[sanado]

Hey guys, was just wondering what is the purpose of the flame in AAS. I no that it atomizes the sample, turning everything to ground state, and then these atoms become divided among different regions of the flame.

However, wouldnt these atoms be in an excited state from the flame. Therefor, wouldn't this reduce the amount of absorbtion by the photons from the light, since the atoms are all ready excited.

Also

Why do the atoms have to be in groundstate, why cant they remain in solution and have the photons passed through it

[Alpha-Omega]

In FAAS you have a flame.  The flame has different regions.  In FAAS a sample is aspirated (a fine spray of sample is aspirated into the flame).  In this flam the sample will form neutral atoms, ions, and molecules.  Basically, there is a mix of states in the flame.  Upon introduction, the sample solution is dispersed into a fine spray, the spray is then desolvated into salt particles in the flame and the particles are subsequently vaporized into neutral atoms, ionic species and molecular species. 

“All of these conversion processes occur in geometrically definable regions in the flame.”

What the analyst wants to do is optimize the number of ground state atoms (neutral atoms in the flame); because, the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame, any factor that affects the ground state population of the analyte element can be classified as interference.


“It is therefore important to set the instrument parameters such that the light from the source (typically a hollow-cathode lamp) is directed through the region of the flame that contains the maximum number of neutral atoms. The light produced by the hollow-cathode lamp is emitted from excited atoms of the same element which is to be determined.”

This is accomplished by adjusting the position of the lamp relative to that region of the flame containing the most neutral (ground state) atoms. 

The radiant energy corresponds directly to the wavelength which is absorbable by the atomized sample. This method provides both sensitivity and selectivity since other elements in the sample will not generally absorb the chosen wavelength and thus, will not interfere with the measurement. To reduce background interference, the wavelength of interest is isolated by a monochromator placed between the sample and the detector.

In flame AAS  a sample is aspirated into a flame using a nebulizer. The flame is lined up in a beam of light of the appropriate wavelength. The flame (thermal energy) causes the atom to undergo a transition from the ground state to the first excited state. When the atoms make their transition, they absorb some of the light from the beam. The more concentrated the solution, the more light energy is absorbed.


Job of the hollow cathode lamp:
Provide the analytical light line for the element of interest
Provide a constant yet intense beam of that analytical line

Job of the nebulizer:
Suck up liquid sample at a controlled rate
Create a fine aerosol for introduction into the flame
Mix the aerosol and fuel and oxidant thoroughly for introduction into the flame

Job of the flame:
Destroy any analyte ions and breakdown complexes
Create atoms (the elemental form) of the element of interest
Fe0, Cu0, Zn0, etc.

Job of the monochromator:
Isolate analytical lines' photons passing through the flame
Remove scattered light of other wavelengths from the flame
In doing this, only a narrow spectral line impinges on the PMT.

Job of the photomultiplier tube (PMT) :
As the detector the PMT determines the intensity of photons of the analytical line exiting the monochromator

To add just a little more per your request:

Explanation of Process in FAAS: 

The spectrum obtained with the flame is due to the following order of events. The solution of the metal salt in question is sprayed into the flame and the solvent evaporates leaving the finely powdered salt. The salt is vaporized, atomized, and a valence electron is raised to a higher energy state. The energy emitted when this electron drops down into a vacant lower level is given off as radiant energy of a wavelength (determined by the Planck-Einstein relationship: Delta E = hv = hc/lambda).

The technique of flame atomic absorption spectroscopy (FAAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 ^oC.

Different flames can be achieved using different mixtures of gases, depending on the desired temperature and burning velocity. Some elements can only be converted to atoms at high temperatures. Even at high temperatures, if excess oxygen is present, some metals form oxides that do not redissociate into atoms. To inhibit their formation, conditions of the flame may be modified to achieve a reducing, nonoxidizing flame.

During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths,

The characteristic wavelengths are element specific and accurate to 0.01-0.1nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device such as photonmultiplier can detect the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.

A cathode lamp is a stable light source, which is necessary to emit the sharp characteristic spectrum of the element to be determined. A different cathode lamp is needed for each element, although there are some lamps that can be used to determine three or four different elements if the cathode contains all of them. Each time a lamp is changed, proper alignment is needed in order to get as much light as possible through the flame, where the analyte is being atomized, and into the monochromator.