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Study Notes: AAS Light Source

You will have used ‘continuum light sources’ in your daily life. A good example is the domestic incandescent light bulb that emits light over a wide wavelength range from about 300 nm (UV end of the spectrum) to 700 nm (infrared end of the spectrum).

In AAS a ‘line source’ is required. That is a light source that produces light at a specific wavelength. Usually a hollow cathode lamp is used which consists of a glass envelope containing a cathode (cup or cylinder containing the element to be excited) and a suitable anode. The tube contains an inert gas, usually argon or neon, at a low pressure.

Hollow cathode lamps incorporate a cathode of the element that is the analyte in the solution - the emission lines of the lamp therefore correspond exactly with the absorption wavelength of the analyte. The end of the lamp contains a ‘window’ that is made of a material that transmits the spectral lines of the element used for the cathode. Usually quartz or Pyrex is used.

Increasing lamp current increases lamp intensity and this decreases baseline absorbance noise. But if the current is too high then ‘self absorption broadening’ occurs. This is where the atoms in the hollow cathode lamp absorb the light being emitted from the hollow cathode itself. This leads to lower absorbance and increased calibration curvature (curvature of the standard curve from the linear).

Spectral Band Width is also important for signal-to-noise ratio. A large spectral band width may result in excellent signal-to-noise ratio but the resonance line (the line of interest relating to the ground state or unexcited state of the atom of interest) may not be isolated from other lines. This leads to a non-linear calibration curve. Conversely a narrow spectral band width will result is good resolution of resonance lines but lead to a poor signal-to-noise ratio resulting from reduced light throughput.

Thus there is a need to use the manufacturer’s recommended values for lamp current and spectral bandwidth at least as a starting point in the development of analytical methods.

Other aspects that are important for effective use of the AAS light source include the following which are often used for troubleshooting purposes.

  • Lamp alignment and wavelength selection
  • Cleanliness of optics
  • Expansion of absorbance scale (equivalent to gain, in techniques such as HPLC).

Common problems often include misaligned lamp and improper lamp current.

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