2. Internal usage only
The problem with TGA
Anything we say now
about the weight losses
is an educated guess
Temperature
Weight
0
m1
m2
T1 T2
3. Internal usage only
A brief history of solutions
1960s – Use of TGA with MS
- Limitations imposed by the vacuum TGA could hold
- Gas were collected and manually transferred initially
1970s – Development of better systems
- Transfers lines improved, alterative direct TGMS system tried
- Other techniques still used “gas bomb”
1980s – Wendlandt listed TCD, GC and MS as coupled to TGA
- Development of FTIRs lead to TG-IR
1990s – Provder et al “Hyphenated Techniques in Thermal Analysis”
- Collected work to date
2000s – Groenewund and other summarized the status
- TG-IR the most popular technique.
- GCMS difficult to do
4. Internal usage only
The problem of EGA
The approach started out simply.
- Hook a tube up to your TGA exhaust and see what's there.
This has some problems
- You have multiple instruments – each with their own eccentric requirements
- You have a gas to transport
- What you see in the TGA may not be the whole story
Small or slow weigh loss
Compositional changes in a weight loss not separated.
- Or be detected in DTG
5. Internal usage only
The error comes the parts
What is optimal for TGA may not be for FTIR, MS, or GC
- All techniques do not allow tracking with time
- Sensitivity vary
- Components have to be transported or stored
All EGA methods represent compromises
TGA
Furnace
Balance
Gas Control All or part
Transfer Line
Heater
Capillary or tube
Gas control
EGA
Cell
Sampling loop
Temperature control
Etc.
6. Internal usage only
The TGA
Sample weight
- Enough to detect by EGA
- Larger than instrument min weight
Gas Flow
- Component concentration in sample
is not what we actually detect
- It is diluted by the gas flow
- All gas flows add to the dilution
effect
- Gas flow must be turbulent to allow
mixing
- Dead zones must be eliminated
- Thermal expansion of gases
Heating
- Eliminate cold spots
- Vary rate as needed
7. Internal usage only
Min Weight Concept and application to EGA
Min Weight
- The amount of weight you can
detect reproducibly under
specific conditions
- Important to understand for
TGA or balance performance
Applied to EGA
- How much do we need to see,
with reasonable reproducibility,
in the chosen EGA technique
- Depending on the technique,
this might be less than the min
weight.
If we have 200 ppm of
X, what sample size is
need when the gas is
diluted by 80cc/min
flow?
8. Internal usage only
Transfer lines
Temperature
- Overall
- Cold spots where things condense
- Hot spots where things degrade
Volume
- Flow rate
- Time Lag
What makes it flow?
- Pressure differential
- Vacuum pumps
- Pushed from TGA
How laminar is the flow?
Flow meters, valves, filters?
TGA
10. Internal usage only
Comparison of techniques- why what?
Gaseous products
are "known"
Gaseous products
are unknown
Masses<300amu
FTIR
GC/MS
Storage
MS GC/MS
11. Internal usage only
IR
• Path length is important
in dilute samples like
gases. Desire as long a
path length as possible
• BUT need to keep
volume low enough that
eluted components don't
mix.
• How long do you need to
purge to remove CO2
and H20?
• Cell needs a relative fast
turnover to track
changes from the TGA
• Designed to prevent
condensation and build-
up on walls.
• Do windows need to be
water resistant?
• How long do I need to purge between runs?
• Does your software support automation?
12. Internal usage only
D
C
B
A
x
x
x
x
Sample 2, 69.2380 mg
Heating rate 10 K/min
Under nitrogen
Gram-Schmidt curve
0.2
TGA curve
%
30
40
50
60
70
80
90
100
°C100 200 300 400 500 600 700 800 900
D
C
B
A
X
X
X
X
DTG curve
1/°C
0.005
9265 sample2 TG-DTG-GS 07.06.2004 17:07:00
SW 8.10e
RTASDEMO Version
15. Internal usage only
MS
"Analysis by smacking things
with a hammer and looking at
the pieces"
Stability of the filament
Mechanism of fragmentation
and ionization
- EI
- CI
- Cold ionization
Highest AMU
- Limited by what your system
can transport
Fragmentation patterns and
libraries
- How complex is the degradation
Pump
Gas from Capillary
Mass Filter Ionisation
Turbomolecular
Pump
Detector
10-6
mbar 10-4
mbar
5 mbar
Time
Current
m/e = x
m/e = y
16. Internal usage only
Data similarities
Or if you know what you are looking for, you can track specific AMU
19. Internal usage only
GCMS
GC requires a greater level of sophistication than other EGA techniques
More options but more ways to mess up
- Choices of detectors besides MS
- Choices of type of MS – type, mass range, resolution, detection limit
- Choice of filament for Ionization
- Choice of column
- Temperature program
20. Internal usage only
Sampling options
Trapping
- Collection on an medium such as a tube or the head of a column
- Pros
Concentrates all the components
Allows for faster runs
- Cons
Need to know what is coming off
Loss of temperature information
Storage
- Collection and storage of 250 µL of evolved gas at defined time (temperature). Up to 16 fraction (loops) of evolved gas can
be stored and analyzed.
- The GC-MS sequence is automatically started once all loops have been collected
- Pros
Separation of evolved gas at specific decomposition temperature
Compound profile according to the TGA curve
- Cons
Relatively long analysis time
Multi-injection
- Only one loop is used
- The IST collects for e.g. 30 s the evolved gas from TGA and injects every e.g. 30 s to the GC one injection every minute
- Isothermal column temperature such as e.g. 250 °C
- Pros
Short analysis duration (same as TGA experiment)
Good for solvent detection and compound profile from specific ion
- Cons
No real GC separation
Injection to the GC may create baseline artifacts on the heatflow curve
22. Internal usage only
Quantification calculation
The content of SBR is individually calculated using loop 9 to loop 15
22
SBR content at different temperatures
360 °C 370 °C 380 °C 400 °C 420 °C 440°C 460°C
2.5% SBR in NR/SBR 2.27% 2.47% 2.76% 2.51% 2.21% 2.08% 1.64%
5.5% SBR in NR/SBR 5.74% 5.90% 5.89% 6.06% 5.30% 4.97% 4.28%
7.5% SBR in NR/SBR 6.30% 6.94% 7.18% 7.66% 5.91% 5.97% 5.55%
SBR content average Formulation
2.28% ± 0.36% 2.5%
5.45% ± 0.64% 5.5%
6.50% ± 0.77% 7.5%
23. Internal usage only
Summary
Technique Advantages Limitations Typical applications
MS
Pfeiffer Vacuum
Thermostar GSD
320 T
- Online technique, typical
resolution1
2°C
- High dynamic range
(> 5 decades) high
sensitivity
- Quantitative evaluation is
possible
- Maximum mass 300 amu
- Interpretation requires some
knowledge about the
expected evolved gases
- Gas inlet may clog with large
molecules (Condensation)
- Detection of small
molecules (COx, NOx, SOx,
H2O, HCl etc.) inorganic
materials
- Residual solvents in API's
FTIR
Thermo Scientific
Thermo Nicolet
iS10 / iS50,
- Online technique, typical
resolution2
2 °C
- Can also be used for the
analysis of solids (requires
an add-on for ATR-
spectroscopy (iS50 only)
- Delivers also information
about the molecular
structure of the evolved
gases
- Dynamic range around 3
decades (DTGS detector)
- Quantitative evaluation is
difficult
- Spectrum interpretation
requires a lot of experience
and some knowledge about
the expected evolved gases
- less sensitive than MS and
GC/MS
- Detection of complex
organic as well as simple
molecules
- Pharmaceuticals
- Polymers
GC/MS
SRA IST16
Agilent 7590 GC
Agilent 5975 MS
- Mixture of unknown gases
can be easily analyzed
(GC separation, MS
identification)
- Quantitative evaluation is
possible (based on the
chromatogram)
- Can be operated stand
alone for analysis of
liquids
- Storage mode: maximum of
16 gas samples during one
TGA scan; time consuming
- Multiinjection mode: poor
separation (GC is bypassed)
- Maximum mass 1050 amu
No restrictions