Monochromator systems

The rocking width or Darwin width mono of a perfect crystal is derivable from dynamical diffraction theory as 

From equation (5.3) we see that 5A/A is approximately constant, though F(h) does vary slowly with A in the presence of absorption edges. The germanium absorption edge at 1.12 A considerably reduces the reflectivity below this wavelength. 

In practice, perfect silicon and germanium are readily available though germanium of adequate quality is quite expensive. Both 

2 Possible configurations of perfect crystals the Du Mond diagram 

There are quite a number of possible configurations of perfect single crystals as monochromators. Detailed reviews have been given by Hart (1971), Barrington-Leigh and Rosenbaum (1974), Beaumont and Hart (1974), Bonse, Materlik and Schroder (1976), Hastings (1977) and Kohra et al (1978). Witz (1969) reviewed the use of monochromators for conventional sources of X-rays. Underwood and Turner (1977) discussed the use of differently shaped materials for producing specific curvature optical elements. 

---

The photolysis of polyhalogenated compounds forms the basis for another monochrome system. Iodoform can undergo photolysis to produce hydrogen iodide, which subsequendy reacts with a di(2-furfuryl) derivative (3) and aromatic amines to produce a colored dye adduct (4) (29). The photolysis scheme and subsequent reactions can be shown by the following ... 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

Luminescence methods are very sensitive, quantities as little as 1 fem-tomole of ATP being detectable and while measurements may be made using scintillation counters, much simpler equipment that requires neither a radiation source nor a monochromating system is satisfactory. 

Figure 2.13 Validity of the Beer-Lambert relationship for different monochromating systems. The absorbance of varying concentrations of cyanmethaemoglobin was measured at 540 nm using a spectrophotometer (A) and a simple photometer (B) with a glass filter.

Stray light is the term given to unwanted radiation that is not removed by the monochromating system of a photometric instrument and is transmitted with the selected wavelengths. 

The absorption spectrum of a dye shows not only an absorption maximum in the visible region of the spectrum but also an absorption minimum which indicates the wavelengths of radiation that are transmitted by the dye (Figure 2.18). Such dyes, in the form of stained glass or gelatin filters, provide the simplest approach to the design of monochromating systems. 

A reflectance grating is the most suitable monochromation system for ultraviolet radiation... 

Despite the measurement of the emitted radiation by these means it is still possible for scattered or reflected incident radiation to reach the detector. To prevent this, fluorimeters require a second monochromating system between the sample and the detector. Many simple fluorimeters use filters as both primary and secondary monochromators but those instruments that use true optical monochromators for both components are known as spectrofluo-rimeters. Other instruments incorporate a simple cut-off filter system for the emitted radiation while retaining the optical monochromator for the excitation radiation. Because the wavelengths of both excitation and emission are characteristic of the molecule, it is debatable which monochromator is the most important in the design of a fluorimeter. 

A flame photometer (Figure 2.31) is designed to cause atomic excitation of the analyte and subsequently to measure the intensity of the emitted radiation. A monochromating system is essential to distinguish between the emission of the test element and other radiation from the flame. 

Any of the monochromating systems described for absorptiometers may be used, although the cheaper models of flame photometer usually employ filter systems. In these cases interference from other elements at wave-... 

The absorption bands due to atoms are very narrow and the use of white light as the incident radiation would swamp even the best monochromating system with unabsorbed radiation on either side of the absorption band. It is fundamental, therefore, to the technique of atomic absorption spectrophotometry... 

Figure 9.2 (a) Simple (+n, -n) double-crystal topography arrangement in the parallel setting, (b) Use of asymmetric reflection to expand illuminated area and eliminate image doubling, (c) Double-crystal topography with a standard duMond-Hart monochromator system... 

For scanning TLC plates by fluorescence, the choice of wavelengths (in a monochromator system) or filters is important. Usually, since background fluorescence is minimized when naturally fluorescent compounds are chromatographed, the excitation and emission maxima are chosen. However, when reactions on the TLC plates are carried out with spray reagents, some alteration in these settings may be required for optimum... 

There is also a standard test method for determination of major and minor elements in coal ash by inductively coupled plasma (ICP)-atomic emission spectrometry (ASTM D-6349). In the test method, the sample to be analyzed is ashed under standard conditions and ignited to constant weight. The ash is fused with a fluxing agent followed by dissolution of the melt in dilute acid solution. Alternatively, the ash is digested in a mixture of hydrofluoric, nitric, and hydrochloric acids. The solution is analyzed by (ICP)-atomic emission spectrometry for the elements. The basis of the method is the measurement of atomic emissions. Aqueous solutions of the samples are nebulized, and a portion of the aerosol that is produced is transported to the plasma torch, where excitation and emission occurs. Characteristic line emission spectra are produced by a radio-frequency inductively coupled plasma. A grating monochromator system is used to separate the emission lines, and the intensities of the lines are monitored by photomultiplier tube or photodiode array detection. The photocurrents from the detector... 

The existing instruments for resonance scattering experiments may be classified according to the monochromator system. As synchrotron radiation is highly polarized in the plane of the orbit, the vertical reflection by the monochromator crystals is preferred. Under these conditions the polarization factor remains nearly constant. With reference to this design feature, the following arrangements exist or are under construction ... 

Fig. 6. The instrument XI5 of EMBL at the storage ring DORIS, DESY Hamburg. The double monochromator system extends 24 to 28 m from the source. The first monochromator crystal (1) deflects the beam to the second crystal which can move between the positibns marked by (2) and (2 ). The sample can be put at different distances from the detector (D)...

Because of significant losses of radiant power on polarization and transmission through the double monochromator system needed to keep stray light to an absolute minimum, the light sources for CD detection must be intense. As a consequence, instrument... 

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. 

Monochromation system (filter, prism) that separates out the anal3ftical wavelength, from other radiations and... 

For SR protein crystallography two main designs for the monochromator system have been utilised to date based on either the bent oblique-cut single crystal or ( +, —) parallel double crystal system. In the next sections (sections 5.2.3 and 5.2.4) we describe their properties in detail. 

---