Spectroscopic components radiation detectors

Absorption, emission, fluorescence, and diffraction of X-rays are all applied in analytical chemistry. Instruments for these applications contain components that are analogous in function to the five components of instruments for optical spectroscopic measurement these components include a source, a device for restricting the wavelength range of incident radiation, a sample holder, a radiation detector or transducer, and a signal processor and readout. These components differ considerably in detail from the corresponding optical components. Their functions, however, are the same, and the ways they combine to form instruments are often similar to those shown in Figure 7-1. 

An example of quasi CW THz detection [86] uses a THz wave parametric oscillator (TPO) consisting of a Q-switched Nd YAG laser and parametric oscillator [87,88], In this technique, MgO LiNb3 is employed as a non-linear material to generate CW THz. Silicon prisms couple the THz radiation from the non-linear crystal where it is detected using a pyroelectric detector. THz images are collected at discrete THz frequencies and then spectroscopically analyzed using a component spatial pattern analysis method to determine sample composition. 

The basic elements of any emission spectroscope are (1) a source that contains the substance to be studied and is capable of energizing that substance so that it can emit its characteristic radiation, (2) a dispersing device to resolve the emitted radiation into its component frequencies, and (3) a detector that can measure the intensity of the radiation at the various frequencies. The choice of devices for each of these elements depends on the region of the spectrum under investigation. 

The double-beam system is used extensively for spectroscopic absorption studies. The individual components of the system have the same function as in the single-beam system, with one very important difference. The radiation from the source is split into two beams of approximately equal intensity using a beam splitter, shown in Fig. 2.28. One beam is termed the reference beam, the second beam, which passes through the sample, is called the sample beam. The two beams are then recombined and pass through the monochromator and slit systems to the detector. This is illustrated schematically in Fig. 2.28. In this schematic, there is a cell in the reference beam that would be identical to the cell used to hold the sample. The reference cell may be empty or it may contain the solvent used to dilute the sample, for example. This particular arrangement showing the monochromator after the sample is typical of a dispersive IR double-beam spectrophotometer. There are many commercial variations in the optical layout of double-beam systems. 

A schematic block diagram of the instrumentation used for AAS is shown in Figure 6.3. The components are similar to those used in other spectroscopic absorption methods as discussed in Chapters 2 and 5. Light from a snitable source is directed through the atomizer, which serves as the sample cell, into a wavelength selector and then to a detector. The detector measnres how much light is absorbed by the sample. The sample, usually in solution form, is introdnced into the atomizer by some type of introduction device. The atomizer converts the sample to gas-phase ground-state atoms that can absorb the incident radiation. 

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