Graphite Furnace for AA
The graphite furnace is used ideally for trace metal analysis. The sample size required is
a mere microliter. Analyte mass however is more important than the actual
concentration. A measure of the sensitivity of the graphite furnace is known as
"characteristic mass." The characteristic mass is defined as the mass of analyte
(picograms) needed to produce a peak height signal of 0.0044 absorbance.
Components of the Graphite Furnace
A graphite furnace contains three major components, the atomizer, the power supply, and
the programmer.
The atomizer consists of a graphite tube, electrical contacts, enclosed water-cooled
housing, and inert purge gas controls. The graphite tube serves two purposes acting as
the heating element and the sample cell. The tube is held in place by two graphite contact
cylinders that provide an electrical connection and allow current to flow through the tube
for heating of both the tube and sample. An external gas flow of argon surrounds the
outside of the tube to prevent air oxidation of the heated graphite. A second, internal
flow of argon purges the tube. There are two end windows that allow light to pass
through the tube and aid in the prevention of air oxidation of the graphite tube. Zeemaneffect background corrections can be applied to transversely heated furnaces elimination
the need for a polarizer in the optical system.
The power supply and programmer are responsible for electrical power control,
temperature program control, gas flow control, and spectrometer function control. The
power supply controls the electrical current that flows through the tube. A temperature
program controls the temperature of the tube. The operator can control variables such as
gas flow, the spectrometer read function, and sequences for atomization through the
furnace programmer.
Graphite Furnace Analysis
Graphite furnace analysis involves the measurement of a known volume of sample. The
sample is then atomized at a particular temperature and the atomic absorption
measurement is made.
Six common steps of a graphite furnace program include:
1.
2.
3.
4.
5.
6.
Drying
Pyrolysis
Cool Down (optional)
Atomization
Clean Out
Cool Down
Sample size will vary based on concentration and amount of sample available. A large
volume of sample will generally contain more analyte that would then generate higher
signals. Varying the sample volume will allow control over the analytical range of the
furnace. Maximum volume of sample depends on the tube configuration. If a platform is
not in place, sample volume can reach 100 L. If a platform is in place then a volume of
50 L or less is recommended. Typical sample size is 20 L for most analyses. Multiple
injections with drying and pyrolysis steps in between can increase the effective sample
size if it is required.
Whenever a sample is inserted into the furnace it must be dried a low temperature (100ºC
- 120ºC for aqueous solution) to avoid spattering of the sample. During the drying step
the internal gas flow is usually set at its default maximum value (250-300 mL per minute)
to purge vaporized solvent from the tube.
The pyrolysis step (or the ashing, char or pretreatment step) volatilizes inorganic and
organic matrix components from the sample leaving behind the sample element in a less
complex matrix, making analysis easier. The ideal goal is to increase the temperature
enough to volatilize matrix components but not to the point where analyte loss would
occur. The internal gas flow rate can be left at the default setting (250 - 300mL per
minute). This internal gas flow will drive off the volatilized matrix materials.
A pre-atomization cool down step is typically not required for transversely heated
furnaces, whereas it would be advantageous for a longitudinally furnace.
An atomic vapor of the analyte element is formed and measured by atomic absorption in
the atomization step. The temperature is increased to the point dissociation of the
volatilized molecular species occurs.
Once the atomization step is over the tube can be heated to an even higher temperature to
insure the removal of analyte residue in the furnace. A cool down step may follow prior
to the introduction of the next sample.
Fast Furnace Analysis