Testing Quality Interpreting Thermograms for Material Purity Quiz

Impurities disrupt the crystalline lattice of a pure substance. This results in a "melting point depression" and a wider temperature range over which melting occurs, which is visible as a broader, lower peak on a DSC thermogram.

In a highly pure crystal, all molecules are held together by uniform forces that break at the exact same temperature. A sharp peak with a narrow width at half-height is a primary indicator of high chemical purity.

The van t Hoff equation relates the depression of the freezing point to the concentration of the impurity. Thermal analysis software uses this relationship to provide a purity percentage by analyzing the leading edge of the melting peak.

Multiple steps usually indicate a mixture or a compound that decomposes in stages. For example, a hydrated salt will show one step for water loss and another for the decomposition of the anhydrous salt.

The onset temperature identifies what is decomposing, while the vertical height of the step tells you how much of that component was in the original mixture. DTG peaks help resolve overlapping steps to see hidden components.

A small peak occurring before the main transition often represents a eutectic mixture (a specific ratio of the substance and an impurity) or a small amount of a less stable crystalline form (polymorph) of the same material.

After all organic components have burned or decomposed, the leftover material is typically an inorganic oxide or carbon char. Measuring this residue is a standard way to determine the mineral or filler content in polymers and coal.

A curving or "drifting" baseline can make it difficult to determine the exact start and end of a peak. This leads to errors when calculating the area of the peak (enthalpy) or the percentage of mass loss.

Components in a blend exit the sample in order of their stability or boiling point. The final major mass loss step typically corresponds to the breakdown of the most robust polymer backbone in the mixture.

Impurities like residual monomers or moisture act as plasticizers, which lower the Tg. Therefore, measuring a Tg that is lower than the literature value is a strong indicator of an impure or "wet" polymer sample.

The DTG curve turns the "shoulders" or "slopes" of a TGA curve into clear peaks. This makes it much easier to distinguish between two different components that decompose at similar temperatures.

A linear plot of temperature versus the reciprocal of the fraction melted confirms that the impurity follows ideal behavior. Consistency and peak sharpness are the hallmarks of reliable thermal purity data.

Sometimes a material melts and then immediately undergoes a chemical reaction or rearranges into a more stable crystal form. This sequence provides deep insight into the chemical stability and "thermal history" of the material.

A large "thermal lag" or a broad transition often suggests that the sample is not pure or that the heating rate is too fast for the sample size, causing the outer layers to melt before the core.

Physical movements or reactions with the aluminum or quartz pan can create peaks that look like chemical transitions. It is vital to ensure the sample is flat and the pan material is compatible with the analyte.