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Combining thermogravimetric analysis with MS
“It’s basically a balance in an oven.” That’s how Kevin Menard, a Texas-based consultant for Mettler-Toledo International and Veritas Testing and Consulting, describes thermogravimetric analysis (TGA). As a material—solid or liquid—is heated, a very precise balance measures the change in mass. A material can also be heated to a specific temperature, and kept there to measure changes in mass over time. Depending on the material and the purge gas, this change can be a weight loss, which is the most common, or a gain. A wide range of industries, including petrochemicals and pharmaceuticals, use TGA to measure the moisture in a material or to analyze its thermal decomposition and other physical characteristics. The gas created during heating can also be analyzed, which can reveal the molecular structures responsible for the changes in mass directly or by their degradation products. These molecules can be detected with various methods, including mass spectrometry (MS). In the latter case, these technologies are called TGA-MS.
Watching the mass changes versus temperature gives some idea of what is happening. “If something comes off around 100 degrees Celsius,” says Menard, “you guess that it’s water.” That’s based on experience, but it could be wrong. “If you’re working with a pharmaceutical, you can get a peak that looks like water, but in MS you might find three or four solvents, like water, alcohol, etcetera.” By adding MS, there’s no need to guess.
According to Xinwei Wang, manager of new development and project management at New York-based EAG Laboratories, TGA plus some form of detection can be used in many applications: thermal degradation processes, such as oxidation and pyrolysis; vaporization and sublimation; detection of additives in a matrix; characterization of starting materials and end products; investigation of chemical reactions, such as catalysis, syntheses, and polymerization; and outgassing and adsorption/desorption behavior.
Particulars of the process
A TGA platform can usually handle up to a gram, and Mettler-Toledo’s TGAs can handle up to five-gram loads. From a sample, says Wang, TGA can “provide high sensitivity, quantitative information on the change in mass of a sample, with high temperature and time resolution.”
When adding MS, scientists typically use a quadrupole mass analyzer, in which ions pass through a pathway created by four rods arranged in parallel. “In EAG Laboratories,” says Wang, “our TGA-MS instrument is also equipped with a long-optical path FTIR [Fourier transform infrared] detector to provide high-sensitivity detection of low molecular species—such as H2O, HF, HCl, SO2, NOx, CO, and CO2.”
The sensitivity of a TGA-MS depends on how well the evolved gas is delivered from the TGA to the MS. “The transfer line is typically heated to 200 to 250 degrees Celsius to enhance the transferring efficiency,” Wang explains. “Nevertheless, it is difficult to detect high boiling components, such as plasticizers encountered in nearly all organic polymer products.”
As Menard says, “You need to tune the technique to the material.” Light molecules can be analyzed with TGA-MS. For heavy fragments, says Menard, use FTIR. Analyzing complex mixtures, he says, requires gas chromatography (GC) (TGA-GC-MS). In addition, an analytical lab should steer customers to what Menard calls “the right solution for their budget.”
Advanced analysis
TGA-MS is applied to many materials, such as siloxane, that are widely used in electronics, photonics, and medical devices. If the siloxane isn’t cured properly, the device can fail. TGA-MS can provide information about curing kinetics and thermal degradation mechanisms that can help manufacturers improve the reliability of devices.
To get the best results from thermogravimetric analysis and mass spectrometry, the devices must be efficiently connected, which often requires a dedicated device. (Image courtesy of Mettler-Toledo.)This analytical technology can also be used in developing batteries. In lithium-ion batteries, safety is a key concern, because they can leak or even explode. TGA-MS can reveal potentially problematic gases created if the battery decomposes under heat.
Scientists use thermogravimetric analysis to explore the evolution of many processes, such as the curing of siloxane shown here in an argon atmosphere and a heating rate of 10 degrees Celsius per minute. (Image courtesy of EAG Laboratories.)Many other materials can be studied with TGA-MS. Scientists used TGA-MS and other techniques to study how pre-ceramic polymers—polysilazane and polysiloxane—turn into ceramics.1 They noted that “thermally induced decomposition of the pre-ceramic polymer is the critical step in the synthesis of polymer derived ceramics (PDCs) and accurate kinetic modeling is key to attaining a complete understanding of the underlying process and to attempt any behavior predictions.” Unraveling the kinetics, in this case, required TGA-MS and a numerical process that the scientists described as “a semiempirical deconvolution method based on the Frasier-Suzuki function.”
TGA-MS and related methods also help researchers study the processing of food waste. Scientists used TGA-FTIR and TGA-MS to analyze the pyrolysis—high-temperature breakdown—of chestnut shells, cherry stones, and grape seeds.2 “In order to make available theoretical groundwork for biomass pyrolysis, activation energies were calculated with the help of four different model-free kinetic methods,” these researchers wrote. “The results are attributed to the complex reaction schemes which imply parallel, competitive, and complex reactions during pyrolysis.” The team used TGA-MS to study the volatile gases released during pyrolysis, and reported that “the main evolved gases were determined as H2O, CO2, and hydrocarbons such as CH4 and temperature-dependent profiles of the species were obtained.”
Pyrolysis is used to decompose other materials, such as polyvinyl chloride (PVC). One study showed that adding electric arc–furnace dust reduced the activation energy to pyrolyze PVC and left valuable metals—including lead and zinc—in the residue that could be recovered. TGA-MS showed that this process can produce gases that could be used as sources of energy.3
Dealing with the data
Just collecting the data doesn’t ensure the best analysis. “You need libraries to interpret the data,” Menard says. “You need to use search programs and libraries, which have gotten very good.”
Very good, yes, but not entirely automatic. “The result doesn’t say, ‘The atomic mass ion is 180 and the molecule is x,” Menard says. Instead, the MS could detect a mass of 180 that can’t be identified definitively by the ion mass alone. “Then, we try to identify it by the fragments and fragmentation patterns, and, depending on how many materials are in the evolved gas, it can get very complicated.” Menard continued, “We run analysis to get the best fit, but if five materials come out the same with 99% probability, you need some skill to decide what is what. … One of the reasons folks move to TG-GC-MS is that the GC separation makes the MS part simpler.”
So, even with advanced search algorithms and libraries, using TGA-MS requires a skilled scientist to accurately analyze a material. With the right equipment, software, and operator, this technology can reveal the characteristics of many materials and processes.
References
- García-Garrido, C.; Sánchez-Jiménez, P.E. et al. Combined TGA-MS kinetic analysis of multistep processes. Thermal decomposition and ceramification of polysilazane and polysiloxane preceramic polymers. Phys. Chem. Chem. Phys.2016, 18, 29, 348–60.
- Özsin, G. and Pütün, A.E. Kinetics and evolved gas analysis for pyrolysis of food processing wastes using TGA/MS/FT-IR. Waste Manag.2017; doi: 10.1016/j.wasman.2017.03.020.
- Al-Harahsheh, M.; Al-Otoom, A. et al. Pyrolysis of poly(vinyl chloride) and electric arc furnace dust mixtures. J. Hazard. Mater.2015; doi: 10.1016/j.jhazmat.2015.06.041.
Mike May is a freelance writer and editor living in Florida. He can be reached at [email protected].