There are some general requirements for the preparation of samples which are equally applicable to DSC, TGA, TMA and DMA.
- To evaluate the temperature gradient in the measured sample, a piece of indium is placed both above and below the sample. The difference in temperature is now measured by assessing the two onsets in the case of DSC, TGA, TMA, and DMA.
- To keep moisture out of the sample while it is waiting on the robot table, the lid piercing accessory is used for DSC, TGA [UC 20/17]
Some factors to be considered when taking a sample include:
- Selecting the production lots which are to be examined
- Choosing the time point or points, if multiple measurements are needed, during the production lot when the sample is to be examined
- Selecting the part or parts of the lot from which the samples should be taken for examination
- Does the point of sampling and the size of the sample represent the product lot or is it nonhomogeneous? Does the sample allow conclusions to be made about the whole sample?
- Determining sample size as regards the volume and number of items in the sample
- The frequency of determination of a given material property, and the number of samples to be examined
- Whether the samples are taken at different points of the lot and then combined to yield an aggregate or composite sample, or processed individually as test samples
- Whether the samples consistently represent the bulk sample
A typical sampling task [UC 29/1] is shown in the figure below:
The recommended sample size for DSC is given in the table below:
Filling a Crucible
The table below shows how various sample types are to be inserted into DSC crucibles:
- At measurements taken above a temperature of 150 °C, a small hole should be made in the lid of the crucible as otherwise evaporation will cause the crucible to explode.
- The crucible’s bottom should remain flat when it is sealed in the crucible press, avoiding overfilling. This flat shape ensures that heat is transferred properly from the crucible to the sensor, optimizing signal strength.
Some points to make sure of regarding the reference crucible are:
- The reference crucible should be of the same type as the sample crucible
- When measuring dilute systems, an inert material like aluminum oxide should be poured into the reference crucible to the brim to make up for the heat capacity possessed by the sample matrix, and should have more or less the same heat flow as the latter [UC 25/21].
The majority of TGA measurements occur with samples weighing between 1 and 30 mg, typically about 10 mg, though this mass is not mandatory.
TGA is a method to assess the loss of mass of the separate constituents of a mixture, so that the mass of the minor constituent which is measured is the most important mass. This mass should exceed the minimum weight of the balance that is used in this method, as illustrated in UserCom [UC 41/14] about USP minimum weight, [UC 21/1] for a calculation example, or the “Validation in Thermal Analysis” handbook.
Sample masses should be as similar as they can be to compare different samples or measure a whole series of samples, so that the difference in sample size will not produce artifacts. If this must be done, as [UC 12/17], and if there must be a comparison of onset temperatures, absolute scaling must be done.
Filling a Crucible
Sample preparation is mostly according to the method described for DSC. The table below is specifically with regard to TGA sample preparation.
Material may tend to jump out of the crucible, which is prevented by using the aluminum oxide lid with a hole already drilled in it. This is an important precaution as otherwise the material may contaminate the TGA sensor.
In general, sample preparation for bulk samples should follow the following pattern:
- Ensure that samples have parallel surfaces
- Samples should be less than 10 mm in height, or if the shorter rest is used, 20 mm as shown in (a) below
- The sample surface should be as smooth as they can be
- Sticky samples or those that peel off the probe should be kept apart using silica spacer discs, and support should be given, as shown in (b)
- The force applied while taking CTE measurements should be kept to the minimum so that the sample is not compressed. Silica discs help to equalize the force over the entire sample surface area as shown in (b).
- The sample size should be about 5 mm for accurate CTE measurement
The important points with regard to DMA samples are:
- Knowing the sample geometry with surety since inaccurate sample measurements can cause the modulus to have a significant error in the modulus
- The sample must be smooth and the surface parallel
- The sample stiffness must be 3-5 times below that of the clamps and the instrument, as otherwise the modulus measured will be unduly low. a calculator for sample stiffness will be available at www.datacomm.ch/mschubnell
- The sample should be clamped with adequate tightness but overtightening will cause the sample to move in the clamp and this will be measured rather than the deformation of the sample which is what is sought. This will yield a falsely low modulus.
- Samples which are clamped above the glass transition temperature before being cooled to below this point need re-clamping at the start temperature
- The position of the thermocouple must be carefully checked and reproducible, since alterations in the position can cause variation in the temperature up to 5 K between measurements
- It is important to keep the thermocouple away from the sample and the furnace
- The clamping direction for non-isotropic samples must be followed
DMA sample preparation in shear mode is detailed in [UC 29/14] and [UC 34/15].
The crucible requirements are as follows:
- The crucible must be made of inert material and no effects must be produced within the measured temperature range
- The crucible should have a melting point which is above the effects that are sought to be observed in the sample
- Crucibles must not react with the sample or to its end products within the temperature range unless the objective is a catalytic process as in the following:
- oxidation measured in a copper crucible
- chemical reaction with a platinum crucible
- Non-volatile samples must be measured using 40-μL standard crucibles of aluminum with a predrilled hole in the lid, and this can also be of use in measurements of TGA/DSC at temperatures up to 640 °C.
- The sample robot cannot be used for crucibles with pin
- The weight before and following measurement should be assessed, so that any difference which is less than 30 μg indicates that the crucibles have held on tight
- In TGA measurements, diffusion rates are affected by the ratio of crucible height to width
An overview of DSC and TGA crucibles is shown in the following table:
For TMA and DMA instruments, no crucible is required but sample clamping is performed directly to the instrument using clamping assemblies of various sorts. The specific type is chosen based on the geometry of the sample and the property to be measured.
The table below shows how different types of materials have their properties measured using various deformation modes:
This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Thermal Analysis.
For more information on this source, please visit Mettler Toledo - Thermal Analysis.