Radiation sources, modulation

This problem is overcome most simply by modulation of the radiation source. Modulation means that the source radiation is switched on and off very rapidly. This can be done by using a rotating mechanical chopper placed directly in front of the source. A chopper is shown in Figs. 6.8 and 6.14. The mechanical chopper is a circle of metal sheet with opposite quadrants cut out. Every quarter turn of the chopper alternately blocks and passes the source radiation. Another way to modulate the source is by pulsing the power to the lamp at a given frequency. When modulated, the signal from the source is... 

Because NEP is roughly proportional to D is more useful for comparing detectors of differing sizes. D depends on the wavelength distribution striking the detector (if it is quantum) and the frequency at which the radiation is modulated, so these measurement parameters need to be included for a D value to have meaning. Often detectivity is written as where Tis the temperature of the blackbody source of radiation or the wavelength of the... 

Figure 4.16. To cover all possible transitions in the absorbing nucleus, the energy of the source radiation is modulated by using the Doppler effect, such that the emitted radiation has an energy E v) = Eo(l + vjc). For Fe the required velocities fall in the range (1 to t-1 cm s k In Mossbauer emission spectroscopy, the sample under investigation is the source, and a single line absorber is...

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

When Walsh started to think about using AAS for analytical purposes back in 1952, one of his key conclusions was that, in order to carry out absorption measurements on luminous atomic vapors, it would be necessary to employ a modulated light source and a synchronously tuned detection system, so that any radiation emitted by the sample would produce no signal at the output of the detection system [2]. This modulation principle, using either an AC-operated radiation source or a chopper in the radiation beam, and a selective amplifier tuned to the same modulation frequency, has ever since been applied in all commercially available atomic absorption spectrometers. It has been considered one of the major advantages of... 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

As an example of a modern commercial interferometer, the optical diagram of a Bruker IFS 66, is shown in Fig, 3.4-1. It allows working in the optical range from 40000 to 20 cm (250 nm to 500 im), to exchange different internal and external radiation sources and detectors, and to connect various accessories, such as a Raman module or an infrared or Raman microscope. 

In most instruments, the radiant flux is modulated periodically. This can be achieved by modulating the current of the primary source or with the aid of a rotating sector (g) in the radiation beam. Accordingly, it is easy to differentiate between the radiant density emitted by the primary source and that emitted by the flame. Both single beam and dual-beam instruments (see also Fig. 77) are used. In the latter the first part of the radiation of the primary source is led directly into the monochromator, whereas the second part initially passes through the flame. In this way fluctuations and drift can be compensated for insofar as they originate from the primary radiation source or the measurement electronics. Furthermore, the spectrometer can be provided with equipment for a quasi-simultaneous measurement of the line and background absorption [253]. 

Virtually all AA spectrometers operate with a radiation source which is modulated (chopped mechanically or electrically at a fixed frequency). The net effect is that the detector receives a modulated signal from the emission source and a constant signal from the flame. The constant signal from the... 

Explain why the radiation source in atomic absorption instruments is usually modulated. 

Why must HDLs or EDLs be used as the radiation source for A AS Illustrate schematically an HDL for lead (Pb). How would you make the cathode Why is modulation of the source necessary for accurate results How is modulation achieved ... 

If the radiation source is not modulated, will emission from the analyte in the atomizer result in a positive or negative error Show your calculation to support your answer. 

Figure 15 Schematic construction of a single-beam AA instrument. Dotted line represents modulated signal from the radiation source and solid line direct current emission from the atomizer...

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