Setting of the Monochromator

The diffracted ray coming from the center of the sample and passing through the center of the receiving slit strikes the center of the monochromator crystal. The angle between this ray and the crystal plane is equal to 6. Signs +, — corresponds to the respective clockwise and counter-clockwise rotations of the 

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In the monochromator, the diffraction grating produces a spectrum in the plane of the exit slit. The exit slit serves as a window to isolate the particular line (wavelength) of interest. When the wavelength setting of the monochromator is adjusted, the grating slowly rotates and the spectrum moves sideways across the exit slit. This adjustment may be done manually, or sometimes, in more expensive automated instruments, under microprocessor control via a stepper motor. 

Check the alignment of the quartz cuvette in the beam (with the background corrector off) as well as the settings of the monochromator. 

If the entrance and exit slits are of the same width (as is usually the case), in theory the image of the entrance slit will just fill the exit-slit opening w hen the setting of the monochromator corresponds to the wavelength of the radiation. Movement of the monochromator setting in one direction or the other produces a continuous decrease in emitted intensity, which reaches zero when the entrance-slit image has moved a distance equal to its full width. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

For an excitation wavelength of 457.9 nm, the maximum intensity of the fluorescence spectrum for auramine O in acrylamide was found to occur at / = 505 nm, so the monochromator was set to record the intensity at that wavelength. Figure 26 presents the output of the monochromator/PMT detector as a function of time during polymerization. 

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