Hollow cathode source

The absorption spectra of Zr atoms isolated in a variety of matrices have been reported. In addition, the diatomic molecule ZrN, prepared using a hollow cathode source and Na, was observed. Other work involving Na included the identification of ThN and Th(Na), and TaN in various matrices. 

This type of interference normally takes place when the absorption of an interfering species either overlaps or lies veiy near to the analyte absorption, with the result that resolution by the monochromator almost becomes impossible, Hollow-cathode-source invariably give rise to extremely narrow emission-lines, hence interference caused due to overlap of atomic spectral lines is rather rare. 

Use of glow-discharge and the related, but geometrically distinct, hollow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

The hollow cathode source in which the plasma is contained inside the cathode (advantage very high temperature). 

The temperature in a Scandinavian source is, for example, around 1000°C. If this is not enough for certain elements, one may use a hollow cathode source which operates at 1500 - 2000°C or even a special high-temperature source using electron bombardment for the vaporization of the material. In that way one may achieve temperatures as high as 3000°... 

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. 

Fig. 78. Hollow cathode source for atomic absorption spectrometry, a Hollow cathode b anode c mica isolation d current supply e window (usually quartz).

As primary sources, continuous sources such as a tungsten halogenide or a deuterium lamp can be used. They have the advantage that multielement determinations are possible. However, because of the low radiant densities saturation is not obtained and the power of detection is not fully exploited. With line sources such as hollow cathode sources and electrodeless discharge lamps much higher radiances can be obtained. Even ICPs into which a concentrated solution is introduced can be used as a primary source, through which multielement determinations become possible. 

From the hollow-cathode source, ions are extracted into a short source drift region filled with water vapor. After passing this small drift section, the H30" ions reach a reaction region which is in the form of a drift section of about 20 cm length and 5 cm inner diameter, filled with the air (pressure... 

Fig. 10.15 Three-channel AAS coupled with a microcomputer for processing of data. H hollow-cathode sources C rotating sector B burner M monochromator units P detectors J beam chopper and start/reset unit I spectrometer control panel A analogue/digital converter K remote terminal V video monitors. (Reproduced from [43] with permission of Elsevier).

A typical single-beam instrumeni, such as that shown in Figure 9-13a, consists of several hollow-cathode sources (only one of w hich is shown), a chopper or a... 

Figure 9- 1.1b is a schematic of a typical double-beam-intime instrument. The beam from the hollow-cathode source is split by a mirrored chopper, one half passing through Ihe tlame and the other half around it. The two beams are then recombined by a half-silvered mirror and passed into a Czerny-Turner grating monochromator a photomultiplier tube serves as Ihe transducer. The output from the latter is the input to a lock-in amplifier that is synchronized with the chopper drive. The ratio between the reference and sample signal is then amplified and fed to the readout, which may be a digital meter or a computer. 

Because the emission lines of hollow-cathode sources are so very narrtrw. interference because of overlapping lines is rare. For such an interference tt) oecur. Ihe separation between the two lines would have Iti be less than about 0,1 A, For example, a vanadium line at. st)H2,l I A interferes in the determination of aluminum... 

Figure V-I.i shows details of an electrothermal atomic absorption instrument, which uses the Zeeman effeci for background correction, Unpoinriz.ed radiation from an ordinary hollow-cathode source A is passed through a rotating polarizer II. which separates the beam into two eomponcnls that are plane-polarized at 90° to one another C These beams pass into a tube-type graphite furnace similar to the one shown in Fig-...

A second type of /eeman effect instrument has been designed in which a magnet surrounds Ihe hollow-cathode source. Here, il is ihe emission spectrum of the source thai is split rather than the absorption specirutn of ihe sample. This instrumen conliguraiion provides an analogous correction, To date, most instruments are of the type illusirated in Figure )-15,... 

Figure 24, Entrance slit of monochromator with image of hollow cathode source superimposed on it.

Walsh has described a system of isolation and detection of radiation by a resonance technique. The system as used for atomic absorption is shown in Figure 6-13. Radiation from a hollow cathode source is passed through the flame sample cell into a resonance detector. The resonance detector contains an atomic vapor of the specific element under analysis. The atomic vapor in the resonance detector may be produced by cathodic sputtering or thermally. The atomic vapor in the resonance detector absorbs a portion of the... 

A resonance detector is a simple device that requires no adjustment as does a conventional monochromator, and thus can be used under quite rigorous conditions. If a multielement hollow cathode source is used, resonance detectors may be aligned in series in the optical path emerging from the sample cell to provide simultaneous determination of several elements. 

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. 

Gaseous discharge lamps have been useful as sources, particularly for alkali metals, because hollow cathode sources for these elements are troublesome. Recent improvements in alkali metal hollow cathode sources have led to their increasing use. 

Use has been made of flame emission spectra as sources for determination of lithium, copper, and some of the rare earth elements. The sensitivity is lower than for a hollow cathode source and the stability also is less. 

Walsh investigated the possibility of using a continuum as a source for atomic absorption spectroscopy and came to the conclusion that it was not a desirable source. He noted that a resolution of over 500,000 would be required and that the intensity of a very narrow segment of a continuum would be extremely low. He therefore advocated the use of the hollow cathode source. Reference to Figure 10-1 will serve as a reminder concerning the problem of measuring the decrease in signal intensity caused by line absorption from a continuum. 

The continuous source is quite useful for certain purposes if it is intense and a monochromator of high resolution is available. Photographic recording of absorption spectra can be made in the same manner as arc or spark emission spectra are recorded. In this manner atomic absorption spectra are readily available for the study of a number of spectral absorption lines, in contrast to the single-line absorption usually obtained with a hollow cathode source. 

The atomic vapor is produced by an action similar to the action of a conventional hollow cathode source. Positive ions of the fill-gas strike the cathode, dislodging metal atoms into the optical path. The sensitivity is a function of the discharge current. Walsh used currents of up to 60 mA, but currents of 200-600 mA have been used by others. Higher currents cause some additional heating of the cathode and some additional atomic vapor is produced by the heating process. 

The power supply to the hollow cathode source is modulated and an ac detection system is used. This arrangement prevents any radiation from the flame or resonance detector from producing an output signal. Random noise is less troublesome than in a conventional spectrophotometer. The resonance detector must, of course, produce a cloud of atomic vapor of the same element being aspirated into the flame. The hollow cathode source also must emit resonance lines of the same element. Analytical calibration curves closely parallel those obtained with conventional atomic absorption systems and sensitivities and detection limits are similar. 

Mitchell et al. describe a multielement atomic absorption system using a multielement hollow cathode source and a vidicon detection system. The monochromator is an 0.25-m grating that permits scanning a spectral range of 1680 A. 

Figure 31. Hollow cathode source for atomic absorption spectrometry...

This approach was introduced by Koirthyo-HANN and Pickett in 1965 [159] and is now provided in almost every AAS system. TTie total absorption resulting from the presence of the element and the background absorption are measured with hollow cathode lamp radiation, but, in addition, a continuum source is used which measures only the background absorption. This is possible as the monochromators used in AAS have a large spectral band width compared with the physical width of the resonance line emitted by the hollow cathode source, and the width of the absorption... 

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