Classification of Techniques Absorption, Emission and Fluorescence

The atomic lines in the spectrum appear as vertical lines or peaks due to the nature of the transition involved. That is, in molecules an electronic transition is usually accompanied by simultaneous changes in the molecule vibrational and rotational energy levels sometimes all the three energy types may change 

spectral interferences in atomic spectroscopy are less likely than in molecular spectroscopy analysis. In any case, even the atomic lines are not completely monochromatic i.e. only one wavelength per transition). In fact, there are several phenomena which also bring about a certain broadening . Therefore, any atomic line shows a profile (distribution of intensities) as a function of wavelength (or frequency). The analytical selectivity is conditioned by the overall broadening of the lines (particularly the form of the wings of such atomic lines). 

1 Atomic Absorption Spectrometry. Principles of Quantitative Analysis 

For quantitative purposes in AAS, a magnitude called transmittance (T) which relates, for a given wavelength, the intensity (measured by the detector) of the light source (Zq) and the intensity not absorbed which has passed through the atomiser or transmitted light (7) is used  

The amount of light absorbed is a function of the so-called absorption coefficient (A ) and of the optical pathlength in the atomiser cell (ft) k depends on the frequency of the selected analytical line and on the concentration of the analyte absorbing atoms. The general absorbance law (Lambert Beer Bouguer law) relates transmittance (and so measured intensities I and If) to k and b through the following equation  

---