Applications of DSC

8/13/2019 Applications of DSC

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Applications ofDSC


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Differential Scanning Calorimetry

PRINCIPLE: It is a technique in which the energy

necessary to establish a zero temp. difference between thesample & reference material is measured as a function of

temp.

Endothermic reaction: In endothermic reaction more

energy needed to maintain zero temp difference betweensample & reference.

Eg: Melting, Boiling, Sublimation, Vaporization,

Desolvation.

Exothermic reaction: In exothermic reaction, less energyneeded to maintain zero temp difference between sample

& reference.

Eg: Crystallization, Degradation, Polymerization.


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Applications

Determination of Heat Capacity Glass Transition Temperature

Crystallization

Determination of Melting Point Degree of Crystallinity

Detection of impurity

Study of polymorphism Drug excipient compatibility study


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Determination of Heat Capacity

When we start heating two pans, the computer will plot the

difference in heat output of the two heaters against temperature

that is plot of heat absorbed by the polymer against temperature


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By dividing heat flow (q/t)by the heating rate (T/t). It ends up

with heat supplied divided by the temperature increase, which is

called heat capacity.

When a certain amount of heat is transferred to the sample, its

temperature increases by a certain amount, and the amount of

heat it takes to get a certain temperature increase is called theheat capacity or Cp .


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Glass Transition Temperature (Tg)

The Tg is the reversible change from a glassy to rubbery state &

vice-versa DSC detects Tgsby a step change in Cp

On further heating the polymer to a certain temperature, plot will

shift downward suddenly, like this:


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Crystallization After glass transition, the polymers have a lot of

mobility. When they reach the right temperature, they

will give off enough energy to move into veryordered arrangements, which is called crystals.

The temperature at the highest point in the peak is

usually considered to be the polymer's crystallizationtemperature, or Tc


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Also, we can measure the area of the peak, which tells us

the latent energy of crystallization of the polymer

But most importantly, this peak tells us that the polymercan in fact crystallize

If you analyze a 100% amorphous polymer, like

polystrene, you wouldn't get this peak, because suchmaterials don't crystallize


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Melting Point

When we reach the polymer's melting temperature, Tmthe polymer

crystals begin to fall apart, that is they melt. It comes out of their

ordered arrangements, and begin to move around freely that can be

spotted on a DSC plot


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Degree of Crystallinity

DSC can also tell us how much of a polymer is crystalline and

how much is amorphous?

If we know the latent heat of melting, Hm, we can figure out the

answer.

Where, mc is the total amount of grams of polymer that were

crystalline below the Tc,

mtotal is weight of the sample


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Determination of Purity

A sharp melting endotherm indicates the relative

purity where as broad asymmetric curve suggest

impurity. The presence of minute amount of

substance (impurity) broadens its melting range &

lowers its mp.

Compare to other thermal methods, DSC is best

method for detection of impurity.


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Polymorphs Determination

The all thermodynamic parameter in the

polymorphism substance is different like melting,sublimation temperature, kinetics, stability,solubility, heat capacity, crystal hardness & shapewhich are extremely important for the dosagesform.

During preformulation, it is important to identifythe polymorph that are stable & also important todetermine whether polymorphic transition arepossible within the temperature range used forstability studies, processing (drying, milling,mixing, granulation etc.) & storage.


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Example: Mannitol occurs in four forms, all melting at

the same temp. & they are non hygroscopic, only form

B shows a small endotherm exotherm process.


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Drug-Excipient Compatibility study

o DSC is widely used to investigate and predict any

physico-chemical interaction between drug and excipients

involving thermal changes.

o METHOD

-The preformulation screening of drug-excipient interaction

requires (1:1) Drug:excipient ratio, to maximize the

likehood of observing an interaction.

-Mixture should be examined under N2 to eliminate

oxidative and pyrrolytic effects at heating rate ( 2, 5 or

100c / min) on DSC apparatus.


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Graph 1 of figure shows peak at 278.330c. (melting endothermic

peak of Ofloxacin).

Graph 3 (Physical mixture of Ofloxacin & Lactose) shows

absence of peak at 278.330c and slight pre shift in Lactose peaks.

DSC RESULT-- INCOMPATIBLE

Example: DSC of Ofloxacin Tablets


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Documents

Applications of DSC