Multichannel wavelength-dispersive

The techniques differ widely in terms of their speed of analysis. The modern multichannel wavelength-dispersive spectrometer is able to produce data from 20-30 elements in less than 1 min. An energy-dispersive spectrometer, or even a relatively simple wavelength-dispersive spectrometer, is able to perform a full qualitative analysis on an unknown sample in less than 30 min. The diffraction technique, on the other hand, can take as much as an... 

Comparison of Fig. 1, which depicts a conventional CD system, and Fig. 4 shows the primary difference in the two approaches to be in the type and location of the wavelength dispersion device. For multichannel detection, the monochromator has been replaced by a polychromator which does not possess an exit slit to define a wavelength interval. For multichannel detection, the location (pixel) on the multichannel device defines the wavelength of interest. In conventional approaches, the monochromator is placed after the source in order to limit the UV exposure of the sample. However, in the multichannel arrangement, the polychromator is located after the sample to allow the sample to be... 

It should also be noted at this point that a multichannel detector can have multiple detector elements along two axes, one parallel to the direction of wavelength dispersion, and one perpendicular. The latter is parallel to the entrance slit in most dispersive instruments. For example, a CCD may have 1024 pixels along the wavelength axis and 256 along the vertical axis, for a total of 262,144 independent elements. This second dimension of the detector may be used in a variety of applications involving Raman imaging, multiple detection tracks, or echelle spectrometers. 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

There are several advantages to a tunable filter system. First, it is unnecessary to have a multichannel detector (for single-point measurements), since only one wavelength is being selected at a time. The size of the detector is also much more flexible, since spectral resolution of the system is not a function of the detector and input aperture as it is in a classical monochromotor, but rather limited only by the functional characteristics of the filter. Second, since focusing and dispersive elements are minimized the spectrometer could be made very small. Third, the entire spectrum does not need to be obtained the random access nature of the filter allows only the spectral features required for a measurement to be made. This can be a significant advantage for routine measurements. 

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