Usually there is a weight loss as a volatile product is produced.
For example when copper sulphate crystal s is heated they lose their blue coloration and there is no complete weight loss.
The addition of water regenerates the initial complex showing that the volatile product in this case is water.
TGA equipment varies considerable. It is often controlled from a single processor.
A typical set up will also have a printer a visual display unit or a plotter, a computer and a controlled supplier gas.
This is the heart of the equipment.
At the top is a sensitive balance that can weigh samples while in their temperature control environment provided by the cylindrical furnace position below the balance.
In this instrument, the balance has two pans.
Above is an easily assessable top pan where empty crucibles can be weighed or tared then re-weighed after the sample has been added.
Beneath the top pan, there is a suspended pan, this is where the sample is weighed during heating.
To heat the sample the furnace is raised around the sample crucible on the suspended pan, typically temperatures up to 10000C can be achieved.
Weight change as small as 0.1micogram can be detected. As with all balances it is important to check that the balance is level.
Since a crucible may need to be re used, it is essential that it is clean at the start of each round, after removal of the lose deposit, this can be achieved by heating the crucible to red heat over the Bunsen.
Any residual discoloration after this treatment will have no effect on the accuracy of the subsequent results.
If the sample produces volatile product, it is normally carried away in an inert gas stream through the exhaust tube; here it is the transparent tube.
Start by checking that all the gas valves to the instrument and the cylinder valves are closed.
Then open the high pressure valve on the cylinder and check whether there is sufficient gas in the cylinder to complete the round.
A reading of more than 500 indicates sufficient gas, in this case di-nitrogen.
Open the low pressure valve and check the gas flow.
Then connect the gas delivering tube to the instrument.
With some samples reactive gases such as O2 or C02 may be used instead of an inert gas.
Adjust the control valve on the instrument to set the required gas flow as shown by the flow meter.
Purge the instrument for at least 5min.
The gases from decomposition may be toxic, so check that the exhaust tube is in place and is secured.
The exhaust tube should pass through a carbon trap to absorb any toxic materials.
Then set the temperature program conditions.
It’s best to start with the trial run using a rapid heating rate in the wide temperature range.
For a copper sulphate sample, set the starting temperature at 500C.
The rate of heating to 500C per minute
And the high temperature limit at 9500C
A long run time has no advantage so do quick calculation of the expected run time with the lower limit set at 500Cand upper limit set at 9500C.
Then a heating rate of 50 deg per minute gives a run time of 18 minutes.
This is a reasonable run time.
Grind the sample to a powder in a pestle and mortal. This ensures homogeneity and quick thermal contact.
Because the sample surface area can affect TGA result, sometimes samples such as catalysts, polymers, and fibers need to be measured in the form in which they are received.
Open the balance window and using tweezers, place a clean empty crucible on the balance pan.
Close the window and set the balance to zero to tare the crucible.
Remove the crucible and add the sample. About 10mg is normal a suitable amount.
Replace the crucible on the balance pan.
Close the window and observe the weight of the sample.
If the weight of the sample is satisfactory, adjust the gas flow if necessary and check that it is constant.
Lower the furnace and carefully transfer the crucible to the suspended balance pan.
Raise the furnace and lock it in position.
Then when the balance is settled down, observe the sample weight again.
Start the heating program and observe the changes on the VDU.
Here, the output has been speeded up to save time
An advantage of the VDU output is that it can be rescaled during the run to optimize he display.
When the run is complete, the data is normally saved on the computer
Note the Final weight of the sample.
The TGA trace called the thymogram shows several points
The first point to note is the onset temperature at which weight loses begins.
This is followed by several steps depending on the nature of the sample
In this trial run, there is high temperature decomposition above 6000C as well as lower temperature effect.
When the instrument is cooled, open up the furnace.
Remove the crucible and note the appearance of the sample.
In this case, the black appearance shows that degradation beyond the simple lose of water molecules has taken place
This degradation is the event of curing above 6000C and the trial thymogram shows that to examine the dehydration effect more closely, a second run at a slower heating rate is needed up to a limit of around 3000C
Set the starting temperature as low as possible - 30 deg,
Set the heating rate at 20ged per minute and the final temperature to 300.
Again, do a calculation to make sure the run time is satisfactory which it is at 13.5 minutes
Now repeat the run using a clean crucible and a fresh sample of copper sulphate using the new run conditions.
The thymogram shows that the weights lose due to dehydrations takes place in 3 stages.
The exact position of the steps is not that easy to see. These changes can be enhanced by taking the first derivative of the plot with the troughs mark the exact position of the steps
Using the first derivative can be particularly helpful in analyzing thymograms in which more than one effect is taking place
Here the computer uses the data to perform a step analysis which clearly shows that the dehydration weight changes have a ratio of 2:2:1
When it is cooled, open the furnace.
Remove the crucible and clean it ready for the next user.
Finally, turn off the gas supply.
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