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Appendix B
B–56
This is particularly true for materials which have
transitions that are overlapped even at very slow
heating rates (such as the sodium/potassium
bicarbonate mixture in the Examples section).
As a general rule, stepwise isothermal heating
cannot be used to reliably separate transitions
which cannot be separated by conventional TGA
at very slow heating rates.
Another problem to watch out for with stepwise
heating is the creation of anomalies in the
weight loss versus temperature curve. These
appear as small unexpected secondary weight
losses following a larger transition. These
anomalies can be caused by two factors.
The first cause is using too high a heating rate in
the ramp which follows the transition. Any
small amount of sample material which did not
finish decomposing during the transition will
now quickly decompose due to the rapid eleva-
tion of the furnace temperature. This will cause
the decomposition rate (%/minute) to rise
substantially. If the rising decomposition rate
crosses the abort threshold for the ramp segment
then a second isothermal period will be intro-
duced which will appear on the weight loss
versus temperature plot as a small unexplained
transition. Generally it is best to use the same
heating rate on both sides of a transition. This
rate can then be accelerated if desired after the
furnace temperature is some distance from the
transition.
The second cause of anomalies is leaving the
isothermal holding period prematurely during a
transition because the percent/minute threshold
for aborting the isothermal segment was set too
high. This situation leaves a significant amount
TA I
TGA 2950
NSTRUMENTS
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