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Double-beam spectrophotometers

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p399-400

2026-11-07

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Double-beam spectrophotometers

Double-beam spectrophotometers (Fig 15.6) use similar high-quality optical components to those in the single-beam instrument. However, the radiation from the monochromator is split into two identical beams by a rotating mirror. One beam passes through the sample and the other through the reference cell, before being recombined to focus on the detector. Each signal is processed appropriately by the detector electronics to measure the absorbance 10 to 20 times per second. The advantage of the double-beam set-up is that the absorbance of the cell and solvent is automatically compensated for so that the resultant spectrum is that of the sample only. A scan motor drives the monochromator to give a constant wavelength change per second, which is synchronised with a recorder or digital plotter to present the spectrum. For broad bands, scan speeds up to 2nm/s can be employed. However, some computer-controlled spectrophotometers with fast data-processing capabilities can scan at rates approaching 20nm/s, and still maintain spectral fidelity even for sharp peaks.