aas (1 Viewer)

[crammy90]

how many ppm can it test
i know it does the .001 of some
and does it test SO42- because i think i wrote down for acidic environment evidence of oxides of S and N are in small amounts 0.001ppm and aas can be used to detect them
is this an expection to the "Cations only" rule?


and to clarify something
when the modified wavelength from the cathode ray lamp is detected after it is passed through the flame, is the amount of wave-rays detected less as some have been absorbed??
Cause then couldnt you do: Wave-rays of wavelength(initially from lamp) - Wave-rays(detected) = wave-rays that were absorbed and from this you can plot it on the calibrated curve and determine the concentration?
yeh?

 

[danz90]

how many ppm can it test
i know it does the .001 of some
and does it test SO42- because i think i wrote down for acidic environment evidence of oxides of S and N are in small amounts 0.001ppm and aas can be used to detect them
is this an expection to the "Cations only" rule?

from what I know, AAS can detect traces as low as parts per billion (ppb), hence it is very accurate and sensitive in determining concentrations of trace elements.
I think that AAS can only test for the presence of cations... due to the cathode lamps that are used... and the fact that the cations release light energy when they are 'ignited' in the acetylene burner.
I wouldn't say in an exam that oxides of sulfur and nitrogen can be detected using AAS.

 

[crammy90]

ah k thanks heaps
do u have an aswer to the quetsion i just added in?
and what is the acetylene burner? is that just like the flame which exites the molecules in the sample of unknown conc to the higher energy levels where they absorb the waves from the cathode ray lamp?

and how do the cations absorb the wavelengths. Like if the acetylene burner is exiting them to higher energy levels, is it at these higher energy levels that they absorb the wavelengths to reach even higher levels and that in this process when then return to their original energy levels release wavelengths of their own frequency which are blocked by the slit and prism?

 

[danz90]

ah k thanks heaps
do u have an aswer to the quetsion i just added in?
and what is the acetylene burner? is that just like the flame which exites the molecules in the sample of unknown conc to the higher energy levels where they absorb the waves from the cathode ray lamp?

and how do the cations absorb the wavelengths. Like if the acetylene burner is exiting them to higher energy levels, is it at these higher energy levels that they absorb the wavelengths to reach even higher levels and that in this process when then return to their original energy levels release wavelengths of their own frequency which are blocked by the slit and prism?

The acetylene burner basically 'gasifies' the cation solution... so that gaseous ions are present for analysis. The high temperature it provides literally boils the water out, and then 'boils' the ionic solid... forming gaseous ions.
It is the cathode lamp that emits light of a certain wavelength/frequency that the gaseous cations absorb, thus 'exciting' them, making them jump to higher energy levels. The amount of light absorbed, then leaves only a portion of the light emitted.. which is then detected by the monochromator. A computer calculation then determines the Absorbance of the gaseous cation, which is proportional to its concentration.

The higher the absorbance (ie the lower the number of light rays detected by the monochromator, since there are a larger number of gaseous cations present absorbing the light), the higher the concentration of that metal cation in the original sample.

Hope that answers your questions.

 

[crammy90]

that was spot on!
thanks for this
really helpful
just to clarify
the ions dont absorb any energy from the acetylene burner, ONLY the wavelengths of the energy produced by the cathode ray lamp yes?
i guess that makes sense because im guessing if they were to be able to absorb energy from the burner that would cause them to rise energy levels and so they wouldnt be able to absorb the lamps energy as they have already found another source for energy?
yeh?

 

[syriangabsta]

wait sorry now im kinda confused....

so, when the light passes through the flame, what happens? does some of the flames electrons absorb some light? and then the detector absorbs the LEFTOVER light, and uses this info against a callibration curve to determine the amount of cation? :S:S

 

[crammy90]

wait sorry now im kinda confused....

so, when the light passes through the flame, what happens? does some of the flames electrons absorb some light? and then the detector absorbs the LEFTOVER light, and uses this info against a callibration curve to determine the amount of cation? :S:S

DIAGRAM:

(cathod ray lamp)------------Flame(acetylene burner)----------(prism/slit)Monochromator--------sensor
EDIT::::::::::::: i just looked it up. the monochromator is the prism/slit which blocks out other wavelengths. The detector is a separate unit called a Photomultiplier Tube PMT which senses the wavelengths exiting the monochromator which would be the wavelength desired (as the monochromator has already remove the unwanted ones)
NOTE: i fixed up my diagram to include this

NOTE: the monochromator aims at only allowing the wavelengths emitted from the lamp (which is absorbed by the cation being tested for-->has been calibrated) to pass through so only those wavelengths are detected. This has something to do with the fact that (as shown in a flame test) when cations move up and down their energy levels (as they absorb the energy from the lamp) they emitt visible light (if they are able to) in the process. The monochromator(slit/prism) prevents this visible light/any other visible light and wavelengths in its environment from passing through and being detected. I guess they must have to calibrate the monochromator to "tell it" which wavelength to allow (i.e. the wavelength absorbed by the cation being tested for).
​

The wavelengths shon from the calibrated cathode ray through the burner(i.e. flame), pass through the monochromator (filters wavelengths) and is recorded by the PMT. Along the way, the sample to be tested is injected into the flame, where it is vaporised (the solvent removed i.e. water) and the cations (we're not concerned with anions) in gaseous form remain. As the sample is vaporised in the flame and the solvent is removed, when the constituents turn into gaseous cations they are quickly "exited" by the wavelengths being shon from the lamp as they aborb the lamp's wavelengths and then (i think once the lamp is turned off or i dont know how long after) they return to their normal energy levels. All the while, the monochromator has been filtering the wavelengths being recieved on the other side of the burner(flame) - there will be less as the cations have absorbed some when they were exited. Hence why the less wavelengths recieved by the sensor(PMT) = the more absorbed = the higher the concentration
Now, from what i gather, the cations do not get exited by the burnet(flame). The flame acts as the utensil which removes the solvent so the cations can become gaseous cations which can then be exited by the lamp's wavelengths.
i think that answers your question.

oh yeh lol the flames electrons dont absorb energy (i think you mean the flames molecules as electrons dont absorb energy, the atom absorbs it and the electrons within the atom are exited i.e. have enough energy to raise a shell)

 

[danz90]

that was spot on!
just to clarify
the ions dont absorb any energy from the acetylene burner, ONLY the wavelengths of the energy produced by the cathode ray lamp yes?

Correct. The purpose of the oxy-acetylene burner is simply to gasify/vaporize the solution into gaseous ions. The cathode lamp then emits the light of certain wavelength.. which the metal cation absorbs... any 'unabsorbed' light is passed through to the monochromator, which as you said, 'filters' out light etc etc.. then the sensor/detector finally detects how much wavelength has reached the end of the column. The amount emitted and the amount of light absorbed are compared... and the difference is the 'absorbance' - relative to the ion concentration.

But don't worry, we don't need to know AAS operation in GREAT detail. Even in a 7 mark question, you'll only use a few lines explaining how it works. The main thing with AAS in our syllabus is, evaluating its impact upon society and envrionemnt (greater understanding of the effect of trace elements).