Atomic Absorption Spectrophotometers

• Manufacturer: Varian Instruments
• Model: AA400 (2 units)
• Options
  • Atomization:
    • Unit 1-Flame: Air/Acetylene or Nitrous Oxide/Acetylene
    • Unit 2-Electrothermal (Graphite Furnace)
  • Autosamplers:
    • Flame-67 position, 15ml tubes
    • Furnace- 44 position, 2ml tubes

General Description

Atomic absorption is a spectrophotometric technique for the quantitative measurement of elements (usually metals) in solution. In this technique, light of a specific wavelength is directed through the sample, which has been vaporized (usually by heating) to convert the element of interest to its ground (atomic) state, and the resulting reduction in light intensity (absorption), which is proportional to the concentration of the element in the sample, is measured by a photomultiplier tube. The light source in these instruments is a lamp made of the element being measured ("hollow cathode" lamp), so the light given off consists only of the specific emission lines for that element, and a high degree of specificity is achieved. In order to further reduce interferences in the measurement, the light reaching the detector is matched to that of the lamp by placing a high precision monochrometer between the sample and the photomultiplier tube.

Sample vaporization/atomization is usually accomplished by heating, either in a flame or electrothermally. (Cold vapor generation is a technique used for some elements, such as mercury.) With flame atomization, the liquid sample is aspirated into the flame by the venturi effect of the combustion gasses flowing through the burner. The flow of sample into the flame is typically in the range of several milliliters per minute. For this reason, flame AA requires milliliter-sized samples. With furnace atomization, a microliter-sized portion of sample is pipetted into a small, open-ended graphite cylinder, which is located in the light beam of the instrument, and the cylinder is then heated by applying a high electrical potential across the cylinder. The vapor cloud thus produced is contained within the cylinder. The result of this method of heating is not only the consumption of much less sample, but the generation of a much more concentrated sample vapor, which translates to much higher sensitivity. The drawbacks of this technology (electrothermal atomization) as currently practiced is that it is more expensive (due mainly to the cost of the graphite tubes), and takes significantly longer to perform an analysis than with flame atomization (minutes versus seconds), and the graphite tubes, which must be replaced relatively frequently, are expensive.