[ajax0604]

I've been reading an online resource which explains why transition metal solutions analysed in UV-visible spectroscopy are coloured but non-transition metal solutions aren't, and I came across a paragraph which puzzled me:

Non-transition metals don't have partly filled d orbitals. Visible light is only absorbed if some energy from the light is used to promote an electron over exactly the right energy gap. Non-transition metals don't have any electron transitions which can absorb wavelengths from visible light.

In atomic absorption spectroscopy, if you had a hollow cathode lamp with a sodium cathode, it would emit a wavelength around 590nm which the sodium atoms in the sample will absorb? So non-transition metals do absorb visible light? 

[marquis]

UV-Vis is commonly done with liquids at near room temperature. 

Under these conditions, you will have trouble detecting non-transition
metals with UV-Vis (There are some ion chromatography instruments
that use a UV-vis detector and a drop in absorbance to detect these ions-
basically because they won't absorb in the UV-Vis). 

In an excited state, you can form the energy transitions that absorb
in the UV and Vis.  You can do this by AA.  These transitions also occur
in plasma.  This is how elements such as sodium are detected in the sun.

Hope this helps.

[ajax0604]

Thanks for your reply. Could you clarify one point? When you said that "in an excited state, you can form the energy transitions that absorb in the UV and vis", does this mean that atoms must be excited by the flame in AAS otherwise the energy associated with UV/vis waves will not be sufficient to promote the electrons to a higher energy level? 

[Corribus]

In AAS, the flame doesn't really excite atoms so much as blast the sample apart (atomize) so that atomic spectral lines can be observed.  In condensed phase, atomic absorption lines are usually "washed out" by the various molecular vibrations and so forth. When the sample is atomized, atoms become well separated in the gas or plasma state, where their absorptions can be easily observed. That's a pretty big simplification but maybe you get the idea.  (There is likely atomic excitation as well, with absorption originating from excited atoms. Starting with the flame temperature you could probably determine this with the partition function approach. I'm not really sure what percentage of atomic absorption lines in the AAS experiment arise from ground-state atoms and excited-state atoms. This will probably depend on the element.)

Compare this to ICP-AES, which uses an RF coil (large electric field) with temperatures four to five thousand degrees hotter. In this case, samples are not only atomized but electronically excited (and ionized) as well, which then decay back to their respective ground electronic states by emission (i.e., fluorescence).