Differential Scanning Calorimetry—A Fundamental Thermal Analysis Tool

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 Differential Scanning Calorimetry—A Fundamental Thermal Analysis Tool

Please see our Differential Scanning Calorimeter (DSC Instrument) and Thermal Analysis Equipment / Thermal Analyzer sections to find manufacturers that sell these products

A differential scanning calorimeter (DSC) is a benchtop instrument used to determine thermodynamic transitions and heat capacities of a variety of substances. A small sample—typically 5–20 mg, around the size of the tip of a #2 pencil—is placed in the instrument’s furnace, the temperature is ramped up (or down), and the amount of heat or power necessary for the sample and the reference to achieve (or hold) a temperature is measured and compared.

Materials absorb energy as they are heated and can drastically change their properties as they reach a given temperature. Ice is a solid that, when heated to 0 ºC, absorbs any additional heat rather than continuing to warm, and undergoes a phase change—a transition—to become liquid water. Many materials undergo transitions including melting, crystallization, and “glass transition, which is when something goes from a solid to a not-quite solid—almost like molasses,” says Paul Rementer, product specialist for physical measurement at Shimadzu Scientific Instruments (www.ssi.shimadzu.com). “This is a very important transition point to determine the purity of a polymer and to see how the properties are affected by different additives.”

Applications and features of DSCs

A DSC tracks the heating curves and heat capacities, with dips, peaks, and other changes being indicative of transitions. A variety of industries use this information for both quality control and R&D, “from the melting point of chocolate—to make sure it melts in your mouth and not in your hand—to solid rocket fuel boosters, and everything in between, like plastics, cosmetics, and foods,” notes Michael Ferraco, business manager for the thermal division in the U.S. of Mettler-Toledo (www.mt.com). The pharmaceutical industry uses DSC to study polymorphic forms of drugs that may be unmasked by heating or cooling.

The term “DSC” as used by most manufacturers refers to an instrument that is capable of a temperature range as low as –180 ºC up to as much as 725 ºC. For practical purposes, “most DSCs have a very similar temperature range,” says Louis Waguespack, product manager for thermal analysis at TA Instruments (www.tainstruments.com). “There are a series of high temperature DSCs that are classified in a little bit different category.” The latter tend to use a slightly different or a hybrid technology to achieve temperatures up to 2000 ºC or so, necessary for characterization of inorganics, ceramics, and some metals, and sometimes are combined with other capabilities.

The lower temperature range is a factor of the cooling system. Lab air can be used to bring a DSC down to ambient temperature, and a DSC can generally be fitted with a manual or automatically controlled liquid nitrogen cooler. Most manufacturers also offer mechanical cooling systems (called intercoolers), “like a little self-contained refrigeration unit with a little cold finger that’s right close to the furnace,” says Ferraco.

Several manufacturers offer a photo DSC accessory to study the effect of light on materials. Typical applications include degradation and decomposition under UV light, and the curing of dental resins. Most employ a mercury vapor lamp as a light source, with filters restricting the wavelength reaching the sample and intensity being software-controlled.

A variety of sample pans and crucibles of different designs and compositions are available (not all compatible with autosamplers). For example, the contents of batteries are highly reactive and must be run in a sealed high-pressure crucible, so that everything that devolves from the sample stays inside the crucible, says Ilir Beta, laboratory applications scientist at NETZSCH Instruments North America (www.netzsch-thermal-analysis.com). A high-pressure crucible is not to be confused with a high-pressure instrument, Beta points out.

DSCs operate under standard atmospheric pressure. Specialized instruments (often called high-pressure DSCs) in which open pans are typically used are available from several vendors. According to Beta, “You can adjust the pressure inside the chamber.”

Advanced DSC techniques

At least 90% of the applications a DSC is asked to do can be accomplished by any of the instruments—“it’s the remaining 10% where you start needing special techniques,” says Kevin Menard, business manager for thermal analysis at PerkinElmer (www.perkinelmer.com).

A DSC’s standard heating and cooling rate is slow enough for a typical sample to heat evenly, go through its various transitions, and achieve good resolution. Yet sometimes those transitions can be masked, for example, by kinetic events. For such troublesome cases, there is a technique in which a nonlinear heating or cooling rate is applied to the sample, allowing difficult-to-find transitions to be ferreted out. TA Instruments’ patent for modulated DSC in the United States expired about two years ago, and a version is now offered by most manufacturers.

Several vendors offer a “fast scan” technique as well, in which the temperature is ramped up or down at rates of up to 750º/min (in the case of PerkinElmer’s HyperDSC). The principal advantage is the ability to prevent a substance from spending enough time at a given temperature for a particular transition to take place. Menard explains, “I can use high-speed heating to kinetically trap unstable forms of material.”

Mettler-Toledo’s specialized Flash DSC instrument has taken the concept of fast scan to an extreme by placing the thermal elements and sensors onto a disposable chip, onto which a nanogram quantity of sample is melted, “so we can heat and cool samples at millions of degrees C per minute,” says Ferraco. Whereas heating and cooling in a standard DSC will allow a sample to recrystallize and change, the Flash DSC will “heat it so quickly we don’t give it a chance to reorganize, and you can actually look at the properties of the material as received from [the] vendor.”

Choosing a differential scanning calorimeter

First and foremost a DSC should meet the user’s needs, both at the time of purchase and into the projected future. Research labs may be running something next year that hasn’t even been invented yet, and it’s important to have a flexible instrument capable of generating the best data no matter what.

Certain accessories are factory installed, meaning that they need to be chosen upon purchase—depending on the vendor, this may include the cooling system and autosampler—while others, like photo DSC, can typically be added at any time. Many vendors offer a choice of different intercoolers as well as liquid N2 coolers, each with their own temperature ranges. Furnaces, too, have a range of temperatures. Thus, it’s important to anticipate what range you may need.

The heating and cooling rates for most instruments are similar. Nonetheless, not all instruments are capable of a fast scan. Similarly, while most provide some sort of modulated technique, not all do, and not all fast scans are created equal, so it’s best to determine whether this is something you’ll need.

Perhaps the elephant in the room is performance specs. It’s not always easy to determine whether an instrument from one manufacturer performs better than one from another. Each may measure resolution, sensitivity, or baseline reproducibility in a different way. One may present raw data while another presents corrected data, for example, or one may calibrate over the width of the entire temperature range, while another quotes only for a sample such as indium (which melts at 156.6º), points out Beta. (That being said, several vendors offer high-sensitivity sensors that can be swapped for the standard sensor.)

Cost, of course, is always a consideration. Menard encourages people to “think about the whole cost, not just the purchase price,” including the cost of warranty and service outside of warranty. The network supporting that service should be taken into account as well, he advises.

Waguespack suggests that the best way to decide on a DSC is to talk to a sales or applications specialist. “We’ll ask about the applications and suggest which DSC will be best. They can send in samples for us to run, or we’ll bring the customer in for a demo or will bring the instrument to them for a demo, to run their sample on the instrument.” In addition to determining whether the instrument is right for the sample, a demo will also give an idea of how user-friendly its operation is.

Anything new in DSC?

“DSC is really a pretty stable science at this point,” says Menard. Many of the latest innovations have to do with things like hyphenating (combining) DSC with other techniques, such as near-infrared (NIR) or Raman to do what’s called spectral DSC.

There are attempts to incorporate light-emitting diodes (LEDs) or lasers into photo DSC, both to increase the irradiation rate and restrict the wavelength, says Beta.

The field also seems to be putting effort into analysis, and connectivity, too. NETZSCH offers a built-in library against which DSC curves can be compared, for example. In addition, most vendors now have packages that can perform post-test kinetics calculations.

Josh P. Roberts has been a full-time biomedical science writer for more than a decade. After earning an M.A. in the history and philosophy of science, he went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology; e-mail: [email protected].

Please see our Differential Scanning Calorimeter (DSC Instrument) and Thermal Analysis Equipment / Thermal Analyzer sections to find manufacturers that sell these products