Monochromators and Polychromators

Pulse dispersion and pulse shift can be avoided by using a double monochromator of the subtractive dispersion type. In this design the second monochromator is turned 180 °, and the gratings are moving in opposite directions. Thus the path length differences for both sides of the gratings and for different wavelength cancel. 

A second problem of monochromators is their relatively low efficiency. Most of the currently available monochromators use a grating as a dispersive element. A grating diffracts the light into multiple diffraction orders on either side of the incident beam, see Fig. 7.19. 

Zero order (specular reflection) Incident light 

Ruled gratings have sawtooth-shaped grooves and diffract 40 to 80% into the first order on one side of the incident beam. The rest of the light is refleeted in the zero order or diffracted into higher orders and into the first order on the other side of the incident beam. This can result in a number of unpleasant effects. 

The unused light can cause considerable straylight problems, and the second diffraction order may overlap with the first one. In TCSPC applications, grating monochromators or polychromators should therefore always be used with a filter that blocks the excitation wavelength. 

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Near-infrared Spectroscopy for Process Analytical Technology 119 5.3.2 The scanning grating monochromator and polychromator diode-array... 

The molecular structure of the anchored Cr(VI) has been a strong point of discussion in the literature, and several molecular structures (monochromate, dichromate, polychromates) have been proposed (see Scheme 3). The nature of the silica support, the chromium loading, and the activation method can all influence the chemical state of the supported chromium. 

In this section the mainstream NIRS technologies in current widespread practical use for PAC applications will be examined. These are scanning grating monochromators, grating polychromator PDA spectrometer, AOTF analysers and FTNIR analysers. 

An instrumentation technique that utilizes the output from a monochromator, and that provides some of the benefits of multiplexed data acquisition, is known as a Hadamard transform spectrometer. This class of instrument can feature either a monochromator or a polychromator (equipped with a detector array). Hadamard transform instruments are available as custom-made devices, but none have been fully commercialized. 

ICP/OES can be conducted either simultaneously or sequentially. Simultaneous instruments rely on a polychromator or direct-reading spectrometer to read up to 60 elements from the same sample excitation. Sequential analyses use a computer-controlled, scanning monochromator system. The light emitted by the sample in the plasma source is focused on the entrance slit of the monochromator and the spectrum is scanned through the region of interest. Typically, it is possible to determine several elements per minute in the sample in a sequential spectrometer. 

Data for ICPAES(USN) are from Boumans (1987b) (DLs, on a 2 basis, are for conventional Ar ICPs operating under compromise conditions with ultrasonic nebulization and include monochromator and combination monochromator/polychromator spectrometers. 

In general, in sequential UVA is spectroscopy the sample is placed between the monochromator and the detector to avoid extreme exposure to high energy UV-radiation. The intensity reduced by the sample falls onto the detector. However, in the case of a diode array spectrometer a sample position behind the polychromator would result in a wavelength gradient (laterally resolved wavelengths) in the sample across the plane of observation, since at the exit of the polychromator a correlation exists between wavelength and space. For this reason, in multiplex spectroscopy the sample is placed directly in front of the polychromator. Under these conditions... 

What is the difference between a monochromator and a polychromator When a sample is introduced into a flame, what processes occur that lead to the emission of radiant energy ... 

FIGURE 3 Schematic representation of the most frequently used grating-based monochromators (A, B, C) and polychromators (D, E, F). A and D are known as Czerny-Turner design. Fastie-Ebert (B) and Littrow (C) have similar designs in that both use a single reflective mirror. A Rowland circle polychromator is depicted in E, and F represents an Echelle spectrograph. 

Figura 3 Grating spectrometers commonly used for ICP-OES (a) monochromator, in which wavelength is scanned by rotating the grating while using a singie photomultiplier tube (PMT) detector (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40 or more exit slits and PMTs can be arranged along the focal plane) and (c) spectrally segmented diode-array spectrometer.

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

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