Thin Layer Chromatography and AAS (1 Viewer)

[Aerlinn]

With Thin Layer chromatography, why is it that it can be used with corrosive s amples and materials as the adsorbent layer because the adsorbent layer is made up of inorganic substances?

Also... with Atomic Absorption Spectroscopy, why can it be used to identify many of the metals that can't be exciting using atomic emission spectroscopy? In fact, does anyone know where I can find some more info about the latter?

If someone can explain these to be, it'd be great... ^_^

 

[xiao1985]

iirc, TLC uses silica + aluminium... hence inorganic...

usually you should not mix corrosive substance with organic substance for safety reasons... first thing comes into mind is acetone + HCl = kaboom + lots of smoke and pple injuired...

in AAS you excite the electron using photon... in AES you excite through other means, such as thermal. Using thermal to excite electrons are not very accurate and rather crude... that's why AES are usually qualitative tests. i speculate, that this is why some elements cannot be excited through AES. they'd decompose before the elecrons are excited.

flame test is a form of AES

 

[Aerlinn]

Don't you mean, usually you should not mix corrosive substances with inorganic substances? Otherwise the logic doesnt follow on.

Using thermal to excite electrons are not very accurate and rather crude

Why's that?
^_^

 

[xiao1985]

corrosive + organic stuff = no no
corrosive + inorg = maybe ok... depends on situations...

therefore, TLC maybe used on corrosive materials... though i myself find this arguement rather handwaving... can you give me more context to the question?!

using flame to excite electrons are not such good ideas... reason being that if the energy given is too much or too little, the atom will not absorb it, and hence will not be excited...

the electron only ever absorb just enough energy for it to jump from one shell to an other...

hence using flame to excite electrons is not a good idea... (not all atoms are excited)

 

[Aerlinn]

Ah, thanks for explaining that, makes much more sense now. Though when we use bunsen burners for flame tests, a flame colour is always produced. Is that because they only select elements that can be excited by the flame, and that the bunsen flame (in this case the hottest blue one) always stays the same temp, n doesn't change? 'Cause you said electrons only absorb an exact amount of energy.

As for more context, well, it just came from a text which was talking about thin layer chromatography, I think in general (you use the same materials for all TLC processes... dont you...?), and I was just curious, 'cause it didn't explain the stuff in much detail. Bit confusing.

 

[xiao1985]

flame temperature flucutates alot... hence the energy it provides to the metal atom fluctutates as well... as a result, it cannot be a reliable method to excite the metal atom, despite the fact that it appears blue at all time to naked eyes.

for TLC, you use the same material, yes... usually you use aluminium coated with silica on one side (i think).

 

[Aerlinn]

So, since flames are not reliable in exciting atoms, do the flame colours that appear mean that only some of the atoms in a sample are excited?

 

[xiao1985]

correct... but even then since all metal atoms will give off a single colour, you still see a unified colour (eg, green for cu, lilac for K etc)

 

[Aerlinn]

I see... that makes sense, but there lies just a small catch.
First, if the atoms excited gave off the same colour, wouldnt that imply that with each atom, the sum of all the photons of light the electrons gave off when going back to the ground state are the same? If the photons are the same, wouldn't each atom's electrons be jumping from excited state across the same energy levels?
The atoms of each element are the same (or else I'm wrong), and the above seems to prove this, but this fact still doesn't seem right when you think of that fact that not all the atoms are excited. If the atoms of an element are the same (and give off a unified flame colour too), doesn't it then imply that each atom would need the same energy to excite?

So why then arent all the atoms excited? Unless I'm completely off the hook (which is possible) or... there isn't enough energy in a bunsen burner to do this...?

(I tend to be a somewhat inquisitive person, heh xD, if you hadnt yet noticed *big grin* )

 

[xiao1985]

i am not sure what your question is, but assume the following cirumstance, see if it makes sense:

we talk about calcium... in an AES camber, there are 20 calcium atoms

we use flame to excite the atoms... but only managed to excite 15 of them

"ok" we say to our selves " fair enough... so only 15 are excited and only 15 will emit a photon"

and now we also noticed, 3 of the 15 are excited from ground level (1st level) to 2nd level; 9 of them from ground level to 3rd level and 3 of them from ground level to 4th level.

which means, when fall back to ground state, 3 of them will emite the energy difference between 1st and 2nd level, 6 of them will emite energy difference bewteen 1st and 3rd level and 3 of them will emite 1st and 4th...

which means you should observe 3 sorts of photons: one with small amount of energy only, one medium and one large respectively.

and indeed we do, most AAS and AES of metal atoms absorb and emit at a number of wavelengths.

so what happen to the 5 which is not fortunate enough to be excited?! they remain unexcited... so they don't glow at all...

 

[Aerlinn]

Ok, so I was going off on tangents It makes sense. So the main thing you are saying is that different atoms in a metal sample randomly absorb different energies
I'm fairly new to this so bear with me... a photon=specific wavelength of light ?

:wave:

 

[xiao1985]

photons are light... a photon can have a specific energy carrying in it... and that energy is reflected through the wavelength of the light

and for the first point, pretty much...

 

[Aerlinn]

I don't quite understand the point about photons...

 

[xiao1985]

it's becoming mroe physics than chemistry now...

but the light (from light bulb etc) you see, are really trillions and trillions of particles called photons...

although you've being taught they are waves, but eistein, in his experiment with photoelectric effect deduced that light has a particle property... so he named it wave particle duality of light particle and called it photon...

in chemistry, all you need to comprehend is that -> giving different energy gaps, light of different wavelength will be produced... larger the gap -> more energy -> shorter the wavelength...

 

[Aerlinn]

K. 'energy gap'=?

 

[Forbidden.]

Damn you're fast you are already up to Chemical Monitoring & Management and I thought my class was fast, unless you are learning ahead of your classmates.

it's becoming mroe physics than chemistry now...

but the light (from light bulb etc) you see, are really trillions and trillions of particles called photons...

although you've being taught they are waves, but eistein, in his experiment with photoelectric effect deduced that light has a particle property... so he named it wave particle duality of light particle and called it photon...

in chemistry, all you need to comprehend is that -> giving different energy gaps, light of different wavelength will be produced... larger the gap -> more energy -> shorter the wavelength...

That's another reason why every senior student should study Physics, I'm not exaggerating.

Anyway photons = particles of light.
Just like protons and electrons are subatomic positive and negative particles respectively.

 

[xiao1985]

say you jump from ground state (energy level = 1) to 3rd state (energy level = 3) the energy gap will be the energy difference between 1st (ground) state and 3rd state...

 

[Forbidden.]

Refresh my memory ... do atoms emit light when electrons jump to a higher energy state or orbit and absorb light when dropping to a lower energy state or orbit ?

 

[Aerlinn]

f3nr15-- The other way around
xiao1985-- I get it... ^^