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Stepwise heating often improves transition
resolution because transitions are time depen-
dent as well as temperature dependent.
Stepwise heating gives transitions more time to
complete, thereby, reducing overlap with
neighboring transitions.
To get the maximum benefit from stepwise
heating it will be necessary to run several TGA
scans to properly "tune" the reaction rate
thresholds used to start and stop heating. Rela-
tively slow heating rates are generally required
to prevent transition overshoot. A rough rule-of-
thumb is to use a heating rate which is about one
tenth the transition temperature difference
between the transitions being resolved. For
example, if the transitions are separated by 10
°
C
°
then use 1
C/min as the heating rate prior to,
between and following the transitions. If precise
reaction temperatures are important, then slow
heating rates should always be used prior to
encountering transitions of interest even though
they may be well separated in temperature, and
the "%/min" limit for the ramp segment should
be set closer to the limit for the isothermal
segment. To avoid excessively long experi-
ments, higher heating rate ramps or equilibrate
segments can be used to skip over baseline
portions of the scan.
A disadvantage to stepwise heating is that most
experiments take much longer in total time to
complete than by a conventional constant
heating rate scan. Another disadvantage of
stepwise heating is that the decision to leave
isothermal mode and continue heating is some-
what arbitrary and may lead to incorrect assump-
tions about the number and size of transitions.
TA I
TGA 2950
B–55
NSTRUMENTS
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