Atomic Absorption

Where g = degeneracy

Lines are broadened by two effects: Doppler Collisional Operating conditions for lamp are chosen so that the Doppler broadening in the lamp (low P, few collisions) is less than the Doppler and collisional broadening in the flame or furnace.

Atomic Absorption

Monochromator is used to select one of the emitted lines Ideally, we want the selected emission line from the lamp to be narrower than the absorption spectrum

Absorption lines are narrow The selected bandwidth of light from source must be narrower than chosen absorption line If a monochromator was able to select a narrow enough bandwidth from the output of a deuterium lamp, the power of the light would be negligible Therefore lamps that emit spectral lines are used

Hollow Cathode Lamp

HCL Inert gas is ionized by discharge Is accelerated to cathode Causes some element to dislodge and form atomic cloud (sputtering) Some are excited (in collisions with ions) and emit line spectra. Usually lamps are for one element – but can be for as many as six.

Electrodeless Discharge Lamp

Electrodeless Discharge Lamp Microwave excited discharge tubes Intensities 10-100 x greater than from HCL Small amount of element or halide of an element in a small sealed tube containing a few torr of inert gas Placed in microwave cavity (2450 MHz) Argon is ionized, the ions are accelerated and excite the metal atoms Less stable than HCL, but more intense. Not available for all elements

High-resolution continuum source AAS: the better way to perform atomic absorption spectrometry

Single xenon arc lamp Today, multiple hollow cathode lamps are no longer used. With the use of a single xenon arc lamp, all the elements can be measured from 185-900 nm. This takes AAS into a true multi-element technique with the analysis of 10 elements per minute.

CCD technology - For the first time in an AAS CCD chips are now available with 200 pixels which act as independent detectors. Simultaneous background correction - Background is now measured simultaneously compared to sequential background on conventional AAS. Better detection limits - Due to the high intensity of the Xenon Lamp there is better signal/noise ratio thus giving better detection limits. In some cases it is up to 10 times better than conventional AAS.

Sample There are a variety of different sampling methods: Flame Furnace (electrothermal atomizer) Arc, spark ICP Cold vapour atomization Hydride generation

Flame Stable Safe Cheap to maintain High temperature Reducing Atmosphere - many metals form stable oxides, not easily atomized just by flame temperatures

Flame Typical system – spray chamber and burner Sample is aspirated into spray chamber using nebulizer (sucked in by Venturi effect) Produces aerosol. Aerosol strikes obstruction – spoiler – to break it into smaller drops Only smallest drops proceed to flame Larger drops go down drain

Sequence of Events in Flame Evaporation of Solvent (leaving fine salt particles suspended in flame) Loss of water of hydration Vaporization of solid particles to free atoms (due to heat and chemical reaction) Excitation Ionization (not always desirable)

Nebulization Controls fraction of sample to reach flame Drop size is governed by viscosity, surface tension, gas flow, density, design of nebulizer Organic solvents have lower viscosity and lower surface tension than water (0.25 - 0.3 x) They also allow preconcentration But change flame conditions – not always so beneficial Salt increases viscosity, decreasing efficiency The smaller the drop, the more easily it is desolvated and vaporized

Ultrasonic breakup of drops High frequency vibrations Uniform and controllable drop size BUT Drops are larger and equipment more expensive

Desolvation Critical to number of free atoms Usually occurs at base of flame Solvent then water of crystallization Depends on droplet size and solvent

Vaporization Atomization to free atoms Ideally – want high temperature and long residence time (slow burn rate of the gases) - lots of time for atomization Depends on nature of molecules and atoms Al2O3 atomizes more slowly than NaCl particle of the same size Important if analyzing mixtures – different conditions are needed for different atoms

Ionization Ions undergo different transitions than atoms Want one or the other Ions not desirable in flame method In ICP, ions are the desired species For alkali and alkaline earths, ions form above 2000K

Ionization Increases at low sample concentration With increasing flame temperature With decreasing ionization potential Prevent by: Low flame temperature Excess of easily ionizable metal eg Li Called a SUPPRESSOR Eg: add lots of Li to solution to be analyzed for K

Premix (Laminar flow) Burner Most common Gases premixed before entering burner Stable flame Use long narrow flames – long path length for light absorption Use at right angles for emission Small (narrow) flame keeps atom concentration high

Width of slot depends on gases Narrow slots prevent flame backing into mixing chamber and causing explosion But must allow enough gas through to support rate of burning Too narrow: Cooling by adjacent air Salt deposition clogs burner Use different burners for different gases

Gases Gas mixtures with high-burning velocities are less safe Also want long residence times C2H2-N2O (220cm/s) is better than C2H2-O2 (1130 cm/s). They have similar flame temperatures.

Air-C2H2 Yellow Colorless Blue C2H2-N2O Red (CN., NH.) Whitish-blue

Flame Atomizer Advantages Convenient Rapid Suitable for all AA-determinable elements Limitations Limited Sensitivity Large Sample Volume Cannot handle some sample types

Sensitivity Limitation

Interferences Spectral Vaporization Chemical

Spectral Mg 285.21 nm Na 285.28 nm Not usually much of a problem – can change to another wavelength Problem worse in emission because more lines – High T – lots of excitation Choice of line dictates concentration range able to be analyzed

Vaporization Interferences When one component of a sample influences the rate of vaporization of the species of interest Physical – changes matrix it vaporizes from Chemical – changes the species to be vaporized

Chemical Vaporization Interferences Metal oxides form Metal ions form thermally stable complexes with anions The effects usually occur during formation of the solid particle CaPO4 formation – a well known example. CaPO4 is harder to vaporize than Ca2+

CaPO4 - Interference Prevention Put light path higher in flame to allow a longer residence time Add releasing agent – La2+ or Sr2+ (added in excess) will preferentially combine with PO43- and leave Ca2+ free to be analyzed Protective agent – add EDTA. Ca-EDTA complex is easily destroyed in flame Glucose – burns easily and helps droplets shatter apart Hotter flame – then need ionization suppressor