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Study Notes: AAS 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. In other words, the light from the beam follows only one path - through the flame. Single beam mirror system

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

The way this is achieved in practice is to run a sample that contains all solvents/reagents minus the analytical species (the ‘blank’) and the readout is adjusted to zero absorbance (or 100% transmission) at the desired incident wavelength setting.

Solution containing the analytical species is introduced into the flame and its absorbance determined. The reading that is observed is attributed to the presence of the analyte alone. Although in practice, if the sample is contained in a matrix that is different to the blank, then matrix effects may affect the reading.

The single beam system relies on the light source remaining stable between blank and sample measurements. The incident beam should not drift or fluctuate. Modern hollow cathode lamps are suitably stable after warm up.

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. The zero absorbance must be reset on the blank solution whenever the wavelength is altered i.e. 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 and this is a significant advantage in smaller laboratories or those with low AAS sample throughput.

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