Analysis of gases emitted by a sample
The majority of liquid and solid compounds continuously emit gaseous compounds.
This emission can be caused by simple liquid/vapor or solid/vapor equilibria. However, they are also often caused by chemical reactions. These reactions are favored by the presence of other reagents or simple temperature variations (thermal degradation, combustion in air, etc.).
In all cases, the analysis of the gas phase makes it possible to identify and quantify the compounds emitted by the sample under controlled conditions. For this, a gas sweep is carried out in an enclosure containing the sample. This gas will carry with it the compounds emitted by the sample. The final and/or regular analysis of the gas mixture recovered at the containment outlet then allows the identification and quantification of the compounds emitted (see article dedicated to gas analysis).
Controlled emission conditions
To carry out a gas emission analysis, the analysis conditions must first be defined. Indeed, whether simple equilibriums or chemical reactions, temperature plays an important role on the compounds emitted and their emission kinetics. It is therefore necessary to control the temperature of the enclosure during the analysis.
The analysis can thus be carried out under isothermal conditions for a determined period. It can also be carried out during more or less rapid temperature ramps, sometimes up to more than 1000°C. More complex programming mixing ramps and isotherms can also be carried out according to the expressed need.
The nature and flow rate of the purge gas are also important. The use or not of an inert gas (N2, Ar, He) will thus prevent the combustion of the samples if this is desirable. The gas flow will have a dilution effect on the gases emitted, which can be sought or avoided to remain within the analytical detection and quantification ranges of the compounds emitted by the sample.
Depending on the experimental conditions, the enclosures used will be different: reactor in a tube furnace, isothermal microchamber, etc.
Final analysis or kinetic follow-up
One of the simplest approaches is the a posteriori analysis of the gaseous mixture obtained at the enclosure outlet. For this, the entire gas mixture is collected at the enclosure outlet in a dedicated bag for the duration of the analysis. At the end of the analysis, it is closed, and the gaseous mixture is analyzed. This makes it possible to identify the compounds emitted and to quantify the total emissions that have taken place during the entire analysis.
Nevertheless, it is sometimes more advantageous to carry out analyzes of the gaseous mixture on a regular basis during the analysis. For this, samples are regularly analyzed. Emission kinetic profiles are thus plotted. They allow in the case of isothermal analysis to know the duration of emission of the compounds. In the case of a temperature ramp, this makes it possible to identify the temperatures at which the various compounds are emitted.