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Study Notes: AAS Monochromator

If you wanted to perform quantitative analysis, monochromatic light would need to be passed through the sample otherwise Beer’s Law would not hold true. Alternatively, if you needed to scan a sample to produce a spectrum, monochromatic light at every wavelength to be scanned would also be required.

In effect, a monochromator produces monochromatic light by removing unwanted wavelengths from the source light beam.

The function of the monochromator is to isolate a single atomic resonance line from the spectrum of lines emitted by the hollow cathode lamp. Essentially it is an adjustable filter that selects a specific, narrow region of the spectrum for transmission to the detector and excludes all wavelengths outside this region.

A monochromator comprises an entrance slit, a dispersion device and an exit slit.

  • The entrance slit selects a defined beam of (polychromatic) light from the source.
  • The dispersion device causes the different wavelengths of light in the source beam to be dispersed at different angles.
  • The exit slit enables selection of a particular wavelength to produce the required monochromatic light.

Dispersion devices can be prisms but diffraction gratings are used in AAS as they are cheaper, easier to make and provide superior performance. Both types have been discussed previously in this unit. The diffraction grating is a block of glass with one surface coated with highly reflective aluminium. The Al surface is scored with fine grooves spaced closely together. Usually 500 - 3000 grooves per mm are used. The grooves must be straight, evenly spaced, parallel and of identical shape.

Light striking these grooves is reflected and dispersed at different angles according to its wavelength. By rotating the grating, a constituent wavelength is focussed on to the exit slit via the second mirror.

Below is a simple representation of a monochromator that uses a diffraction grating.

Optical geometry of monochromator

Ideally a monochromator produces truly monochromatic light but in practice it emits an optically symmetrical band of a certain narrow wavelength range. The width of the band at half its height is called the instrumental bandwidth. This can be as low as 0.01 nm for gratings but usually a 0.02 nm bandwidth is typical for AAS.

Transmitted light peaks ranging in colour from yellow only, ie a narrow bandwidth, to yellow with some green and orange mixed in, ie a wide bandwidth.

Spectrophotometers will often have a ‘slit width’ adjustment that the analyst will need to check or set depending upon the nature of the work to be done. Adjusting the slit width alters the instrumental bandwidth. The question then is, under what circumstances would the slit width need to be adjusted?

For instance, if the slit is too wide, the light throughput will be high and the signal-to-noise ratio may be excellent BUT the resonance line may not be isolated from other lines and the calibration line may be badly curved. On the other hand, if the slit is too narrow, the resolution may be excellent but the signal-to-noise ratio may be poor due to the reduced light throughput.

 

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