass of the slit (usually 0.2 to 0.7 nm). The intensity of the hollow cathode lamp (/hd) is equalized to the intensity of the deuterium lamp (/ i) before the determination. When the ratio /di//hd = 1, no reading shows on the display. When a... 

Figure 10-3 shows the basic features of a hollow cathode lamp source. Here A is the anode (the plus electrode) and C is the cathode, terminated in the lamp as a hollow cup. The anode can be a wire, such as tungsten, and the cathode cup may be constructed from the element whose spectrum is desired or it may be an inert material into which the desired element or a salt of the desired element is placed. The lamp envelope is made of glass and IT is a window of suitable properties. If an ultraviolet line spectrum is desired, the window may be quartz or a high silica glass. The hollow cathode has an inert gas present, usually neon or argon, at low pressure. 

Several examples of possible spectral line interferences include sodium at 2852.8 A with magnesium at 2852.1 A, iron at 3247.3 A with copper at 3247.3 A, and iron at 3524.3 A with nickel at 3524.5 A. Spectral interferences also are possible from hollow cathode lamps. The fill-gas of a hollow cathode lamp is commonly argon or neon and the lamps emit the line spectrum of the fill-gas as well as that of the hollow cathode material. The fill-gas therefore must be one that does not produce an emission line at the desired wavelength of the hollow cathode element. 

Fig. 1.44 Optogalvanic spectrum of a uranium hollow-cathode lamp filled with argon buffer gas. In the upper spectrum (a) taken at 7 mA discharge current, most of the lines are argon transitions, while in the lower spectrum (b) at 20 mA many more uranium lines appear, because of sputtering of uranium from the hollow cathode walls [125]...

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