Atomic Absorption Spectroscopy:

AA has the following essential features:

  • Must be capable of producing flame. Flame is produced by burning acetylene & air and also a hotter flame by burning acetylene with nitrous oxide. So essentially we have the luxury of two different flames. Acetylene/air (2100-2400*c) or acetylene/nitrous oxide (2600-2800*c). Compressed gas cylinders of acetylene, nitrous oxide and air are attached to AA for production of these flames.
  • Hollow cathode lamps are used as source of energy. For every metal you wish to run you must have its lamp. Each specific lamp is capable of emitting energy at specific wavelength. There are also multi element lamps available.
  • Two types of burner heads are used. Long burner head is only capable of handling acetylene/air flame. Shorter burner head can be used with both acetylene/air and acetylene/nitrous oxide flames. Metals like vanadium; aluminum; silicon must be run with acetylene/nitrous oxide flame burner head, however sodium; copper can be run with acetylene/air flame and hence either of the two burner heads can be used. The ASTM methods clearly advise about the right use of flame, burner head and lamp required.
  • Every AA is capable of aspirating the sample (aqueous or organic) through the available aspiration system. The sample flow through AA can be adjusted by proper use of the nebulizer of the aspiration system.
  • Finally, there is a detection system for measuring the absorbed energy. These days software is available help develop the method controls in each case.
Principle of Operation:

After getting the AA optimized with all necessary steps including the use of right lamp, proper wavelength and slit setting, optimizing the energy, making proper alignment of the lamp, a set of known concentrations (standards) are run through the aa. The absorption measured in each case and a nice linear calibration curve is obtained. The curve is then verified by running a check standard of known concentration. You are now ready to aspirate the sample containing the metal of interest and from the measured absorption the concentration is recorded using the available calculations stored in the software system for that metal. This may sound a bit confusing but once a tech is properly trained with the operating functions, he can comfortably run the metals.

Necessary Precautions When Running AA:
  • Make sure you have the right burner head.
  • Ensure that the right lamp has been installed.
  • Gases cylinders have enough gases to take you through the analysis. Acetylene gas cylinder must be changed once the tank pressure falls below 50 lbs. This is a safety feature because there is acetone at the bottom of acetylene tank and this should not be sucked into the burner head. Acetone can explode.
  • Take all necessary precautions including but not limited to wearing safety glasses.
  • Run the exhaust system to let the flame vapors escape.
  • Lamp alignment, proper flows and flame conditions are critical for getting good absorption.
  • Standards and sample preparation are key to getting reliable and repeatable numbers.
  • Standards must be prepared within the linear range for each of the metal of interest.
  • Do not attempt to run aa without the safety system for discharge of the waste collection not attached. This may cause flash back.

In summary, AA is a highly reliable method for running metals in ppm levels. The equipment is simple and easy to learn, however never attempt to run AA without proper training. There is other equipment like ICP and ICP-MS for running metals to extremely low detection limits in PPBMS. This information does not qualify any one to read and be ready to run AA. This is very basic and general information of interest. AA runs one metal at a time and is a bit time consuming especially when the samples have to be ashed and then brought into aqueous phase. Aluminum and silicon take extra time in sample preparation which includes ashing, fluxing and dissolutions. Operations personnel must share with their clients the fact that running metals with AA is a time consuming exercise.