What is an Atomic Absorption Spectrophotometer (AAS)

History of the AAS

In 1940 Alan Walsh was a young physicist working at the British Non Ferrous Metals Laboratories during World War II. His work included determination of metal composition from the debris of enemy bombers. He developed a number of methods for rapid spectrographic analysis of metals.

After the war ended, he accepted a position at CSIRO’s, Fishermen’s Bend laboratories in Melbourne. He continued his research to find a fast alternative spectroscopic analysis to the slow colorimetric method for metal identification. His dilemma was that, trying to observe the metal emission lines from metal electron quantum transitions, was only observable for the Group 1, alkali elements such as Sodium, 589.0nm. The Bunsen burner temperatures were too cool for significant emission line strengths.




Walsh’s solution was to irradiate the low temperature flame with the metal wavelength of interest. A photon amplifier sensor on the other side of the flame, then measured the signal through the flame.



The sensor measured the signal before the flame was aspirated with the unknown metal in solution and then again after the sample was aspirated into the flame.

The sensor was synchronized with a chopper set up between the wavelength source and the flame. This allowed the sensor to measure the wavelength intensity before and after the flame.


Theory of operation.

The theory of operation is that the aspirated metal atoms electrons are ‘excited’ by the flame energy but do not have the necessary energy to ‘jump’ to the next quantum energy level. The wavelength source provides the wavelength energy, exactly, that is required by the ‘excited’ electron. The electron ABSORBS this wavelength photon and lifts to the next energy level. This photon does not fall on the amplifier sensor and the signal strength is reduced by the photon loss. The greater the concentration of metal atoms in solution, then the more photons that are absorbed in the flame, from the wavelength source.  The signal strength after the flame then decreases with increasing concentration. This follows the Beer-Lambert law.

The wavelength sources evolved to become Hollow Cathode Lamps, where the metal of interest is incorporated into the cylindrical cathode and the anode and cathode are sealed in a glass envelope.

 A voltage is applied between the electrodes and the fill gas ions are attracted to the metal cathode and their impact releases the metal wavelengths required for the absorption in the flame, of that specific metal.

To analyse for different metals, requires a different hollow cathode lamp for each metal of interest. The specific cathode is made of that metal. That is, if the determination is for Copper, then a copper hollow cathode lamp was used. For magnesium, a magnesium hollow cathode lamp is used.

The mechanism for sample introduction includes:

  1. Some form of venturi where the air sucks in the sample liquid.
  2. The sample vapour is atomized by the vapour impacting on a glass bead.
  3. The air vapour is mixed with a fuel (acetylene) to form a flame heat source.

The optics at the sensor end comprises:

  1. An enclosed monochromator that is able to spread and separate all the wavelengths coming from the hollow cathode and the superimposed flame.
  2. A suitable set of slits to exclude extraneous light falling on the sensor
  3. A detector that is able to amplify the photon signal (normally a photo multiplier) and convert it into a measurable electric signal.

Typically the wavelengths are spread in the monochromator by the wavelengths falling on a diffraction grating.


Determination of metal concentration

In order to quantify the concentration of metal in solution, it is required to calibrate the AAS (Atomic Absorption Spectrophotometer). This is achieved by first establishing the flame and stabilizing the optics.

Initially the detector is ‘zeroed’ on a solution that is devoid of any metal. This is to measure the flame noise and other absorbances that will be subtracted from each measurement.

 A series of known concentration metal solutions is then aspirated into the flame            

 and the absorbance used to develop a calibration graph. Typically at least

 3 calibrant points are required as an AAS calibration is curvilinear.

The unknown metal solution is then aspirated, and the resulting absorbance is

used on the calibration graph to determine the metal concentration.


AAS Detection limits

Each quantum energy transition is unique and the photon wavelength associated with that transition is also unique to that element. This in turn creates a range of sensitivities for the elements in the periodic table.

Referring to the periodic table below, which lists the type of orbit of the outer shell electron, in general the elements with outer electrons in the S orbits are the most sensitive followed by the D orbits and P and F orbits are the least sensitive. Flame sensitivities range from 0.00003 parts per million (ppm) for magnesium to 40 ppm for Phosphorous. The alkali earth elements have the best sensitivity ~0.0005, the transition elements average ~0.001 ppm, while heavy metals range higher.



During the atomization process, (liquid ->aerosol - > individual metal atoms) less than 1% of the sample mass enters the flame.

 An elongated burner, long- thin, is used to ensure the maximum                                                            

 number of atoms are exposed to the wavelength passing

 through the flame.         



Enhanced AAS Detection Limits

An alternative heat source to the flame, is to have the sample loaded into a hollow graphite tube that is part of a high voltage electrical circuit. The application of the high voltage atomises the sample in less than a second at a temperature of 2,600 deg C. This improves sensitivity by 1,000 fold compared to flame sensitivities.

A small amount of sample, 20 uL is loaded into a sample hole in the                        

 longtitudinal graphite tube. The furnace is gradually heated up to an     

 ashing temperature of ~ 700 deg C and then atomized very quickly to

2,600 Deg C. The furnace tube being hollow has the hollow cathode                                       

lamp passing through the tube.     



Sample Self Absorption

A key measure of any analytical technique is that the detector response is sensitive and linear with increasing concentration. Most techniques have working ranges that are an order of magnitude i.e. 1-100ppm, 1-10%.

With Atomic Absorption Spectrophotometers, the working range is relatively narrow because the calibration is not linear.


This is due to the self absorption phenomena. In very dilute metal solutions, the sample is atomized and enters the flame where the free metal atoms take on the extra energy and the electrons absorb the wavelength from the hollow cathode lamp source. As the atom continues to rise through the cooler part of the flame, the electron becomes unstable and has to release the extra energy as a photon with the original wavelength it absorbed. This is released into the surrounding flame area in a random direction.

With increasing concentration of the metal atoms rising through the flame, there is an increasing probability that an electron-photon absorption will occur from a previous electron release rather than the hollow cathode lamp source. If this occurs then the instrument will not ‘see’ this absorption because it has not been a decrease in the hollow cathode lamp intensity falling on the detector.

The solution is to decrease the self absorption step by:

  1. Diluting the metal sample solution.
  2. Rotating the burner so that less of the sample is in contact with the hollow cathode wavelength.



Atomic: Atomic Absorption spectra are very simple compared to emission spectra, however there are still a few lines that overlap. Either an alternative line can be selected or if the interferent is at very low levels, then the interference will be minimal.


    λ (nm)


λ (nm)


























Non-atomic : (Continuum) absorption is caused by molecular vibration of matrix in the sample or wavelength scattering off sample particles. Correction for this anomaly is to :

  1. Do a sample blank with the matrix, less the element of interest
  2. Use a secondary wavelength source such as a D2 continuum source. The AAS is able to modulate the signal between these 2 sources and subtract the absorbance measured with the continuum from the total absorbance

Chemical: Chemical interference depresses the number of free metal atoms reaching the optical path. This can result from precipitation or stable compound formation such as for the alkali earth elements. Correction involves:

  1. Using  a higher temperature nitrous oxide flame
  2. Addition of a releasing agent such as lanthanum or strontium to the sample.

Ionization: this occurs with metal elements that are easily ionizable such as the alkali and alkali earth elements. The solution is to add a sacrificial, easily ionizable element such a caesium to the sample.

If caesium is the analyte, then sodium is added.



In order to get the best sensitivity, there are several instrument parameters that need to be optimized.

These include:

  1. Burner height relative to wavelength optic path through the flame.
  2. Fuel/oxidant ratio. Some elements analyse best with a lean(oxidizing)flame Cu, Au, Ca.

Others prefer a stoichiometric flame, (no excess fuel or oxidant) Mn, As.

Also there is a group that prefer a reducing flame(excess fuel) Cr, Sc, Ti.

  1. Nebuliser sample uptake rate; air-acetylene does best with high uptake rate while nitrous-acetylene requires a slower uptake.
  2. Burner rotation angle; can be used to detune high concentration samples.
  3. Hollow cathode lamp position.


New Release Atomic Absorption Spectrophotometers