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.