Eclectic Techniques Measure Thermal Properties

Thermal analysis is the broad category of at least 20 techniques that measure some fundamental property of matter as a result of adding heat. For example, dilatometry measures volume changes upon heating, thermomechanical analysis quantifies the change in dimension of a sample as a function of temperature, and thermo-optical analysis detects changes in optical properties on heating or cooling.

This discussion applies mostly to two techniques, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), although many of the ideas presented here apply to other types of thermal analysis. DSC and the related micro-DSC measure the amount of heat required to change the temperature of a sample. DSC is most often used with materials that undergo phase changes.

Homogeneous or nonhomogeneous materials that melt or freeze, or that undergo transformation from one solid form to another, are excellent candidates for DSC. Notable examples include the glass transition temperature of polymers and the interconversion of crystalline polymorphs in the pharmaceutical industry. Approximately 60 percent of thermal analyses involve DSC.

TGA is often used to quantify residual solvent or moisture content of a sample (for example, in foods and pharmaceuticals). A related technique, evolved gas analysis, analyzes gas decomposition products.

Trends

From the perspective of Kenneth Aniunoh, Ph.D., senior product specialist at Shimadzu Scientific Instruments (Columbia, MD), thermal analysis is trending toward user-friendly software, smaller and more compact instruments, availability of auto-samplers, and greater value. “The same is true for many other laboratory instrument types,” says Aniunoh. “Bench space is at a premium, and overly complicated software can be a real detriment to productivity.”

Simplification without sacrificing features is a trend observable in most lab instruments. Thermal analyzers are no exception. “Thermal analysis used to require an operator with a Ph.D. in calorimetry or rheology,” notes Kevin Menard, global product manager for thermal, elemental, and hyphenated analysis at PerkinElmer (Shelton, CT). “Now it is a more general-purpose tool. Most of the activity is no longer directed at new instrumentation, but at applications and the development of standardized methods.”

For instrument manufacturers, this means producing systems that are more accessible to technicians with a B.S. degree or even just a high school diploma. Vendors must provide a level of support based on users’ expertise and applications.

This is not as straightforward as it sounds, because many industries test the same types of samples. Polymers, which are targets for up to 70 percent of all thermal analyses, are tested not just at the raw plastics processing plant, but in nearly every manufacturing industry, especially packaging.

Users want thermal analysis systems that deliver results quickly and test several parameters at once. The first requirement is achieved by the use of autosamplers and lab automation. “Years ago, a major oil company might have had twelve DSCs running in one room, and enough scientists to run all of them. Today the number of instruments is far smaller, and there might be just one person in the room,” Menard says.

Multi-parameter analysis is achieved through “hybrid” thermal techniques that pair TGA or DSC with standalone chromatographs, GC-MS, LC-MS, or spectrometry. Thus a TGA or evolved gas analysis (for gas decomposition products) will provide not just the temperature of evolution, but an unequivocal identity for the evolved gas. As Menard observes with respect to TGA, “You can’t automatically assume something that comes off at 100 degrees is water. A lot of things come off at about 100 degrees. If you’re working on a drug, the FDA will ask about the identity of impurity, and you’ll need a secondary method to identify it.”

Customers are demanding higher-quality measurements and greater ease of use, according to Steve Aubuchon, Ph.D., product manager for thermal analysis at TA Instruments (New Castle, DE). “Instruments are becoming more ‘walk-up,’” he explains, “and users don’t want to spend half their careers learning to use them.”

TA has focused its efforts on improving the user experience, through both software and sample prep tools. The company also maintains a YouTube channel where it uploads weekly “Tech Tips” on thermal analysis techniques and sample preparation.

Interface, which is based on software, has become nearly synonymous with user experience. TA takes this aspect of product development as seriously as the instrument side, according to Aubuchon. Some features it has introduced include a touchscreen interface, macros and wizards, and the ability to schedule after-hours calibration.

“People don’t realize it, but sample prep is key to the success of a thermal analysis,” Aubuchon tells Lab Manager Magazine. TA has recently introduced a sample-loading tool for filling DSC pans with a powdered sample and for tamping down the sample to provide good thermal contact.

Purchase decisions

Kenneth Aniunoh suggests that purchasers, particularly first-time buyers, first speak with an expert about the specific materials and applications that make up the lab’s workflows. “This step costs the customer nothing,” he says. “Most vendors of thermal analyzers will gladly put customers in touch with one of their experts to discuss needs and requirements.” Second, customers must understand the properties they are trying to study, as this provides insight into what types of thermal analysis methods are most suitable. Two other factors are instrument sensitivity and the temperature range of interest, and the likely heating and/or cooling rates likely to be encountered during the lifetime of the instrument.

“And finally, to prevent after-sale buyer’s remorse, customers should request a demo, using their own material,” recommends Aniunoh, “to confirm that the instrument chosen is appropriate for the given application.”

“Price is an issue,” says Kevin Menard, “but you really want to look at the cost of ownership, support, and service. If you take many samples per day, you’ll want a good service contract.” An FDA-regulated lab that turns on a TGA once a month, on the other hand, would be more interested in sensitivity and the accuracy of hybridized techniques like GC-MS.