Concerns about AAS (1 Viewer)

[Ganondorf69]

Ok so electrons can get excited out to a higher energy level of their normal energy levels. After a short time the excited electrons will fall back to their ground state, but also release the same amount of energy absorbed.

For AAS, what causes the electrons to get excited? Is it the heat source from the nebuliser or the light source from the hollow cathode lamp?

I will have a follow up question after this ^.^. Thank you

 

[ProdigyInspired]

I'm pretty sure the lamp heats it up, and as it cools down it emits the energy.

"As they return to their ground state, the energy originally absorbed is released in the form of a photon (light energy)".

Because the combusted metal/mixture in the fire is turned into atoms rather than ions.

What you want really is the intensity of the light compared to another controlled intensity.

 

[Ganondorf69]

Alright, follow up question.

So the concentration is determined by how much light is absorbed by the atoms. What happens to energy released from the atoms? Does it get dispersed, and is there a chance that it can affect the results?

Thank you again!! Notify me if you don't understand the question

 

[ProdigyInspired]

Actually I'm unsure now, I think its the flame.

http://www.ausetute.com.au/aas.html

"Light with specific frequencies is absorbed by different metals when they vaporize in a flame.
The energy absorbed excites electrons, moving them from their ground state to a higher energy state."

Which is confusing. So I'm guessing the heat energy excites the electrons, and then it is further released in the form of photons to the lens/monochromator.

 

[ProdigyInspired]

Alright, follow up question.

So the concentration is determined by how much light is absorbed by the atoms. What happens to energy released from the atoms? Does it get dispersed, and is there a chance that it can affect the results?

Thank you again!! Notify me if you don't understand the question

AAS usually occurs in a closed environment, so I'm guessing the energy doesn't get dispersed as it's concentrated into the beam of light.
The change in energy is what defines AAS as it measures how much concentration you have for your metal after passing it through the flame.

 

[ProdigyInspired]

I did a bit more research and I'm now sure its the light.

The flame is purely used to do atomisation. The lamp excites the metal ions and emits a light beam in excess. The flame then absorbs the light through its atomised metal ions.

The metal atom in the fire has its electrons excited since it absorbs (absorption spectrum).
The metal in the cathode lamp has its electrons excited since it emits (emission spectrum).

 

[ProdigyInspired]

I never type like this sorry, but its easier to get my point across.
Clarification

The lamp emits photons from the excitement of its electrons in the atom it energies through voltage. The fire then absorbs this light and the specific wavelengths SPECIFIC to that metal. So therefore, the wavelengths or frequencies that the metal should have are decreased. The monochromator then chooses one of these REDUCED wavelengths/frequencies and puts it into a photomultiplier. The difference between the standard wavelength/frequency is compared to the reduced wavelength, thus how much was absorbed is the measure of concentration.

 

[Ganondorf69]

Thank you!! I appreciate the amount of dedication you put into researching just to answer my question . One last query sorry haha, does the monochromator choose a specific wavelength? In my notes it says "The wavelength selected is the one at the strongest intensity, because there will be a greater variety of intensities with different concentrations in the sample, and thus greater accuracy." Is this true?

 

[ProdigyInspired]

Thank you!! I appreciate the amount of dedication you put into researching just to answer my question . One last query sorry haha, does the monochromator choose a specific wavelength? In my notes it says "The wavelength selected is the one at the strongest intensity, because there will be a greater variety of intensities with different concentrations in the sample, and thus greater accuracy." Is this true?

It's fine, I'm revising for Chemistry myself.

Yes the monochromator separates the light into different wavelengths and selects a specific one.

I'm not 100% sure, but different concentrations and different substances do indeed have different wavelengths, so this may not be true for every substance.

If you take into account the emission spectrum, the wavelength of highest intensity would be when emission is strongest right?
Similarly the absorption spectrum would have the highest intensity wavelength at the point of highest absorption, therefore the highest intensity wavelength would show the extent of concentration as it shows how much has been absorbed by the flame.

If you look at both an emission and absorption spectrum, they'll match up. This is similar to how the lamp works, as it provides energy to an atom, the same amount of energy is released. This means that the same wavelengths/frequencies are released.

So therefore if absorption is the most intense at one wavelength, emission is the most intense at the same. Therefore the most intense emission wavelength i.e. the light beam would also give you the greatest point of absorption, which would be the wavelength that was mostly absorbed since they are the same atoms.

I'm not exactly sure what Shah is talking about for the "greater variety of intensities with different concentrations", as some versions of AAS simply place the metal in the fire. I think he's considering the sample to consist of other chemicals, so the lower intensity wavelengths would be of the contaminants and not the desired substance.

 

[Ganondorf69]

Yeah I see. I don't think we have to go in too much detail about the the monochromator anyway haha. Thanks!