Question

Why don't atomic emission spects. detectors have to be positioned 90 degrees like a molecular fluor. spect detector?

Answer 1

The source lamp for atomic/molecular fluorescence is mounted at an angle to the rest of the optical system, so that the light detector sees only the fluorescence in the flame and not the light from the lamp itself.

It is advantageous to maximize lamp intensity since sensitivity is directly related to the number of excited atoms which in turn is a function of the intensity of the exciting radiation.

Atomic emission spectrometry measures how much of a particular wavelength of light gets emitted by a sample. The wavelength of the atomic spectral line gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element.

In atomic emission spectrometry atoms moving toward the detector emit wavelengths that are slightly shorter than the wavelengths by atoms moving at right angels to the detector. This difference is a cause for doppler shift but can be reversed for atoms moving away from the detector.

The net effect is an increase in the width of the emission line which is more important to determine the quantities of elements in the sample by using the intensity of light emitted by plasma, spark, flame or arc.. For precisely the same reason, the Doppler effect also causes broadening of emission lines. This type of broadening becomes more pronounced as the flame temperature increases because of the increased rate of motion of the atoms.