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Signature: Dhanlal De Lloyd, Lab Tech 1, Chem. Dept, UWI St. Augustine campus
The Republic of Trinidad and Tobago.
Copyright: delloyd2000© All rights reserved.

Graphite Furnace Atomic Absorption Spectroscopy

GFAAS


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Related links :
Flame Atomic Absorption
Hg cold vapor and As hydride generation
Interferences in Aanalysis
1000 ppm AA standards

Graphite Fufnace AA


The graphite furnace is an electrothermal atomiser system that can produce temperatures as high as 3.000°C. The heated graphite furnace provides the thermal energy to break chemical bonds within the sample held in a graphite tube, and produce free ground state atoms.

Ground-state atoms then are capable of absorbing energy, in the form of light, and are elevated to an excited state. The amount of light energy absorbed increases as the concentration of the selected element increases.

Flame AA can only analyse solutions, but graphite furnace can accept very small absolute quantities of solution, slurry or solid samples.

New Instruments

Fully automatic atomic absorption spectrometers are now available. All operations are controlled through the PC, with the AA-Win software based on a WINDOWS platform. The design with the flame burner and graphite furnace in one compact unit enables automatic interchanging of two analytical methods : Flame atomic absorption and Graphite Furnace

In order to make GFAAS more efficient and user friendly, improvements have been made in features such as automated flame-to-furnace switching (as indicated above), computer-coded source lamps (automatic wavelength and slit width) self-aligning graphite tubes, automatic sample dilution and automatic standard preparation.

Description

The heart of the Graphite Furnace is the atomization chamber which is made up of a graphite tube with a hole in the center for sample introduction. It is encased within graphite electrical contacts at both ends and the sample is electrothermally vaporised into atoms. A supply of water circulates the outside of the tube to keep the furnace cool, and an external stream of inert gas flows around the tube to prevent air from entering the chamber. GFAAS instrunents comprise the following basic components:


Light source
The light source is the lamp that emits radiation of the resonance line.
Atomization chamber
This is the graphite tube where the sample is vaporised. It serves as a sample cell and a heating element.
Electrical contact cylinders
These provide electrical connection to the graphite tube. Current flow heats up tube and sample.
Water Cooling Housing
This is a circulating water flow that serves to cool the assembly.
Inert GAS
This protects the heated tube from atmospheric oxidation. An external gas flow surrounds the outside of the tube and an internal flow purges the tube.
Quartz windows
These seal both ends of the tube and allow light to pass through.

Power supply
This controlls current supplied to the tube according to the programming being used
Detector
This is the photomultiplier tube that measures the amount of light intensity absorbed in the vapor.
Signal processor
This is a computer managed data and display system.
Solid Sample Accessory
Sampling cup. This is a small pyrolytically coated graphite cup. This cup is loaded with a solid sample, weighed and inserted into the graphite tube through a sampling hole. Matrix modifiers can be added.
Slurry Sample Accessory
This accessory can analyse materials directly and automatically. The desired amount of materiol is weighed out, and a solvent added to form slurry or solution.
This sampler reduces sample preparation and contamination, and can easily be switched over from slurry to solution sampling.

Operation

GFAAS involves 4 stages, Drying, Pyrolysis, Atomization, and Cleaning. This is done by running a sequential temperature program to perform the following:
Drying
This is a low temperature drying to remove water without sputtering.
Ashing
This is pyrolytic method in which the temperature rises to 400 - 800 degrees centigrade. Matrix components and high boiling volatile compounds such as oils, fats organics are removed.
Atomization
This is where the atomic vapor is formed, with a rapid increase in temperature up to 3000 degrees centigrade.
Radiation from a Hallow cathode lamp is directed through this vapor and a signal trace of absorbance vs time is measured.
The amount of energy absorbed is proportional to atomic concentrations. Microliter quantities are used and analysis time per sample is 1 to 5 minutes.
Purge gas
This is an internal stream of inert gas that flows through the tube to clean and carry away vapors from sample matrix.
Gas Stop
This is a programmable step that shuts down the gas flow in the sample chamber just 1 - 2 seconds immediately before atomization.
This provides a stabilized gas environment in the graphite tube, increases residence time of the atom cloud, prevents atom loss from the tube and improves the signal output.

Graphite Furnace and Flame AA Methods


Graphite FurnaceFlame AA
Use small sample size
0.05ul-100ul or 0.005ml-0.1ml
Use larger sample size 3-5ml
Analyse sample in pbb rangeAnalyse sample in ppm range
Longer analysis time 5mins.Fast. 10 secs. for the nost.
Single measurement technique.
A fixed vol of sample is analysed at one time
Sample is continuously aspirated.
Mamy measurements can be taken during an aspiration
Sample prep is minimised.Involve sample prep. Extraction, digestion etc.
Sample type can be solid, slurry, powder or soln.Sample must be in solution only.
Longer residence time for atoms in furnaceShorter residence time for atoms in flame.
Matrix destroyed.Need matrix clean up.
Zeeman background correction.Deterium lamp correction.
Electrothermal analyser - Graphite tube.Flame atomiser-Burner and nebulizer.
Temperature programming.Depends on temperature of flame.
Can analyse refractory elements.Need Releasing agents.
Can analyse about 40 elements from periodic table..Can analyse about 70 of the elements.
High costLower cost
Measure ground state atoms.Also measure ground state atoms.
Base on Principles of AA spectroscopy.Also based on the Principles of AA spectroscopy.




Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
Copyright: delloyd2000© All rights reserved.