During a DSC run of a single element material, its melting peak was found to be split into two (say peak-371 C and peak 2 - 371.8 ). what can be the reason for such distinct splitting of the melting peak?
Hi,
This double peak could be due to temperature gradients in the apparatus. This phenomenon may happen when the sample moves into the crucible during melting and spread onto the whole surface of the underneath sensors.
Try one more run to see if it still happens once the material was melted.
It can also be due to some impurities...
Regards.
The DSC only provides us with an information on a heat exchange which is either endo or exothermic. Because multiple reasons can be behind a heat exchange, it will never be possible to answer this question from a DSC curve alone. Use other equipment, such as hot stage microscopy, X-ray diffraction, solid state NMR, or whatever seems the logical choice for the phenomenon under study to look at the underlying causes of the peaks that we observe by DSC.
That said, if the described peak is in fact a single peak of a single crystal with a badly defined max causing two maxima to appear, then it is likely caused by heat transfer problems and therefore one should integrate the entire peak and take the onset of the lowest peak as the melting value. But be aware that there is never a single rule, when it comes to interpretation of DSC data.
The following is no more than a tentative explanation, since neither the sample was identified, nor the thermogram is shown. Instrument and operational parameters were also not mentioned.
The peak due to melting is endothermic. The peak may be sharp; please check if it is distorted (tilted). Due to heat absorption at the melting front, the sample may possibly first have melted incompletely, close to the pan inner surface, while the core might still be not melted, and perhaps even cooled slightly due to heat transfer to the melting front. That perhaps slightly decreased the average sample pan temperature for a short period of time, what may have inhibited further progress of melting for a glimpse of time. Upon further heating, melting (at the sample pan core) could possibly be resumed and completed shortly after.
That being the case, and to avoid what seemingly might be an experimental artifact; you may either: (i) reduce the quantity of sample; (ii) select temperature of the standard pan for the horizontal axis (rather than the sample pan temperature); (iii) select time for the horizontal axis; (iv) select a suitable metallic pan material (rather than alumina), for higher thermal conductivity.
It also seems advisable to check the subsequent cooling plot for the reverse peak (crystallization).
You may want to check also my post elsewhere at this forum for a related query; except that for crystallization (exothermic peak), rather than for melting (endothermic): https://www.researchgate.net/post/2nd_spike_on_DSC_curve_can_anyone_tell_me_what_is_it
It would have been better if you would have mentioned the nature of the crystal, organic, organic. semi organic !
In general, the answer of Stéphane seems me appropriate. But sorry, the information given by you is vague. If you work on a single crystal of a chalcogen, then we have S (melting 115°C, before phase transition at 95°C --> could give double peak), Se (melting 221°C), Te (melting 450°C). There is simply no chalcogen melting at 371°C, as you wrote!
- Badges
- Science topic
- Similar topics
- Biological Science
- Kinesiology
- Running