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Study Notes: Single Beam Design

As the name suggests this instrument comprises a single beam of radiation emanating from the source and travelling through the various components and sample until ultimately reaching the detector.

Simplified internal layout of spectrophotometer components from light source to detector. White light is emitted from the source and light of different colours are transmitted to the detector.

Sample absorbance is determined by measuring light intensity without the sample in the beam and comparing it with the intensity after passing through the sample.

The way this is achieved in practice is to fill a sample cell with solution that contains all solvents/reagents minus the analytical species (the ‘blank’). The cell is then placed in the sample compartment of the instrument and the readout is adjusted to zero absorbance (or 100% transmission) at the desired wavelength setting.

Solution containing the analytical species is replaced in the cell and its absorbance determined. The reading that is observed is attributed to the presence of the analyte alone.

The single beam design is well suited to measuring absorbance at a particular wavelength however it is not as well suited to multiple wavelength measurements or indeed to scanning. The zero absorbance must be reset on the blank solution whenever the wavelength is altered ie there is no automatic blank correction. There is also the possibility that drifts in lamp intensity could mean significant errors over long periods of time.

The single beam spectrophotometer is relatively low in cost.

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