Cathode lamp, emission spectrum

Silicate, nickel, and cobalt tend to interfere in the air-acetylene flame, although nickel and cobalt are rarely present in sufficient excess to cause a problem. Silicate interference may be eliminated at modest excesses by the use of lanthanum as a releasing agent or by using a nitrous oxide-acetylene flame. Very careful optimization is sometimes necessary, for example in the analysis of freshwaters, when concentrations are very low. It is important to use a narrow spectral bandpass and to make sure that the correct line is being used, because the hollow cathode lamp emission spectrum of iron is extremely complex. If you have any doubts about monochromator calibration, check the sensitivity at adjacent lines ... 

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). 

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. 

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. 

Since the hollow-cathode lamp spectra used in AAS are relatively simple, spectral bandwidths narrower than 0.1 nm are seldom if ever used. In atomic emission analysis, however, higher resolving power is often essential, particularly when the excitation source (e.g. the nitrous oxide—acetylene flame) is producing a complex spectrum. The instrument should, therefore, provide a wide range of slit settings and a convenient digital display of the wavelength in use for the operator. 

Cathode lamps comprise an anode and a cathode in the form of a cylindrical cavity containing the element of which the emission spectrum is required. The two electrodes are sealed in a closed glass container with a quart/ window. The container is filled with a pure rare gas (neon or argon), at a pressure between 100 and 500 Pa (Fig. 2.4). 

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. 

An electric beam chopper and a tuned amplifier are incorporated into most AA instrument. Operationally, the power to the hoUow-cathode lamp is pulsed so that the light is emitted by the lamp at a certain number of pulses per second. On the other hand, aU of the light coming from the flame is continuous. When light leaves the flame, it is composed of pulsed, unabsorbed light from the lamp and a small amount of unpulsed flame spectrum and sample emission. The detector senses all light, but the amplifier is electrically tuned to accept only pulsed signals. In this way, the electronics in conjunction with the monochromator discriminate between the flame spectrum and sample emission. 

The narrow emission lines which are to be absorbed by the sample are generally provided by a hollow cathode lamp—ue, a source filled with neon or argon at a low pressure, which has a cathode made of the element being sought. Such a lamp emits only the spectrum of the desired element, together with that of the filler gas. The considerations afiFecting the design and choice of lamps will be treated later in some detail. 

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... 

Figure 22 The emission spectrum of the nickel hollow cathode lamp at 230-233 nm and 340-344 nm (Perkin Elmer Corp.)...

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... 

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. 

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