stonecold's chem questions :)

[stonecold]

thanks longy.  when you did exams, was there much drawing of complicated organic molecules?

[longy1991]

nah nothin mate. We had to draw a chloroalkane, and i also think there was aspirin or something. But nothing as complex as the full structure of a nucleotide or anything. Expect something a little simpler like amino acids, or a small amino acid sequence.

[stonecold]

okay so is this all i need to know about about the monochromator and light pulsing:

-monochromator ensures only a specific chosen wavelength of light reaches the detector
-light pulsing/chopping allows for the detector to distinguish between light that originated from the light source, and light that was emitted by excited atoms

i'm just a little confused as the text goes into very little detail about these things.

i think i get the monochromator, but i don't really understand how light chopping works, just the definition.  how is the detector able to tell where the light comes from, and discard the light that has come as photons from excited electrons?

obviously some of the light emitted form atoms will not match the light input, so it will be filtered out by the monochromator, however some photons will be released that are of the same wavelength as the light source.  i'm pretty sure it is light chopping that eliminates this, but i just don't really know how...

[superflya]

yea im also a lil unsure as to why the light is chopped/pulsed through...

heres the reason monokekie gave:
After absorbing the light energy, an atom would then release the quantum of energy, which sometimes emits light. In AAS, we want to know the amount of light absorbed by the atoms, but not the amount of light released by them. The light emitted may lead to biased experimental results. To eliminate the bias, the light is therefore chopped. So that less light released by the sample will be detected.

[stonecold]

yeah, i just want to know HOW it works, it's doing my bloody head in lol!

[stonecold]

okay, think i've got it.  feel free to add or correct me if i'm wrong.
part of this is from tsfx notes...

the detector only measures pulsed light, so say your light source releases 100 units of pulsed green light.  Your sample then absorbs 10 units of this pulsed light, and releases 5 units as red light and 5 units as green light.  the light emitted as photons is continuous, and not pulsed, so the detector ignores it even though it may be of the same colour.  So the amount of pulsed light reaching the detector is 90 units, which means 10 units were absorbed. 

Also the red light would be removed by the monochromator.

i think (hope) that is it lol...

[m@tty]

Yeah, that's what the TSFX lady said at the free lecture. Sounds right.

[simonhu81292]

BINGO...
yep .. that's right stonecold .. if you read the TSFX notes ..
there analogy was quite comprehensible ...
the detector only detects the intensity of the light emitted from the sample ....
therefore you'll need a monochromator to right freselect the quency that was emitted by the original device at the start.
hope that helps  

[stonecold]

okay, thanks all.  and is light pulsed for both AAS and UV-Vis Spec?

[vexx]

okay, thanks all.  and is light pulsed for both AAS and UV-Vis Spec?

Pretty sure just AAS.

[fady_22]

okay, thanks all.  and is light pulsed for both AAS and UV-Vis Spec?

Pretty sure just AAS.

Nope, pulsed for both. Have a look at pg. 86 in heinemann.

[vexx]

^ it says the light is a "narrow beam" i assumed a light beam meant it didn't pulse and was just a beam.. doesn't say anything about pulsing

[stonecold]

i think it would have to be both also, because it just makes sense.  they both absorb light, and both therefore have the potential to release light, which could enter the detector and alter the results.  

edit: maybe not, as AAS effects the valence electrons, whilst UV-Vis effects the electrons in the molecule.(what exactly does electrons in the molecule mean?)  Does that mean that it can still emit light?

[simonhu81292]

it still holds the principle that they both absorb light as stonecold said ...
therefore what fady said is right . .

[fady_22]

The book says this:
(about the double beam scanning spectrophotometer) "the beam of light is rapidly chopped and passed alternately through the reference and sample cells".