Monochromators

[shinny]

I'll keep this short; why is the monochromator put after the sample in AAS, but before the sample in UV-Vis? I got a rough idea but I'ld like some certainty

[Mao]

the monochromator is a wavelength selector that passes through only a specific frequency of light.

In UV, the wavelength that has the most absorption is used for the actual tests, hence we need to select the wavelength prior to it passing through the sample [as a mixed range of wavelengths would mean some of each would be absorbed to different levels, i.e. very confusing end result]
also keep in mind that this occurs in a completely "dark" environment [no ambient radiation], so monochromator is not needed to single out a frequency for the detector [whereas AAS does]

in AAS, only specific frequencies are emitted by the cathode lamp [that matches the metal], which matches the exact quantum of energy that promotes electrons [to different levels], hence the worry about competiting wavelength is no more, and there is no exact need for the monochromator to be put before the sample [the cathode lamp is specific].
however, the sample is sprayed in flame, and flame [combustion] releases light [as we can see], this is just one amongst many ambient "noise" added to our specific frequency tested for absorbance in AAS, hence we add the monochromator to ensure that ONLY this frequency is recieved in the detector, not any ambient radiation.

[kj_]

i agree with Mao above

but from what i've learnt, in AAS, you have a cathode lamp which provides specific frequencies of the atom under analysis - but this is not one specific wavelength;
like any other absorption technique, we are required to pick the specific wavelength which absorbs the most - although the lamp emits frequencies which only match the metal - this is not one specific frequency

.... -hopes i'm not completely off the plot here-