Monochromators, selection

Figure 18-20 Essentials of a luminescence experiment. The sample is irradiated at one wavelength and emission is observed over a range of wavelengths. The excitation monochromator selects the excitation wavelength (X ) and the emission monochromator selects one wavelength at a time (Xem) to observe.

Heitmann et al. [11] designed a very compact double monochromator, consisting of a 300 mm prism pre-monochromator and a 400 mm echelle grating monochromator, both in Littrow mounting, which is shown schematically in Figure 4.3. The prism monochromator selects the part of the spectrum that is of interest, and the echelle monochromator provides the high dispersion of the selected spectral interval, which is better than 2 pm per pixel at 200 nm (see Welz et al. [10]). 

Schematic diagram of an ultraviolet spectrometer. In the ultraviolet spectrometer, a monochromator selects one wavelength of light, which is split into two beams. One beam passes through the sample cell, while the other passes through the reference cell. The detector measures the ratio of the two beams, and the printer plots this ratio as a function of wavelength.

The primary monochromator selects the excitation wavelength and removes unwanted radiation. The analyzing monochromator selects from the luminescent radiation the required wavelength and discriminates against scattered primary radiation. Monochromators need not be of high resolution (X/S ss 5000) but should be of large aperture. 

For normal fluorescence scanning, a high-intensity xenon continuum source or a mercury vapor hne source is used, and a cutoff Alter is placed between the plate and detector to block the exciting UV radiation and transmit the visible emitted fluorescence. For fluorescence measurement in the reversed-beam mode, a monochromatic Alter is placed between the source and plate and the monochromator between the plate and detector. In this mode, the monochromator selects the emission wavelength, rather than the excitation wavelength as in the normal mode. 

The components of the fluorimeter include (Fig. 7.5) (1) power supply—powers the lamp (2) light source—UV -vis lamp (3) excitation monochromator—selects a particular wavelength of light from the lamp to excite the sample (4) sample—solution in a cuvette in a holder (5) emission monochromator—scans through a set of wavelengths where the sample emits light (6) detector—a photomultiplier tube PMT detects the number of... 

Figure 9.14 Simplified scheme of the optical path of a simple beam, sequential mode spectrophotometer. 1. Two co-existing sources, though only one is selected for the measurement. 2. The monochromator selects the measurement wavelength. 3. The measuring cell containing either sample or control blank is placed in the optical path. 4 and 5. Diode detector and control diode.

Fig. 26.6 The setup for an X-ray experiment. The X-ray generator produces a powerful beam. The monochromator selects X-rays of a single wavelength (1.54 A for copper targets) and the collimator limits the diameter of the beam to 0.3-0.5 mm. This beam hits the crystal and some of the X-rays are diffracted by the crystal. Most X-rays pass straight through and are stopped by a small piece of lead, the beam stop. The diffracted X-rays are detected by an area detector, an imaging plate, or by other detection systems. The goniometer, shown here as a big black circle, has four rotation axes and allows the crystal to be positioned in any orientation with respect to the X-ray beam.

The WD spectrometer (Figure 1) behaves as an X-ray monochromator. Selected crystalline materials are used to diffract lines in the fluorescence X-ray spectrum. When a beam of X-rays is directed at certain crystalline materials. X-ray photons are reflected off the various atomic layers in the crystal. Destructive interference occurs between almost all reflected photons, except those that satisfy the Bragg equation ... 

In the atomic spectroscopy experiment in Figure 20-1, a liquid sample is aspirated (sucked) through a plastic tube into a flame that is hot enough to break molecules apart into atoms. The concentration of an element in the flame is measured by absorption or emission of radiation. For atomic absorption spectroscopy, radiation of the correct frequency is passed through the flame (Figure 20-2) and the intensity of transmitted radiation is measured. For atomic emission spectroscopy, no lamp is required. Radiation is emitted by hot atoms whose electrons have been promoted to excited states in the flame. For both experiments in Figure 20-2, a monochromator selects the wavelength that will reach the detector. Analyte concentrations at the parts per million level are measured with a precision of 2%. To analyze major constituents, a sample must be diluted to reduce concentrations to the ppm level. Box 20-1 describes an application of atomic emission for space exploration. 

In the spectrofluorimeters, a monochromator selects the excitation wavelength from the band spectrum of either a xenon lamp or a deuterium lamp. Some instruments use a band-pass filter and others a second monochromator on the emission side. The deuterium lamp gives better signal at the lowest wavelengths (190-400 nm), while the xenon lamp can also be used in the visible region (200-850 nm). 

The monochromator selects and passes only the radiation in a narrow bandpass of known wavelength. This can be done with prisms, or gratings, or a Fourier transform spectrometer. Commercial instruments are listed in a table by Zissis and LaRocca (1978, Tab. 20-7). The same reference discusses the various instrument types in detail. Moore et al. (2009) discuss optical dispersing instruments and provide a table comparing the various types. Shannon and Wyant (1979) also discuss spectral dispersing... 

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