Thermogravimetric Analysis.

The essential parts of the modern thermogravimetric instrument remain basically the same as those employed by the early Victorians (and, for that matter, the alchemists). These essential components include, a container in the form of a crucible to hold the sample, a furnace that can heat the sample to a high temperature, and an appropriate balance that can continuously monitor the sample. However, that is where the comparison ends – modern equipment has now become far more sophisticated providing much greater sensitivity, accuracy, precision and speed. It is interesting to note that the main advantage of modern analytical instrumentation is not so much the improved sensitivity, precision and accuracy (although these are extremely useful, particularly in forensic analysis) but the advantage of much greater speed. Faster throughput of sample generates much more data with quicker interpretation and, consequently, betters quality control and faster scientific progress. A block diagram of a modern thermo-gravimetric instrument is shown in figure 1.

Figure 1. A Block Diagram of a Modern Thermo-Gravimetric Analysis Instrument

The sample container consists of a small platinum crucible suspended from the arm of a microbalance and situated in a small oven, the temperature of which is carefully monitored by a highly accurate thermocouple or some other appropriate temperature measuring sensor. The oven can be evacuated so the sample can heated and weighed in a vacuum (this can also improve thermal stability) and all volatile substances that are evolved are rapidly removed. Alternatively the atmosphere can be made inert by the introduction of a rare gas such as helium or argon, can be made oxidizing by the introduction of oxygen. or reducing by the introduction of hydrogen. In the more sophisticated and expensive instruments, the products evolved from the crucible can also be led into a capillary gas chromatograph, or a mass spectrometer (or a tandem instrument containing both) to help identify the products being generated. The sample can also be examined at elevated pressures if required.

The temperature programmer and the temperature sensor together with the microbalance are all under control of a microprocessor and the output can be fed to a monitor and/or a printer. The crucible temperature can be programmed linearly or by a variety of different time-temperature functions that are provided by the manufacturer. The instrument normally gives an output in the form of a curve relating sample mass to temperature but software is available to provide other forms of output for example the differential form of the mass/temperature curve. The normal maximum balance load may be one or ten grams and the average mass resolution about 0.1 g. Special instruments providing wider mass ranges and greater sensitivity are available if required. Photographs of two different thermo-gravimetric instruments that are commercially available are shown in figure 2.

Figure 2. Two Commercial Models of Thermo-Gravimetric Instruments.