Detectors, absorption photometric requirements

Photometric Moisture Analysis This analyzer requires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near-infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

The technical specifications identified and described by most of the manufacturers of absorption photometers for medical use include wavelength accuracy, spectral half-width of spectral radiation flux at the detector, photometric accuracy, percentage of wavelength integrated, false radiation, and photometric short-time repeatability. As discussed previously [2], the Instrumental Performance Validation Procedures, issued by serious manufacturers of analytical instruments, indicate the methods and the reference materials required to test and to maintain optimum spectrometer performance in daily routine analysis. 

UV photometric. Such detectors are applicable only to molecules with a UV chromophore. They can either use a filter to select a particular wavelength or be tunable to any wavelength. Radiation is absorbed at that wavelength to an extent determined by the extinction coefficient of the molecule and thus calibration is required for individual molecules. By using highly purified solvents, it is possible to extend their use down to 210 nm or below, where many molecules with no chromophore absorb weakly. An alternative approach is to incorporate a chromophore as part of the solvent and monitor a reduction in absorption (indirect photometric detection). 

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