Atomic Absorption Spectra

The fundamental relationship in AAS is the Beer-Lambert Law, which relates the absorbance ($A$) to the concentration ($c$) of the analyte.

The law is expressed as A = \log_{10} \left( rac{I_0}{I} ight) = \epsilon b c where $I_0$ is the incident light intensity, $I$ is the transmitted intensity, $\epsilon$ is the molar absorptivity, and $b$ is the path length.

Energy absorption occurs when the photon energy matches the difference between two electronic energy levels: \Delta E = h u = rac{hc}{\lambda}.

Because atomic energy levels are quantized and specific to each element, AAS is highly selective, meaning it can detect one element in the presence of many others.

Check Linearity: Always ensure the absorbance values fall within the linear range of the calibration curve; values above $0.8$ absorbance units often deviate from Beer's Law.

Blank Correction: Remember that a 'blank' (solvent without analyte) must be used to zero the instrument to account for any absorption by the solvent or flame.

Sensitivity vs. Detection Limit: Understand that sensitivity refers to the slope of the calibration curve, while the detection limit is the lowest concentration that can be reliably distinguished from the blank.

Unit Consistency: When using the Beer-Lambert Law, ensure that the units for path length ($cm$) and concentration ($mol/L$ or $mg/L$) match the units of the absorptivity constant.

Chemical Interference: This occurs when the analyte does not fully atomize due to the formation of stable compounds (e.g., calcium phosphate). This is solved by adding a releasing agent like Lanthanum.

Ionization Interference: At high temperatures, atoms may lose an electron to become ions, which do not absorb at the atomic resonance wavelength. Adding an ionization suppressor (like Potassium) prevents this.

Spectral Overlap: While rare in AAS, students often mistake background noise or molecular absorption for atomic absorption. Using a deuterium lamp for background correction is the standard solution.