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Tech Intelligence
by Mike May AL
AL
A Double Mass Measurement Combining thermogravimetric analysis with MS
“It’s basically a balance in an oven.” That’s how Kevin Menard, a Texas-based consul- tant for Mettler-Toledo International and Veritas Testing and Consulting, describes thermogravimetric analysis (TGA). As a mate- rial—solid or liquid—is heated, a very precise balance measures the change in mass. A material can also be heated to a specific tem- perature, and kept there to measure changes in mass over time. Depending on the mate- rial 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 dur- ing 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 tempera- ture gives some idea of what is happening. “If something comes off around 100 degrees Celsius,” says Menard, “you guess that it’s wa- ter.” That’s based on experience, but it could be wrong. “If you’re working with a pharma- ceutical, you can get a peak that looks like water, but in MS you might find three or four solvents, like water, alcohol, etcetera.” By add- ing 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
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.)
AMERICAN LABORATORY 38 JUNE/JULY 2017
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 po- lymerization; 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 Laborato- ries,” 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 mo- lecular species—such as H2 CO, and CO2
O, HF, HCl, SO2 .”
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. “Nev- ertheless, 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 tech- nique to the material.” Light molecules can be analyzed with TGA-MS. For heavy fragments, says Menard, use FTIR. Analyzing complex mix- tures, 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,
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