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METTLER TGA/sDTA851e ThermoGravimetric Analyzer
With Blazers ThermoStar 300 AMU Mass Spectrometer
The TGA/DTA/MS at the MRL is a great tool for the qualitative
and quantitative analysis of a sample as it is heated. The TGA measures
the change in mass of a sample as a function of temperature and time.
Physical changes in the sample that do not involve a change in mass but do
involve a change in energy, such as a phase change, are captured in the DTA
curve. The evolved gas can be routed to the MS for identification.
The system is a Mettler STARe ThermoGravimetric Analyzer,
TGA/sDTA851e, coupled to a Balzers ThermoStar Mass Spectrometer. The TGA
can go up to 1100°C in almost any atmosphere. Its balance resolves to 1
microgram. Typically the sample size is around 10 milligrams. The MS can
measure the gas evolved from the thermal decomposition of the sample up to
300 AMU. We also have a Cahn TherMax 400 TGA
that can heat to 1500°.
The Mass Spectrometer is a Balzers Quadrupole. It typically
measures integer mass units and can detect between 1 and 300 AMU. It has a
very low detection limit, typically picking up materials in the PPB range.
It can easily resolve the different isotopes of an element.
This TGA-MS system is flexible, robust, and very easy to use.
The temperature ramp is fully and easily programmable. There is a sample
robot that allows up to 34 samples to be run automatically. Almost anyone
can be trained to use the TGA in less that two hours. Learning to use the
MS adds another two to three hours.
The MRL Chemistry Facility does not have the staff resources to test TG-MS samples
on a contract basis nor is it our intention to compete with private sector analytical
services.
Specifications for the TGA
| Balance Resolution | 1 micro gram |
| Typical sample size | 10 milligrams |
| Temperature range: | 1100 °C |
| Heating rate | 25° C to 1100 °C: 5 min |
| Cooling rate | 1000° C to 100 °C: 20 min |
| High temperature accuracy: |
±0.25 °C/±0.5 °C (up to 1100 °C) |
| sDTA resolution: | 0.005 °C |
| Sample robot/Autosampler holds 34 samples |
| Gas-tight measuring cell |
Specifications for the MS
| Range | 0 - 300 AMU |
| Resolution | 1 mass unit (actually much less) |
| Transfer line | can be heated to 200° C |
| Minimum Detection | low PPB range |
Because of the one microgram resolution of the TGA balance,
very small samples can be tested. The typical sample size is 5 to 15
milligrams, but changes in the mass of a 1 milligram sample would be
evident to 0.1%.
The sample chamber in the furnace is sealed to allow full
control of the atmosphere to which the sample is exposed. The active part
of this chamber is made of fused silica, so that samples can be exposed to
many reactive gasses. Samples can also be run under vacuum. To keep the
system assessable and easy to use, all control of protective and reactive
gas is manual. Dry nitrogen, dry air, and UHP helium are provided for
routine analysis. Arrangements can be made for the use of other gasses.
Samples are always loaded onto the TGA balance by a Sample
Robot. This allows multiple samples to be run without the user needing to
be present. Since tests frequently take from one to three hours, this can
save the user a lot of time and trouble.
Setting up the TGA temperature program is completely intuitive.
Sample preparation consists of weighing a crucible, loading the sample into
the crucible, weighing the full crucible, and setting it on a tray.
Configuring a test takes only minutes. Getting a test into the queue for
the autosampler is trivial. Raw data may be printed, exported, or analyzed
within the software.
The TGA analysis software is powerful and flexible. The needs
of most users will be easily accommodated by the software. Final results
may be printed on a color ink jet printer in the lab or exported to several
formats.
The evolved gas from the TGA is routed to the MS inside a fused
silica capillary. This capillary begins right on top of the sample inside
the TGA furnace to maintain the maximum integrity of the evolved gas. The
capillary can be heated up to 200°C to keep the evolved gas from condensing.
Typically, the MS is used to look at integer atomic mass units within the range
of 1 to 150, but it can measure up to 300 AMU. The resolution is usually set at
one mass unit. For example, the difference between the 16 O and 18 O in
ordinary water is easily seen.
The software for the MS is relatively easy to use and does permit full
integration of the MS into the TGA autosampler sequence.
If you have any questions, please feel free to contact Joe Doyle at x7925 or at
jdoyle@mrl.ucsb.edu.
Anasys Thermal Methods Consultants has a fine introduction to thermal
analysis at
http://www.anasys.co.uk/library/macrota.htm.
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