Photometers and Photometric Systems

The purpose of a spectrophotometer is to record the spectral variation of energy transmitted through the sample. The photometric system is the means of establishing the variation and relating it to a reference, e.g., the energy transmitted through the sample in regions of no absorbance. 

Single-Beam Systems. The earliest work in infrared spectroscopy utilized single-beam systems. Although still prevalent in some applications, primarily related to infrared physical measurements, these systems play only an infrequent role in analytical chemistry. 

In contemporary single-beam systems the radiation is interrupted at a frequency compatible with the detector (e.g., 13 Hz ) to utilize ac amplifier systems. These have many advantages over the dc amplifiers which would otherwise be required, including increased stability and freedom from drift. Nevertheless, the stability requirements of the source and the amplifiers in a single-beam system exceed those of the double-beam systems to be described below. 

In order to measure transmittance bands in a single-beam system it is necessary to scan the region at least twice—with and without the sample cell. To ensure that amplifiers and source are adequately stable, it is preferable to record the background scanned without the sample 

Double-Beam Optical Null Systems. Virtually all instruments used for analytical chemical applications utilize a double-beam optical null photometric system. In these instruments an electrooptical. servo-system continually attenuates the energy in the reference beam so that there is no net signal difference between the reference beam and the sample beam. The recording pen indicates the position of the reference beam attenuator and therefore the relative transmittance of the sample. 

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