Using DSC for the Identification of Polymorphism

Onsorbitol is regularly used as a sweetener in food products and is a sugar alcohol found in fruits. It exists in four anhydrous crystalline phases plus the hydrate. With regard to melting and to water absorption, each form acts differently as the polymorphism has an affect on the characteristics of the substance.

How to Measure

Sigma-Aldrich prepared a sorbitol sample (mass: 3.81 mg) in a Concavus crucible and measured with the DSC 204 F1 Nevio. During a heating rate of 10 K/min three heatings were undertaken between -80 °C and 150 °C:

• 1^st heating: sample as received
• 2^nd heating: after 1^st heating and cooling at 10 K/min
• 3^rd heating: after 2^nd heating, cooling at 10 K/min and 24 hours at room temperature

The DSC measurements were taken in a dynamic nitrogen atmosphere.

What the Curves Look Like

Figure 1 shows the DSC curves of sorbitol throughout the three heating runs.

• 1^st heating: The endothermal peak with an extrapolated onset temperature of 91 °C is due to the melting of the sample. This temperature is usual for the change which is known as a gamma form, it is the most stable one so it is the best suited to commercial applications.
• 2^nd heating: No melting peak is identified: The sample is in an amorphous state with the glass transition at -1 °C (mid temperature) and no longer shows any crystalline phase.
• 3^rd heating: One day at room temperature is adequate for crystallization to happen. Yet, the peaks identified at 57 °C and 81 °C (peak temperatures) demonstrate that it is a different crystalline form than the one detected during the initial heating.
• This DSC curve is usual for the modification known as crystallized melt. This form is more hygroscopic than the gamma one. Although, it is often employed commercially because of its glassy and transparent appearance, in the manufacture of hard candies for example.

Figure 1. DSC curves of sorbitol as received (blue), after controlled cooling (pink) and after one day at room temperature (green). The differences in the curves indicate different polymorphic modifications.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH.

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