AAS

[Shark 774]

Hi guys,

Why is it necessary to turn the ions into free atoms in order for them to be analysed by AAS??

I used to know this, but seem to have forgotten after a year

Thanks.

[funkyducky]

I'm not 100% sure, but as far as I understand, it's got to do with the fact that ions of the element under analysis will have fewer electrons than the atoms of the same element (fewer because you only use AAS to analyse metals), so it would not necessarily absorb and emit the same wavelengths of light, because the outermost electron shell in the ground state will be empty. Since you are using the atomic element in the lamp to select and pulse only the wavelengths that will excite electrons for that element, your sample must be atomised.

I'm really sorry if that's confusing or vague...I can elaborate if you need.

[thushan]

I think funkyducky has hit this on the head -

So let us take the example of barium chloride (BaCl2) and we want to work out its concentration in a solution. The species present are Ba2+ and Cl-, but we are interested in the barium. Atomising the sample renders Ba (g) and this is the stuff that absorbs the light that we are interested in. The thing is - Ba2+ and Ba have different electron configurations and would hence have different electronic transitions and hence would absorb light differently.

I am not sure about this, but my theory is that since say Cs+ and Ba2+ have the same electronic configurations, they would have the electronic transitions and hence very very similar absorbance characteristics (i dont think it's the same as core charge of Cs and Ba are different - the electrons would be closer to nucleus in Ba2+). That means it would be very hard to differentiate btwn Cs and Ba if the samples were not atomised and were hence absorbed as ions.

Same goes for Na+/Mg2+ , etc.

[Mao]

I think funkyducky has hit this on the head -

So let us take the example of barium chloride (BaCl2) and we want to work out its concentration in a solution. The species present are Ba2+ and Cl-, but we are interested in the barium. Atomising the sample renders Ba (g) and this is the stuff that absorbs the light that we are interested in. The thing is - Ba2+ and Ba have different electron configurations and would hence have different electronic transitions and hence would absorb light differently.

spot on.

I am not sure about this, but my theory is that since say Cs+ and Ba2+ have the same electronic configurations, they would have the electronic transitions and hence very very similar absorbance characteristics (i dont think it's the same as core charge of Cs and Ba are different - the electrons would be closer to nucleus in Ba2+). That means it would be very hard to differentiate btwn Cs and Ba if the samples were not atomised and were hence absorbed as ions.

Same goes for Na+/Mg2+ , etc.

Almost.
Firstly, protons makes a big difference in absorption spectrum. The absorption spectrum for Cs+ and Ba2+ are quite different. Electronic configuration is a important factor, but not the only important factor. Each atom has a unique spectrum, a similar thing can be said for ions.
Secondly, due to the cathode lamp we are using as a source, if we were to measure the concentration of Ba2+, our lamp must also be Ba2+. This then implies we need a vapourized sample of Ba2+ in the cathode lamp as the radiation source. This then mean our source must be either be a charged gas (very unstable), or a vapourized salt (very high temperature). Thus not feasible.

[Shark 774]

Ok thanks guys. So Mao, is it a lot easier to have a vaporised sample of Ba atoms rather than Ba2+ ions???

[funkyducky]

I think the Ba atoms would be in the form of a solid metal filament, similar to the tungsten filaments in regular light globes.

[Shark 774]

Ok. And so the reason you can't really analyse non-metals with AAS is because they can't be ions, as stated above, and instead of simply 'giving' the ions electrons to become atoms, you need to rip electrons off them which is a lot harder, right?

[Water]

The flame wouldn't be strong enough to excite the electrons.

I had this graph, I"ll pick it up sometime next week. But it shows the various spectroscopy techniques for various atoms/molecules/ions,  AAS isn't apprioriate for ions.

PS: UV Would be appropriate though, as in can occur for atoms, ions or molecules.



PS2: I just watched this AAS Video, you'd have a sample, say Fe2+ Ions, as it goes through the process, before it reaches to become atomic vapour, it is atomized to become Fe Atoms.



PS3: Mao's Answer to the question

Also, removing an electron from anions tend to be much more difficult than adding an electron to a cation.




I suspect that the transfer of electrons occurs because, there isn't enough strength in the light*to actually excite the Fe2+ Ions.

[Shark 774]

Doesn't the "excitement" come from the cathode lamp, not the flame? In AAS the flame is just purely there to cause atomization, I thought..

[Mao]

I think the Ba atoms would be in the form of a solid metal filament, similar to the tungsten filaments in regular light globes.

I stand corrected. This is closer to the actual design of hollow cathode lamp. A solid metal is used. As electricity pumps through, uncharged species of the metal is then vapourized, giving the gas I was talking about. (the working of a HCL is quite different to a light bulb, which uses thermal excitation thus gives a broad spectrum)

The main reason why we want to have ground state atoms is because of the light source. It is very hard to get a sample of cation, then excite them sufficiently to create an adequate light source. We atomize the ions in the sample because the light source is able to generate the spectrum of the uncharged species.

As for why we can't do non-metals, their ground state tend to be very unstable, and I don't think we have the light sources. Also, removing an electron from anions tend to be much more difficult than adding an electron to a cation.

The flame/atomizer cause atomization. The light is emitted by the excitation of metal atoms in the lamp. This light is emitted in excess. The emitted light is then absorbed by the atomized metal atoms in the flame, and an absorbance reading is given.