1. Thermogravimetric Analysis Theory, Operation, Calibration and Data Interpretation
Prepared by Kadine Mohomed, Ph.D
Thermal Applications Chemist
TA Instruments

2. Agenda: TGA Theory, Operation and Calibration
Definitions and review of instrument
Balance, furnace and heat exchanger review
Mass and temperature calibration
Purge gas considerations
Baseline considerations
Sample preparation and pan selection
Method development

3. TGA: The Technique
Thermogravimetric Analysis (TGA) measures the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere.
Measurements are used primarily to determine the composition of materials and to predict their thermal stability at temperatures up to 1000°C.
The technique can characterize materials that exhibit weight loss or gain due to decomposition, oxidation, or dehydration.

4. What TGA Can Tell You Thermal Stability of Materials
Oxidative Stability of Materials
Composition of Multi-component Systems
Estimated Lifetime of a Product
Decomposition Kinetics of Materials
The Effect of Reactive or Corrosive Atmospheres on Materials
Moisture and Volatiles Content of Materials

5. Calcium Oxalate Example

6. Mechanisms of Weight Change in TGA
Weight Loss:
Decomposition: The breaking apart of chemical bonds.
Evaporation: The loss of volatiles with elevated temperature.
Reduction: Interaction of sample to a reducing atmosphere (hydrogen, ammonia, etc).
Desorption.
Weight Gain:
Oxidation: Interaction of the sample with an oxidizing atmosphere.
Absorption.
All of these are kinetic processes (i.e. there is a rate at which they occur).

7. Features of the Q500/ Q50 TGA
The Q500 is a research grade thermogravimetric analyzer, whose leading performance arises from a responsive low-mass furnace; sensitive thermobalance, and efficient horizontal purge gas system (with mass flow control). Its convenience, expandability and powerful, results-oriented software make the Q500 perfect for the multi-user laboratory where a wide variety of TGA applications are conducted and where future expansion of analytical work is anticipated.

8. Features of the Q500 TGA 1. Q Series Two Point Mass Adjustment
200mg range
1000mg. range
*No need to do a mass recalibration
when switching from regular Pt pans to
Pt pans with Al hermetic pans.
*Mass Loss Reference Materials
Materials with nominal 2%, 50% and 98%
mass loss are available for verification of TGA
weight calibration.
2. Curie Point Transition Temperature
Calibration
ASTM 1582
*Curie Temperature Reference Materials:
TA Instruments is the exclusive worldwide
distributor for a set of six certified and traceable
Curie temperature materials developed by ICTAC
Photodiodes
Infrared LED
Meter movement
Balance arm
Tare pan
Sample platform
Thermocouple
Sample pan
Furnace assembly
Purge gas outlet
Heater
Elevator base
Purge gas inlet
Sample pan holder

9. Q50/Q500 Features and Options
Feature Q Q50
Furnace – low mass Standard Standard
Furnace – EGA Option Option
Temperature Range RT-1000°C RT-1000°C
MFC / GSA Standard Option
Autosampler Option NA
Hi-Res TGA™ Option NA
Modulated™ TGA Option NA
Touch-screen display Standard NA
TGA / MS operation Option Option
TGA / FTIR operation 3rd Party 3rd Party
NA = Not Available

10. TGA Furnaces Standard Furnace Low mass Used for Hi-Res Runs
Cools down in <20min
EGA Furnace
Higher Mass
Used for EGA runs due to quartz liner
Cools down in ~40min

11. TGA: Purge Gas Flow 40ml/min 10ml/min 90ml/min 60ml/min EGA Furnace
Standard Furnace

12. Standard Furnace

13. Low internal Volume ~15ml
EGA Furnace Schematic
Quartz Liner
Off-Gases
Balance Purge
Sample
Thermocouple
Pan
Furnace Core
Purge Gas In
Low internal Volume ~15ml

14. TGA: How the balance works
The balance operates on a null-balance principle. At the zero, or “null” position equal amounts of light shine on the 2 photodiodes.
If the balance moves out of the null position an unequal amount of light shines on the 2 photodiodes. Current is then applied to the meter movement to return the balance to the null position.
The amount of current applied is proportional to the weight loss or gain.

15. TGA: Q Series MFC and GSA
MFC and GSA standard on Q500 and optional on Q50

16. TGA: Q-Series Purge Gas Plumbing
Instruments w/o MFC
The gas 1 port purges the sample area only.
The gas 2 port purges the balance area only.
Instruments w/ MFC
The gas 1 port purges both sample and balance areas.
The gas 2 port is used when a different purge gas is required or gas switching is used.
Selection of gas on NOTES page is critical for proper use of MFC calibration tables.

17. Heat Exchanger – Cleaning
Check cleanliness (no algae growth) once every 3-6 months.
To clean dump old water, fill with new and add conditioner (algae growth suppressor) if available.
For Q series, after filling, in software choose “Control \ Prime Exchanger”.
For 2xxx, after filling, continue starting a dummy run until error 119 (heat exchanger – no flow) goes away.

18. TGA Performance Criteria
Baseline
Drift
Affected by TGA construction, balance quality, and buoyancy effect (minimized through proper construction techniques and purge gas control)
Sensitivity
Affected by TGA balance quality
Reproducibility
Affected by balance quality, temperature control, and construction quality
Temperature Accuracy
Affected by thermocouple placement, calibration stability, purge gas interaction

19. TGA Performance
TGA Performance is primarily a function of balance sensitivity and baseline stability
Balance sensitivity is optimized through design and construction techniques
Baseline stability is a function of instrument design, as well as purge gas control
TGA resolution is primarily a function of heating rate, but can be optimized using Hi-Res TGA

20. Quantifying TGA Baseline Performance
Drift
Unnormailzed Sample Mass
Temperature or Time

21. Measuring Q500 TGA Baseline Performance
Drift ~19 mg
Q500, 20°C/min Ramp

22. TGA: Calibrations Mass (Verify monthly) Temperature (Verify monthly)
Platform (Perform if there is a problem picking up pans.)
Q series instruments w/ MFC will also have options to calibrate the sample and balance MFC’s. These have been calibrated by TA Instruments and should not require further calibration. Contact TAI if a problem arises.

23. TGA: Mass Calibration Two point mass adjustment: 2050, 2950, Q50, Q500
100mg. (2XXX modules) or 200mg (Q series) range (use 100mg. weight)
1000mg. range (use 1000mg. weight)
Q5000IR – 100mg
Run TGA weight calibration routine
Follow screen instructions to tare and mass calibrate using two calibration weights (if known, enter exact mass of calibration weights)

24. Mass Loss And Residue Validation
P/N TGA / SDT Mass Loss Reference Materials Kit $1,760
2.4 %
49.7 %
99.1 %
0.017%
Mass Loss Reference Materials: Materials with nominal 2%, 50% and 98% mass loss are available for verification of TGA weight calibration.

25. Temperature Calibration: Curie Point Transition
Paramagnetic - a material that is susceptible to attraction by a magnet
Curie Point Temperature - that temperature where the material loses its magnetic susceptibility (defined as offset point)
Requires a magnet and well characterized transition materials
ASTM Standard Practice for Calibration of Temperature Scale for Thermogravimetry

26. TGA: Temperature Calibration
Vertical Balance Configuration - TGA 2050/2950/Q50/Q500
Tare
Sample %
Offset
Furnace
temp
Attraction of Sample to Magnet
Results in Initial Weight Gain
Magnet

27. TGA: Temperature Calibration Important Points
Clear the ‘Temperature Table’ before performing the calibration runs (TGA only).
Choose method end condition of “Furnace Closed”. This prevents the potential of the furnace opening onto the magnet at the end of the run and damaging the TGA.
Start run and then put magnet under furnace. This allows capture of the weight increase (decrease) at the beginning.
Use of a small labjack is recommended for holding the magnet in place under the furnace.

28. Standards Can Be Run Simultaneously
Alumel C
Nickel C

29. Calcium Oxalate “Standard” Analysis
Although Calcium Oxalate is not generally accepted as a “Standard Material,” it does have practical utility for INTRA-laboratory use
Carefully control the experimental conditions; i.e. pan type, purge gases/flowrates, heating rate
Particularly control the amount (~5mg) and the particle size of the sample and how you position it in the pan
Perform multiple runs, enough to do a statistical analysis
Analyze the weight changes and peak temperatures and establish the performance of YOU and YOUR instrument
When performance issues come up, repeat the Calcium Oxalate analysis

30. Calcium Oxalate Decomposition
1st Step CaC2O4•H2O (s) CaC2O4 (s) + H2O (g)
Calcium Oxalate Monohydrate Calcium Oxalate
2nd Step CaC2O4 (s) CaCO3 (s) + CO (g)
Calcium Oxalate Calcium Carbonate
3rd Step CaCO3 (s) CaO (s) + CO2 (g)
Calcium Carbonate Calcium Oxide

31. Calcium Oxalate Repeatability
Overlay of 8 runs, same conditions

32. Calcium Oxalate Repeatability

33. General Considerations (Experimental Effects)

34. TGA Curves are not ‘Fingerprint’ Curves
Because most events that occur in a TGA are kinetic in nature (meaning they are dependent on absolute temperature and time spent at that temperature), any experimental parameter that can effect the reaction rate will change the shape / transition temperatures of the curve. These things include:
Pan material type, shape and size.
Ramp rate.
Purge gas.
Sample mass, volume/form and morphology.

35. Effect of Sample Size on Decomposition Temperature

36. Effect of Heating Rate on Decomposition Temperature

37. Mass Effect – Semi-crystalline PE

38. Shift in Onset with Ramp Rate

39. Typical Applications Thermal Stability Compositional Analysis
Oxidative Stability

40. Thermal Stability of Polymers

41. 25.18mg of an adhesive @ 10°C/min
TGA of an Adhesive
25.18mg of an 10°C/min

42. Inset View Shows Strange Result
Is this real?

43. Use time based derivative of temperature to plot the heating rate

44. Aberration in Heating Rate
Usually means that the sample touched the thermocouple

45. Typical Applications Thermal Stability Compositional Analysis
Oxidative Stability

46. PET w/ Carbon Black Filler
How much Carbon Black was in this sample?

47. PET

48. Comparison of Filled & Un-Filled PET

49. Filled Polymer Analysis
Carbon Black
Air
(A)
(B)
"Light" Oil
Polymer +
"Heavy" Oil
(C) \\
100
WEIGHT (%)
TEMPERATURE (°C)
Inert filler
Inert filler
Inert filler

50. Kinetic Analysis
The rate at which a kinetic process proceeds depends not only on the temperature the specimen is at, but also the time it has spent at that temperature.
Typically kinetic analysis is concerned with obtaining parameters such as activation energy (Ea), reaction order (k), etc. and/or with generating predictive curves.

51. Kinetic Analysis, con’t.
Activation energy (Ea) can be defined as the minimum amount of energy needed to initiate a chemical process. Ea
State 1
State 2
With Modulated TGA, Ea can be measured directly.

52. TGA Kinetics 1st Order Kinetics based on Flynn and Wall method
Lifetime Estimation based on Toops and Toops method
PTFE tested at 1, 5, 10 and 20 deg/min
Sample sizes constant
Nitrogen purge
Conversion levels selected at 1, 2.5, 5, 10 and 20%

53. Common Thermogram with TGA Scans

54. Log Heating Rate versus 1/T
Check for linearity

55. Activation Energy by MTGA

56. Sample of TGA Application Briefs
H “Thermogravimetry-Mass Spectrometry Using a Simple Capillary Interface”
TA023 “Thermal Analysis Review: High Resolution TGA - Theory and Applications”
TA075 “High Resolution TGA Kinetics”
TA 122 “Determination of Carbon Black Pigment in Nylon 66 by TGA”
TA 125 “Estimation of Polymer Lifetime by TGA Decomposition Kinetics”
TA231 “TGA Evaluation of Zeolite Catalysts”
TN6 “Consideration of Subtle Experimental Effects (Simultaneous TGA-DTA)”
TN24 “TGA Temperature Calibration Using Curie Point Standards”
TN40 “Optimizing Stepwise Isothermal Experiments in Hi-Res TGA”
TS13 “Clarification of Inorganic Decomposition by TG-MS”
TS39 “Characterization of Polyurethane by TGA and Hi-Res  TGA”

57. Common TGA Parts & Accessories

58. Common TGA Parts & Accessories

59. Need Help? Check the online manuals and error help.
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