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In single-beam uv-vis absorption spectroscopy, obtaining a spectrum requires manually measuring the transmittance (see the Beer-Lambert Law) of the sample and solvent at each wavelength. The double-beam design greatly simplifies this process by measuring the transmittance of the sample and solvent simultaneously. The detection electronics can then manipulate the measurements to give the absorbance.
The dual-beam design greatly simplifies this process by simultaneously measuring P and Po of the sample and reference cells, respectively. Most spectrometers use a mirrored rotating chopper wheel to alternately direct the light beam through the sample and reference cells. The detection electronics or software program can then manipulate the P and Po values as the wavelength scans to produce the spectrum of absorbance or transmittance as a function of wavelength.
Schematic of a dual-beam uv-vis spectrophotometer
Pictures of Lambda 3A and 4B dual-beam uv-vis spectrophotometers
Larger picture of the Lambda 3a spectrophotometer
Close-up of the control panel / Close-up of the sample compartment
Pictures of a Hitachi dual-beam uv-vis spectrophotometer
Larger picture / Picture of the sample compartment
Picture of the sample cells
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