L6: Atomic Spectroscopy

Atomic Spectroscopy

an analytical technique used to identify elemental components of a sample by measuring the amount of light it absorb or emits

Fundamental Principle of Atomic Spectrometry

Emission of a photon enables a transition from a higher atomic energy level to a lower atomic energy level

Optical Atomic Spectrometry

AES (emission), and AAS (absorption), AFS (Fluorescence) require atomization

Mass Atomic Spectroscopy

toms are converted into positive ions who

charge is analyzed by m/z

X-Ray Absoprtion Spectroscopy

No atomization necessary, based on fluorescence, emission or absorption

Energy Diagrams

• outer electrons are excited, increasing number of outer electrons result in larger number of energy levels

• not every transition is allowed, selection rules from quantum chemistry

• Emission lines observed depend on source temperature

• Lines from ionic species are also visible

Spectra

• Room temperature all atoms in the ground state

• Outer valence electrons of elements are excited to higher energy levels by flame atomization, plasma, electric arc/spark.

• Return to ground state and emit a photon

• Intenser lines correspond to transitions from excited state to ground state

Atomic Absorption Spectra

In hot gaseous medium elements may absorb radiation

at wavelengths corresponding to electronic transitions to excited states

Atomic Fluorescence Spectra

Atoms or ions in a flame may be excited to emit fluorescence radiation by irradiation with a strong light source

Usually measured with a 90° to the excitation radiation

Line broadening Effects

• Uncertainty Effect

• Doppler Effect

• Pressure effects due to collisions

• Electric and magnetic field effects

Uncertainty effect

he Heisenberg uncertainty principle states that the exact energy of an atom cannot be found with perfect precision when the lifetime of the excited state is finite. A shorter excited-state lifetime produces greater uncertainty in energy, resulting in a broader range of possible energies and therefore a broader spectral line.

Doppler Effect

Atoms are constantly in motion. When they move toward the transducer, it shortens the observed wavelength, and when they move away from the transducer, it lengthens the wavelength. This, along with the differing speeds of theatoms, broadens the wavelength.

Pressure

Higher pressure increases collisions. Those collisions can cause energy exchange, which changes the wavelengths that are absorbed and emitted, therefore broadening spectral lines.

Electromagnetic Fields

Electric or magnetic field can alter energy levels of electrons in atoms, or where an electron is in an orbital. This can cause single spectral lines to split into slightly different wavelengths, which may cause line broadening.

Atomization

• A sample has to be converted into gaseous atoms or ions before AAS or AES can occur

• Temperature influences the ratio between ground, excited and ionized atoms

Sample Introduction

In order to ensure the best accuracy and reproducibility, the sample has to be introduced into the atomizer with efficiency.

Solids can be introduced as slurries and introduction can be continuous or discrete