Difference between AAS and UV-Visible Spectroscopy?

[Chromeo33]

They're the same!



But the book suggests otherwise, but I just don't see it..

[Greatness]

AAS analyses metal atoms, whilst UV spec analyses moelcules. - The idea behind each are similar tho.

[azngirl456]

AAS analyses metal atoms, whilst UV spec analyses moelcules. - The idea behind each are similar tho.

To be specific, the light source in the uv spec provides an appropriate wavelength for the sample to absorb while AAS, provides the exact wavelength for the metal sample to absorb

[vea]

AAS analyses metals while UV-Visible Spectroscopy analyses coloured compounds and organic compounds. UV-visible spec is only able to provide an appropriate wavelength, compared to AAS which can provide a specific wavelength from its cathode lamp. This makes AAS more accurate than UV-visible spec.

[luken93]

AAS analyses metal ions only, and the light absorbed by the ion-specific lamp excites the electrons to a higher energy level, the intensity of the light on the electrons' way back to ground state is proportional to the concentration. Also, the cathode lamp used is SPECIFIC to the ion being analysed, as it must be a specific wavelength that corresponds to the electron jumps

In UV, the compound itself absorbs the light. It is different to AAS in that the absorbance readings given is due to the bonds in the molecule the same as AAS in that it absorbs a specific wavelength of energy that will excite electrons. The only difference is, some ions absorb wavelengths from the visible spectrum, whereas others absorb wavelengths from the UV Spectrum. Also, the wavelength used is one that is absorbed STRONGLY, but it is not a specific wavelength like AAS. Also, it can be used some ionic compounds (especially transition metals), as well as SOME coloured organic molecules.

Both methods are similar though...

[Chromeo33]

So, from what I've gathered, the differences are:
- The types of samples under analysis (AAS = Metal ions only, UV = compounds)

- The wavelengths 'shot' at the sample (AAS = A specific wavelength of EM radiation, UV = An appropriate 'range' of wavelengths)

- What actually absorbs the EM radiation (AAS = Metal sample's electrons absorb, UV = The bonds of the compounds absorb)

- The method/instruments used to carry out their analysis (AAS = Cathode lamp, UV = No cathode lamp)


Would someone be able to confirm that? Or even add to that?

[luken93]

AFAIK, I'd say that's a very good summary

[Water]

Luken, is it the bonds that absorbs the energy? For UV, as the molecules would require discrete electronic energy in absorbance to change to higher energy levels. This would be designated by the monochromator. So its the molecules that absorbs the energy, how is it particularly the bonds?

While the Infrared Spectroscopy, would be the movement of change in the bondings of Molecules , via vibrational energy.


PS: Seems Bonds is something new to learn for HS

[luken93]

Luken, is it the bonds that absorbs the energy? For UV, as the molecules would require discrete electronic energy in absorbance to change to higher energy levels. This would be designated by the monochromator. So its the molecules that absorbs the energy, how is it particularly the bonds?

While the Infrared Spectroscopy, would be the movement of change in the bondings of Molecules , via vibrational energy.

My bad, you are correct. I was thinking about IR 

Sorry Chromeo - AAS and UV are similar for the "What actually absorbs the EM radiation", the only difference there is that some ions absorb energy in the visible spectrum (AAS), whereas others absorb a specific energy that is from the UV Spectrum

And yes, IR absorbs according the bending/stretching of bonds

[Asx4Life]

UV analysis depends on the bonds of the molecule while in AAS, it depends on the energy required to excite electrons. Also UV is performed under room temperature while AAS is performed under high temperatures.
Another thing, in AAS sample is dissolved in strong acid while in UV, sample is dissolved in a solvent

[Mao]

I see a lot of misconceptions in this thread. I will go through each post individually and point them out. But here is a summary (I shall use the same concise format as Chromeo33, with a bit more explanations)

First and foremost, UV-VISIBLE shines VISIBLE light (most of the time), only occasionally do we use it in the UV range. Absorption mostly happens in the VISIBLE region. It is not restricted to UV.

- The types of samples under analysis (AAS = Metal ion solutions only [if a solid sample is being analysed for composition, it must be dissolved in solution first, usually in acid], UV-vis = solutions of coloured compounds)

- The wavelengths 'shot' at the sample (AAS = the exact emission spectrum of the metal being analysed [note that analysis occurs as neutral atoms, not as ions], UV-vis = a specific wavelength in the visible and UV region)

- What actually absorbs the EM radiation (AAS = Metal [neutral atoms] sample's electrons absorb, absorption promotes an electron in ground state to an excited state, UV-vis = The electrons in bonds of the compounds absorb, absorption promotes an electron in ground state of the bond to an excited state of the bond***)

- The method/instruments used to carry out their analysis (AAS = Cathode lamp, UV-vis = tungsten filament, similar to a light-globe, emitting a large range of wavelengths, a narrow range selected with a monochromator)


***The principle behind UV-vis is as follows, (university level theory)

-Remember atomic orbitals (1s, 2s, 2p, 3s, 3p, 3d, etc), these are discrete energy levels associated with ATOMS

-When atoms come close enough to each other, these atomic orbitals overlap to form molecular orbitals. These 'overlapped' orbitals (or rather, combined orbitals) often have lower energy than their atomic counter-parts, thus more stable. The result of this energetically-favoured overlapping is what we call a 'bond'.

-In a molecule, it can be thought of as a bunch of electrons needing to be allocated to a very complex organization of nuclei. That is, the electronic structure of a molecule is found in a very similar manner to an atom (in fact, the same quantum physics equation is used). There are orbitals just like atomic orbitals, electrons fill in these orbitals the same way. Electrons are excited and jump between energy levels in the same way

-Thus when we shine light at these electrons, the absorption we observe are just like the mechanism in AAS. The difference is that all atoms in the molecules can vibrate around their position. Thus the molecular orbitals are not 'fixed', these change over time due to vibrations in the molecule, and so the energies vary. This means we do not see discrete absorption levels. Rather, we see somewhat broad ranges of absorption due to contribution by molecules that are not at their optimal conformation. We generally select the wavelength of the optimal conformation, which tends to be peaks in the absorption spectrum.

--Simplified version: there are atoms in bonds. These atoms absorb visible light to jump up energy levels in the bond. UV-vis shines visible light and measures absorption.

[Mao]

AAS analyses metals while UV-Visible Spectroscopy analyses coloured compounds and organic compounds. UV-visible spec is only able to provide an appropriate wavelength, compared to AAS which can provide a specific wavelength from its cathode lamp. This makes AAS more accurate than UV-visible spec.

I don't completely agree with that. AAS is definitely more sensitive (has a very low detection limit), but in terms of accuracy UV-vis and AAS can both give very accurate results. UV-vis can't achieve the same sensitivity as AAS due to limitations in the light-source (the source isn't very bright after you filter out 99.99% of the light with the monochromator, whereas AAS doesn't have the same problem).

AAS analyses metal ions only, and the light absorbed by the ion-specific lamp excites the electrons to a higher energy level, the intensity of the light on the electrons' way back to ground state is proportional to the concentration. Also, the cathode lamp used is SPECIFIC to the ion being analysed, as it must be a specific wavelength that corresponds to the electron jumps

Not ion. Specific to the neutral species of the element. Otherwise correct.

In UV, the compound itself absorbs the light. It is different to AAS in that the absorbance readings given is due to the bonds in the molecule the same as AAS in that it absorbs a specific wavelength of energy that will excite electrons. The only difference is, some ions absorb wavelengths from the visible spectrum, whereas others absorb wavelengths from the UV Spectrum. Also, the wavelength used is one that is absorbed STRONGLY, but it is not a specific wavelength like AAS. Also, it can be used some ionic compounds (especially transition metals), as well as SOME coloured organic molecules.

-It is the electrons in bonds that absorb
-You have correctly recognised that some molecules can absorb in the visible and UV spectrum.
-The wavelength used is generally one specific to the optimal structure (see above post). It is generally the case that this is the most strongly absorbed wavelength.
-It can be used on ionic compounds such as coordination complexes (e.g. Cu(OH2)6), since there are orbital overlapping/electron sharing between the metal centre (Cu) and ligands (H2O).
-It can almost be guaranteed that all coloured soluble compounds can be analysed by UV-vis. Some insoluble compounds may even be analysed if a dispersed colloid can be prepared.

UV analysis depends on the bonds of the molecule while in AAS, it depends on the energy required to excite electrons. Also UV is performed under room temperature while AAS is performed under high temperatures.
Another thing, in AAS sample is dissolved in strong acid while in UV, sample is dissolved in a solvent

Dissolving in strong acid applies for AAS if you want to analyse a solid sample (since solid metal doesn't dissolve in water, and we require an aqueous ion solution). If you were looking for the mineral concentration in mineral water (already aqueous metallic ions), you wouldn't need to add any acid. That sample can almost be put under analysis by AAS without any further preparation.