A High-Sensitivity Sensor for DSC

A dramatic increase in sensitivity at low signal noise without a significant loss in signal resolution has been achieved with the HSS7 high-sensitivity sensor (Mettler-Toledo Inc., Columbus, OH). Instead of using 56 thermocouples (as in the FRS5 sensor, Mettler-Toledo Inc.), the sensor measures the heat flow with 120 thermocouples. The thermocouples are arranged in three levels in the form of a multiple star.

The sensor uses a ceramic material of higher thermal resistance. This allows an additional increase in sensitivity in the low temperature range. Because the star-shaped heat flow measurement design has been retained, the heat flows into the sample, and reference pans are individually measured with multiple thermocouples below and around the pans. This minimizes influences from temperature gradients across the sensor, especially during heating or cooling, and thus provides flat baselines.

The sensor can easily be removed and replaced, and all previous differential scanning calorimetry (DSC) modules of the STARe system can be upgraded with the sensor. The Multi-STAR™ sensors combine high sensitivity, resolution, baseline stability, and minimum signal-to-noise. This allows DSC to be applied to thin coatings and weak glass transitions (polymers), very small sample amounts (i.e., for forensics and pharmaceuticals), and diluted solutions (i.e., bio-transitions). Examples for applications in these fields will be given.

Experimental

The measuring system was a DSC822e with an HSS7 sensor (Mettler-Toledo Inc.). Nitrogen with a flow rate of 60 mL/min was used as the purge gas.

Results and discussion

Melting of indium

The melting peak of indium is a thermal event frequently used to characterize DSC sensors (see Figure 1). To demonstrate the sensitivity of the HSS7 sensor, a very small sample of indium (8 μg) was measured at a heating rate of 0.012 °K/min). The extremely sharp melting peak and the very low noise level are immediately apparent in the DSC curve.

Figure 1 - Melting of 8 μg indium at 0.01 °K/min.

The indium peak has a height of 5.08 μW with a width at half-height of 0.0038 °K. The largest peak-to-peak noise is less than 0.3 μW; the root mean square (RMS) noise is 0.06 μW.

Unfolding of dissolved lysozyme

Biopolymers in dilute solution have traditionally been studied in microcalorimeters. Such studies are now possible with the DSC822e equipped with the high-sensitivity HSS7 sensor. Figure 2 shows the unfolding of lysozyme dissolved in a buffer solution (pH 3). The endothermic peak (7.8 μW) can still be clearly measured, even at the very low concentration of 0.1%. The curves for the lysozyme samples were corrected by subtracting the curve of the buffer solution.

Figure 2 - DSC curves of lysozyme buffer solutions with 1.8 wt% and 0.1 wt% lysozyme, respectively.

Liquid–liquid transitions of MHPOBC

Figure 3 shows transitions in a liquid crystal 1-methyl heptyloxycarbonyl phenyl-octyloxybiphenyl-4-carboxylate (MHPOBC). The sample was heated very slowly (0.05 °K/min). MHPOBC is known to exhibit different smectic phases with transition temperatures at 118.4 °C, 119.2 °C, 120.9 °C, and 122.0 °C. The transition peaks are extremely sharp and occur within a fraction of a second. The data demonstrate the high-temperature resolution and sensitivity of the HSS7 sensor.

Figure 3 - DSC curves of the liquid–liquid transitions of MHPOBC measured at 0.05 °K/min.

Forensic studies

Highly sensitive analytical instruments must be used for forensic investigations because usually only very small amounts of sample are available. In such work, the thermal behavior of forensic material is compared with that of relevant reference samples. Figure 4 shows how a DSC822e equipped with the HSS7 sensor can unequivocally identify extremely small samples (typically 50 μg) of different plastic materials (poly(imino-1-oxohexamethylene) [PA6], poly(iminohexamethyleneiminoadipoyl) [PA66], and polyethylene terephthalate [PET]) on the basis of melting peak temperatures.

Figure 4 - DSC curves of different polymer fibers with a mass of several micrograms.

Glass transition of silicone rubber

If silicone rubber is rapidly cooled (by placing the sample in a DSC cell that was previously chilled to –145 °C), there is no time for crystallites to form, and the material is in a glassy or amorphous state. The heating curve measured afterward exhibits a well-defined glass transition followed by cold crystallization (Figure 5). When an identical sample is cooled more slowly (e.g., at 20 °K/min), it undergoes partial crystallization. The height of the glass transition step decreases accordingly. Under these conditions, it may then be difficult to detect the glass transition, especially with small samples (here, 717 μg). In such cases, a high-sensitivity sensor that can measure very small heat flows (here, about 13 μW) has to be used.

Figure 5 - DSC curves silicone rubber cooled in different ways. The small glass transition step in the heat flow with a step height of approx. 13 μW is magnified.

Characterization of self-reinforced polymers

As an alternative to glass or carbon fibers, liquid crystal polymers (LCP) can be incorporated into the polymer structure to reinforce polymers. In the microscopic LC domains, the molecules orientate themselves according to the direction of processing. Materials then have very high tensile strength in this direction (self-reinforcement). Figure 6 shows DSC curves of polyethylene that has been reinforced with LCP. After the PE melts, a small endothermic peak is observed at about 217 °C. This is due to a phase transition of the LCP component. In the first heating run, the peak is accompanied by a small shoulder at 50 °C that does not appear in the second run. This indicates that the first heating run eliminates internal stress frozen in the material during the production process.

Figure 6 - First and second heating of a film made from a PE/LCP blend. The weak transition of the LCP component at 217 °C is magnified.

Conclusion

The multilayer thermocouple technology described here made it possible to arrange thermocouples on top of one another in several layers. This further amplifies the measurement signal, resulting in very low noise at the sub-μW level. The weakest effects in the μW range can now be measured, which was previously possible only with microcalorimeters. The HSS7 sensor is an extremely sensitive DSC sensor that exhibits high resolution.

Mr. Weddle is a Technical Representative, Mettler-Toledo Inc., 1900 Polaris Pkwy., Columbus, OH 43240-4035, U.S.A.; tel.: 847-608-1355; fax: 614-985-9091; e-mail:[email protected]. Dr. Riesen is a Senior Research Scientist, Mettler-Toledo GmbH, Schwerzenbach, Switzerland.

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