Dispersion Instrument readings

For dispersive instruments, the wavenumber or wavelength readings on the instrument are not to be taken at face value. It is recommended that the instrument be calibrated periodically. The time involved in calibration... 

The NO analysis involved considerable problems. Eventually we used a Beckman NDIR (non-dispersive infrared) for NO measurements because of good signal stability and because the instrument read NO directly, not total NOj,. However all of the water had to be removed from the gas because moisture was recorded as NO. The NO-NO2-H2O interaction was eliminated in the water trap by removing the NO2 before the trap. After trying several approaches, we finally used a saturated solution of sodium sulfite at room temperature as the NO2 absorber... 

Almost all companies producing spectrometers offer direct readers. They vary from quite small instruments, capable of determining 6-12 elements simultaneously, to large dispersion instruments that can determine as many as 50 elements simultaneously. Many instruments also can interchange photographic recording and direct reading. 

The physical distance between the jaws of the slit is called the mechanical slit width. Instruments used to have a micrometer scale attached so that one read off the mechanical slit width directly modern computer-controlled instruments set and read the slit width through the software that controls a stepper motor operating the slit mechanism. In UV absorption spectroscopy, mechanical slit widths are of the order of 0.3-4 pm. In IR spectroscopy, slit widths between 0.1 and 2.0 mm are common for dispersive instruments. There are no slits in FTIR spectrometers. 

A dispersive instrument is patterned after a double-beam spectrophotometer. Radiation at two fixed wavelengths passes through a cell containing the process stream to provide a continuous measurement of the absorption ratio. At one wavelength, the material absorbs selectively at the other wavelength, the material does not absorb or else exhibits a constant but small absorption. The ratio of transmittance readings is converted directly into concentration of absorber and recorded. This type of instrument can handle... 

The simplest application of these multiplexing methods involves the so-called direct-reading spectrometer , which was used with some success for a short period in atomic spectroscopy [42]. This instrument consists of a dispersion system with an array of exit silts arranged at appropriate locations. Behind each silt Is a photodetector —usually a photomultiplier. These multiplexing methods have also been used In UV-vIsIble spectroscopy, although to a lesser extent they have been Implemented on automatic discrete analysers featuring an optical system of this type with 5-10 channels or wavelengths... 

The dispersed radiation is photographically recorded on film or plate providing a permanent record for analysis. The slowness of the photographic process and subsequent evaluation led to the development of instruments known as direct-reading spectrometers. 

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