Sample preparation process using METTLER TOLEDO melting point tools:
Step 1: First, the sample needs to be dried in a desiccator. Then a small portion of sample is finely ground in a mortar.
Step 2: Several capillaries are prepared simultaneously for measurement with a METTLER TOLEDO instrument. The capillary filling tool perfectly assists the filling as the empty capillaries are securely held in a peg-like grip. Collecting a small sample portion from a mortar is easily done with the assistance of the tool.
Step 3: The small amount of sample at the top of the capillaries is then moved down the capillary by releasing the grip and gently bouncing the capillaries on the table several times. This action packs the sample tightly down into the bottom of the capillary. The 'bouncing effect' causes tight packing of the substance and avoids the inclusion of air pockets.
Step 4: The correct filling height can be checked with the engraved ruler on the capillary filling tool. Generally the filled height should not exceed 3 mm.
7. Instrument Setup
Along with proper sample preparation, the settings on the instrument are as well essential for the exact determination of the melting point. Correct selection of the start temperature, the end stop temperature and the heating ramp rate are necessary to prevent inaccuracies due to a heat increase in the sample that is incorrect of too fast:
a) Start Temperature
Melting point determination starts at a predefined temperature close to the expected melting point. Up to the start temperature, the heating stand is rapidly preheated. At the start temperature the capillaries are introduced into the furnace, and the temperature starts to rise at the defined heating ramp rate.
Common formula to calculate the start temperature:
Start Temperature = expected MP – (5 min * heating rate)
b) Heating Ramp Rate
The heating ramp rate is the fixed rate of temperature rise between the start and stop temperatures for the heating ramp.
Results depend strongly on the heating rate - the higher the heating rate the higher the observed melting point temperature.
Pharmacopeias apply a constant heating rate of 1 °C/min. For highest accuracy and non-decomposing samples use 0.2 °C/min. With substances that decompose, a heating rate of 5 °C/min should be applied. For exploratory measurements a heating rate of 10 °C/min may be used.
c) Stop Temperature
The maximum temperature to be reached in the determination.
Common formula to calculate the stop temperature:
Stop Temperature = expected MP + (3 min * heating rate)
d) Thermodynamic / Pharmacopeia Mode
There are two modes for melting point evaluation: Pharmacopeia melting point and thermodynamic melting point. The pharmacopeia mode neglects that the furnace temperature is different (higher) during the heating process than the sample temperature, meaning that the furnace temperature is measured rather than the sample temperature. As a consequence, the pharmacopeia melting point depends strongly on the heating rate. Therefore, measurements are only comparable if the same heating rate is applied.
The thermodynamic melting point on the other hand, is obtained by subtracting the mathematical product of a thermodynamic factor ‘f’ and the square root of the heating rate from the pharmacopeia melting point. The thermodynamic factor is an empirically determined instrument-specific factor. The thermodynamic melting point is the physically correct melting point. This value does not depend on heating rate or other parameters. This is a very useful value as it allows melting points of different substances to be compared independently of experimental setup.
9. Influence of the Heating Rate on the Melting Point Measurement
Results depend strongly on the heating rate - the higher the heating rate the higher the observed melting point temperature. The reason is that the melting point temperature is not measured directly within the substance, but outside the capillary at the heating block, due to technical reasons. Therefore, the temperature of the sample lags behind the furnace temperature. The higher the heating rate, the more rapid the rise in oven temperature, increasing the difference between the melting point measured and the true melting temperature.
Due to the dependence of the rate of heat increase, measurements taken for melting points are comparable with one another only if they are taken using the same rates.
12. Mixed Melting Point Determination
If two substances melt at the same temperature, a mixed melting point determination can reveal if they are one and the same substance. The fusion temperature of a mixture of two components is usually lower than that of either pure component. This behavior is known as melting point depression.
For mixed melting point determination, the sample is mixed with a reference substance in a 1:1 ration. Whenever the melting point of the sample is depressed by mixing with a reference substance, the two substances cannot be identical. If the melting point of the mixture does not drop, the sample is identical to the reference substance that was added.
Commonly, three melting points are determined: sample, reference and 1:1 mixing ratio of sample and reference. The mixed melting point technique is an important reason why all high-quality melting point machines accommodate at least three capillaries in their heating blocks.