Control of the monochromator

The use of high-resolution grating monochromators for the Isolation of the selected wavelengths Is of crucial relevance to spectrochemical measurements. The remarkable advances In the functioning of monochromator control systems have considerably improved the precision with which these units can select a given wavelength. 

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Monochromators. Replica gratings and narrow band-pass filters are used commonly, more so than quartz prisms. Computer control of the monochromator is available in some instruments, so that optimum intensity at the desired wavelength or maximum absorption by the examined substance can be obtained. 

The control of the monochromator stepping motor, the PEM retardation level, and the lock-in amplifiers, are carried out by a standard IBM-compatible personal computer. It communicates with the PAR lock-in amplifier via a 1200 baud serial link,... 

Accuracy of wavelength calibration is maintained by thermostatic control of the monochromator. Water vapor may be harmful to optical components and in all instruments the internal atmosphere must be controlled by drying, gas filling, or evacuation. Carbon dioxide and water vapor absorb in the infrared and in single-beam instruments separate recordings of blank and sample spectra must be made. This is inconvenient, and double beam instruments, with automatic blank compensation and improved stability, are more commonly used. 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

The fluorescence spectra of dilute solutions (A < 0.05) of the compound and the standard must be recorded under exactly the same experimental conditions (slits of the monochromators, high voltage of the photomultipliers, gain of the electronic devices). The temperature of the sample holder must be controlled because the... 

The addition of a single monochromating crystal gives the device great flexibil ity, since one has independent control over the collimation, by the number of reflections, and the bandpass, controlled by the width of the reflection of the monochromating crystal. Figure 2.16 shows one variant, a three-reflection... 

For high-throughput data collection, control of the X-ray wavelength must be rapid and reproducible. Current state-of-the-art monochromators have mechanical precisions on the order of 0.0001 angular degrees. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

The prism at the outlet of the laser serves to separate the laser emission of the gas fluorescence and allows for a clean excitation of the sample. For excitation using solid-state lasers, this element is dispensable. The lens (element 5) collects the fluorescent signal and focuses on the aperture of the monochromator. The filter is used to eliminate excitation that is spread over the surface of the sample. The optical chopper serves to modulate the light at a defined frequency, which serves as reference for the lock-in amplifier. A data acquisition system controls the pace of the monochromator and reads the signal of the lock-in, generating the sample s emission spectrum. 

Mechanical backlash of the wavenumber reading is another problem that may be encountered when measuring a spectrum on some instruments. Finally, the temperature of the monochromator should be kept constant because band position may vary by as much as 3 cm-1 with temperature fluctuations. Although the monochromator normally is thermostated above room temperature, it is recommended that a laboratory temperature be reasonably controlled. 

When accuracy of 1 cm-1 is required, internal standards may be employed. These can be frequencies of solvent bands or the bands of added noninteracting solutes. Bands due to the compounds being measured are compared with the frequencies of the internal standard. However, care must be taken so that significant band shifts do not occur because of chemical interaction between the substance under study and the reference itself. In addition to its simplicity, this method has a distinct advantage over the other methods in that the frequencies determined from the position of a band relative to the internal standard are essentially temperature-independent. It should be noted that the absolute readings from the monochromator may change from day to day as much as 2-3 cm-1 if the temperature control inside the monochromator is malfunctioning. 

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. 

A computer controls the instrument s automatic operation and the positioning of the monochromator, the sampler changer, certain generator settings, etc., in addition to recording and treating the analytical data. 

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. 

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