USING ATOMIC ABSORPTION SPECTROPHOTOMETER

The atomic absorption spectrophotometer is widely used to measure low concentrations of metal ions in aqueous or organic solution.
The parts of the spectrometer are the source of light, which is specific to the element to be determined.
A flame into which solutions are sprayed, converting the meter of their constituent atoms which absorb light at their characteristic wave length.

This absorption is measured by the reduction in the light falling on the detector that lies to the left of the flame.
Fumes from the flame are drawn off by an extractor.
The very hot flame is formed by burning acetylene with compressed air, this cause the temp. 24000C needed to create atoms.
Such a flame must never be left burning unattended so it is important to know how to turn in on or off safely.
To start, turn on the extractor, then at the instrument switch on the compressed air, and adjust the flow to the rate advised in the laboratory note.

Turn the acetylene on at the cylinder, check that the pressure is sufficient to sustain the flow to the flame; here it should be in the white region of the diagram.
Note of the red region must be avoided as this high pressure can cause acetylene to polymerize.
Also note that the cylinder pressure must be at least to 100 pounds per square inch, otherwise the acetone in which the acetylene is dissolved will escape affecting the result.
Switch the acetylene on at the instrument, and adjust the valve to the recommended settings.
Allow 5 seconds for the acetylene to flush out the pipe work, then press the ignite button to light the flame.
Varying the acetylene flow rate varies the characteristic of the flame.
 A higher flow rate gives a rich luminous flame.

 For the determination of copper, select a lean flame. With distilled water aspirating leave the flame burning for 10min, to enable the flame temperature and instrument conditions to stabilize.
Check the correct lamp for the element to be determined is fitted.
The cathode of this discharge lamp is made from element of interest, and its shape gives its name the hollow cathode lamp.
The current through the lamp is set to that recommended by the manufacturers.
A monochromator is used to isolate the light from the lamp from that due to the flame. Set it to the characteristic of the wavelength of the element.

Switch the meter from the lamp to read energy, and fine tune the wave length watching the energy meter as you do so until the meter reads a maximum when adjustment is exactly right.
But enough light may not be reaching the detector; the energy meter needle should be read in the green band.
If not, adjust the flame until it does, too much light will burn out the detector, too little cannot be measured reliably.
The instrument is calibrated using a set of known concentrations of the elements to be analyzed.

The concentration of these standards should be chosen so that the expected concentration of the unknown lies in the middle of the range.
Working with trace concentrations, it is vital that the glassware, water and chemicals used are scrupulously clean and pure.
The absorbencies of the standard solutions and that of the unknown are measured relative to a blank.
This contains exactly the same materials use to make the standard solution except the element to be analyzed in this case, pure deoinised water.

When the instrument stabilizes and aspirating the blank solution, set the instrument absorbance reading to zero by pressing the auto zero button.
Then aspirate the weakest standard solution within a few seconds the reading will stabilize which should read greater than zero, since there are now atoms absorbing in the flame.
Record this value, and then aspirate the remaining standard in turn re- zeroing the instrument between measurement by aspirating the blank solution and pressing auto zero again.
Now aspirate the unknown solution and record the absorbance reading.

Plot a graph of the absorbance readings of the standard solution against their concentrations.
Plot the absorbance of the unknown on the calibration graph and read off the concentration.
Before switching off, aspirate distilled water for 5min to rinse up the capillary and the mixing chamber.
Turn down the acetylene slowly to zero on the instrument fuel tank, and then close the main valve on the cylinder.
Switch off the compressed air and then the extraction fan.
Finally turn down the lamp current, and then switch off the power to the instrument.