ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: casbanjo on October 28, 2014, 06:08:12 pm

Title: Why are ions vapourised in AAS?
Post by: casbanjo on October 28, 2014, 06:08:12 pm
Why do we have to vapourise the metal ions in AAS before they will absorb light energy? It seems that in UV Vis spec, the compounds will absord light energy without being vapourised.

Please explain the vapourisation process in AAS

Thank you.
Title: Re: Why are ions vapourised in AAS?
Post by: psyxwar on October 28, 2014, 07:28:51 pm
I'd imagine its because the hollow cathode lamp is made from metals in the atomic form rather than ionic form, and thus the light it produces is only going to be absorbed exactly by the atomic form of the atom (ions absorb light differently, for example difference in color of say Cr3+ and Cr2O7 2-)
Title: Re: Why are ions vapourised in AAS?
Post by: casbanjo on October 28, 2014, 08:42:21 pm
Quote from: psyxwar on October 28, 2014, 07:28:51 pm
I'd imagine its because the hollow cathode lamp is made from metals in the atomic form rather than ionic form, and thus the light it produces is only going to be absorbed exactly by the atomic form of the atom (ions absorb light differently, for example difference in color of say Cr3+ and Cr2O7 2-)

Ah okay I was beginning to think it must be that. Thanks  ;D
Title: Re: Why are ions vapourised in AAS?
Post by: lzxnl on October 28, 2014, 08:43:50 pm
Quote from: casbanjo on October 28, 2014, 06:08:12 pm
Why do we have to vapourise the metal ions in AAS before they will absorb light energy? It seems that in UV Vis spec, the compounds will absord light energy without being vapourised.

Please explain the vapourisation process in AAS

Thank you.

As psyxwar has hinted, AAS works only if what you're analysing absorbs at the same wavelength as that which is emitted from a metal lamp. These metal lamps themselves only work on pure metals. What your flame does is reduce the metal ions to elemental form. In UV Vis, you're analysing for the metal ions; in AAS, you're analysing for the metal atom. UV Vis would be using different wavelengths to AAS. Problem with UV Vis is that you'd have to run a test of which wavelengths in solution would be absorbed well as these can change even if the ion is the same.

As anyone doing uni chem would know, copper (II) nitrate dissolved in water has a distinctly different colour to [Cu(NH3)6](NO3)2 (aq). So, using UV Vis, you would have to scan and find the right wavelength. Using AAS, you just inject the sample without any extra modifications.

I've done AAS; it is REALLY simple xD