Thermogravimetry is a classic technique that enables a precise quantitative analysis of a sample, without identifying the nature of the components, but only by measuring how much weight is lost from the sample at a certain temperature. The amount of sample is small (a few Milligramm).
In some cases there is the possibility to identify gases that are emitted bu thermogravimetry (TGA coupled with FTIR, for example).
Sample do not need particular preparation, but the weight is very low (10-20 milligrans). For this reason care must be taken to be sure of the representativeness of samples.
Instrumentation is conceptually simple, but practically very delicate and precise and requires periodic inspection and calibration: in essence iy is a balance of extreme precision in which the sample is progressively heated from room temperature to over 1000 ° C with ramps in temperature programmable. From the analysis of when the sample loses weight (and of what it loses), it is possibile to have suggestions on what the sample loses, and it is measured precisely how much is released. The instrument requires a flushing with gas in the combustion cell. The gases that are released from the sample can be analyzed by interfacing the TGA with a detector such as an infrared spectrometer (TGA-FTIR).
The measure is quite simple and takes a maximum of 1 hour but the only thermogravimetric analysis generally not allow a determination of the components of a substance unknown composition. However, in particular for quality control requirements where the composition is known, it can provide precise quantifications on % by weight of the different components of a substance.
The differential scanning calorimetry enables the analysis of the phase of the thermal transitions of a sample both qualitatively (analysis of the temperature at which the transition occurs) and quantitative (analysis of the area of the transition spike). The sample should generally weigh a few tens of milligrams, and then also this is an analysis which can be particularly subject to possible lack of homogeneity in the samples. It is typical of the use of calorimetry to measure the melting of a crystalline part or the glass transition of a polymer. For this it is a valuable aid for the recognition of substances, although, as for the thermogravimetry, a single DSC analysis does not allow the certain identification, but rather the exclusion of the presence of other components.
Preparation is simple: a small quantity (10-30 mg) must be closed in a small metal container, by taking care of a good contact, and then inserted in the instrument.
The DSC instrumentation is now very compact and fully automated via computer. It requires only a correct flushing with adequate gas (usually nitrogen and optionally oxygen) and a periodic calibration control for temperatures. It is possible to work at low temperatures with liquid nitrogen.
The analysis has a variable duration, generally from 10 minutes to about 2 hours. However, it is necessary to check with different measures for the presence of "frozen" transitions in the sample, due to particular thermal histories (for example quenching), that do not occur more if the measurement is carried out a second time, or that, vice versa, occur only with a second measurement or with different thermal histories.
Good indications on the presence and amount of inorganic substances can simply get by analyzing qualitatively and quantitatively the combustion residues of a sample left to burn at certain temperatures. This procedure is indicated for quality controls.
Pratically no preparation is needed. Sample is direcrly inserted in the muffle, after weighing it.
It is sufficient a muffle that can reach 1000 °C. Microoven muffles allow very quick analysis and a precision that is not reachable with standard muffles.
The residual weight is measured, and eventually the residual substances are analyzed with other techniques.
It is a fundamental characteristic, but, by an analytic point of view, it has not great interets. It is a technological test. Different procedures are possibile, depending on the materials, the temperatures and the range of vaules expected.
Dr. Maurizio Veronelli - Specialist in Polymer Science
RDLAB137 srl - Milano
Last revision: 01/22/2017