[Baileys]

I'm a little confused about how monochromaotrs are used in AAS.

I understand that a monochromator isolates a narrow range of wavelengths and I understand how this can be applied to AES, but in AAS, if the analyte of interest (e.g. Sodium) is absorbing all of radiation of a specific wavelength based on the fixed energy transitions characteristic to the sodium, then radiation of this specific wavelength will not be present to pass into the monochromator since it's being absorbed. So, the only light passing into the monochromator must be light that hasn't been absorbed by the sodium. Is this correct?

[marquis]

First of all, sodium is a very good element to analyze by AES, but a very poor element to analyze by AAS. 

In AAS, you have a single element lamp in your instrument that generates the specific lines for the element you are looking for.  When you pass the light from that lamp through the flame, you absorb some of this light.  The instrument measures that light and allows you to calculate concentration. 

Most elements have a variety of lines produced by the lamp.  The purpose of the monochromator is to select the specific line you are need.  And the flame will generate some noise (light from the wavelength you are looking at) that the monochromator will help eliminate.

In AES, you measure the light produced by the flame.  The single element lamp is not needed.

Hope this helps.