Atomic Fluorescence Spectroscopy

Background l First significant research by Wineforder and Vickers in 1964 as an analytical technique l Used for element analysis –Example: Trace elements in ground water l Has not found wide spread success because there does not seem to be a distinct advantage over established methods, i.e. atomic absorbance

What is Atomic Fluorescence? l “Atomic fluorescence spectroscopy (AFS) is the optical emission from gas-phase atoms that have been excited to higher energy levels by absorption of radiation.” l “AFS is useful to study the electronic structure of atoms and to make quantitative measurements of sample concentrations.”

Why use AFS? l it is a quantitative technique for determination of a large number of elements l used mostly in analysis of metals in biological samples, agricultural samples, water, and industrial oils

Jablonski Diagram

Instrumentation l Virginia Tech website maintained by Professor Brian Tissue Department of Chemistry

General process for AFS l nebulization - converts the sample solution into a mist made up of tiny liquid droplets l atomization - flow of gas carries sample into heated region where sample molecules are broken into free atoms –desolvation - the solvent is evaporated to produce a solid molecular aerosal –dissociation - molecules dissociate to produce an atomic gas –The atoms dissociate to produce ions and electrons

General Process continued l Excitation due to light source l Fluorescence of sample l This fluorescence can be selected for certain wavelengths by a monochromator l Then the detector reads the emission and amplifies the signal l Then the readout device relays the data

Problems with Technique l Precision and accuracy are highly dependent on the atomization step l Light source l molecules, atoms, and ions are all in heated medium thus producing three different atomic emission spectra

Problems continued l Line broadening occurs due to the uncertainty principle –limit to measurement of exact lifetime and frequency, or exact position and momentum l Temperature –increases the efficiency and the total number of atoms in the vapor –but also increases line broadening since the atomic particles move faster. –increases the total amount of ions in the gas and thus changes the concentration of the unionized atom

Interferences l If the matrix emission overlaps or lies too close to the emission of the sample, problems occur (decrease in resolution) l This type of matrix effect is rare in hollow cathode sources since the intensity is so low l Oxides exhibit broad band absorptions and can scatter radiation thus interfering with signal detection l If the sample contains organic solvents, scattering occurs due to the carbonaceous particles left from the organic matrix

Interferences continued

Detection Limits l Are similar to those for Atomic Absorption and Atomic Emission l Varies for different elements

Final conclusion l This technique offers some advantages for some elements while other atomic spectroscopy techniques may be better for other elements l Future work on light sources and atomizers will increase the analytical uses of this technique