TGA (Thermal Gravimetric Analysis) generally refers to a combination of gravimetric and mass-spectrometry analysis. Thermogravimetric Analysis (TGA) is a thermal analysis technique that measures the change in mass of a sample as a function of temperature or time under a controlled atmosphere. It is widely used in materials science, chemistry, and engineering to study decomposition, oxidation, moisture content, and thermal stability. As such, it can be used to detect the presence of chemisorbed gases. This is typically negligible on most carbon adsorbents due to van der Waals repulsion, but it can occur in unique geometries like single-walled carbon nanotubes. 

Basic Concept

TGA records weight loss (or gain) due to processes like:

• Decomposition (e.g., polymer degradation).

• Evaporation (e.g., solvents, moisture).

• Oxidation/Reduction (e.g., metal corrosion).

• Gas adsorption/desorption (e.g., CO₂ capture).

and usually use below basic core instrumentations like:

• Sample pan (placed on a microbalance).

• Furnace (heats the sample at a controlled rate).

• Gas control system (inert N₂, oxidizing O₂, or reactive gases).

• Data recorder (plots mass vs. temperature/time).

TGA can be used effectively in tandem with isotope substitution to characterize and validate hydrogen spillover. An example for a spillover sample dosed first with H2 followed by D2, is shown in Figure 1. This has been interpreted as successive H2 and D2 dissociation on the spillover sites, recombination at the interface between the two isotope layers, and desorption at the same spillover sites.

Figure 1: TGA desorption trace for a spillover sample that was dosed first with H2 followed by D2.

The decomposition of the off-board rechargeable chemical hydride polyamino borane (Second dehydrogenation step of Ammonia borane) is given by: 

Equation 1: (NH2BH2)n → (NHBH)n + H2

Typical TGA-curves for this decomposition at different heating rates (1, 5 and 10 °C min−1) are shown in Figure 2a). A single mass loss step was observed at all heating rates. It is clearly evident that a mass loss is detectable in the same temperature range in which hydrogen release and heat evolution were determined. The final value of the mass loss depends significantly on the heating rate used. With rising heating rate the final mass loss increases from 7.1 wt.% at β = 0.1 °C/min to 20.3 wt.% at β = 10 °C/min. The formation of boron nitride at a temperature of 250°C is not probable, as follows from the volumetrically detected release of only 1.1 mol hydrogen per mol H₂BNH₂.

In Figure 2b) results of thermogravimetric and volumetric investigations are compared. Volumetric results were converted into mass loss data. The release of 1.1 mol H2, which was detected by volumetric investigations, corresponds to a mass loss of only 7.6 wt.%. The results for the mass loss from thermogravimetric and volumetric investigations are nearly in agreement at a heating rate of 1 °C/min. The amount of gaseous products evolved in addition to hydrogen should be very small at 1 °C/min. The thermal decomposition of polymeric aminoborane (H₂BNH₂)x is accompanied by the evolution of different gaseous products as follows by volumetric and thermogravimetric investigations.

Figure 2: a) Mass loss on the thermal decomposition of polymeric aminoborane (H2BNH2)X versus temperature b) Experimental data for the mass loss (TG, line) in comparison with the mass loss data calculated from the released amount of hydrogen (volumetric measurements, points), (heating rates 1 and 10 °C/min).

Gravimetric measurements are often used for measuring capacity for many hydrogen storage materials. However, in ammonia borane it is very difficult to obtain accurate information about the capacity using simple TGA measurements. The key issues that could skew measurements are:

• Foaming: Ammonia borane foams extensively in the range of 90-110 °C during hydrogen release. This often creates mechanical errors in the instrument balance thus reporting an erroneous weight loss.

• Sublimation: Sublimation of the storage material is more common on samples which are heated under TG conditions. Undetected loss of some of the sample via sublimation may result in a significant error in estimating hydrogen capacity.

• Non-hydrogen volatiles: Other volatile intermediates or byproducts such as ammonia and borazine also contribute to an overestimation of hydrogen capacity if not properly accounted for.

These errors though significant under certain conditions can be minimized by use of smaller sample sizes in larger sample cups equipped with a lid (with a small orifice) that can minimize sublimation and foaming issues. Volatile byproducts or intermediates can be quantified by other techniques mentioned in section dealing with gas composition analysis and corrected for those measurements.

Figure 3: Comparison between mass loss data detected thermogravimetrically (line) and calculated from volumetric results (points) (heating rate 1 K/min (a), 5 K/min (b))

TGA is less accurate than either the volumetric and gravimetric methods and is not typically used for measuring capacity.

Major Application Fields of TGA

A. Polymer Science & Plastics

• Decomposition behavior (e.g., thermal stability of PVC, PET).

• Filler content analysis (e.g., carbon black in rubber).

• Curing & cross-linking studies (e.g., epoxy resins).

B. Pharmaceuticals & Drug Formulation

• Moisture & solvent content (e.g., drying processes).

• Drug stability testing (e.g., decomposition temperature).

• Excipient compatibility (e.g., interactions in tablets).

C. Metals & Ceramics

• Oxidation resistance (e.g., TGA in air for alloys).

• Carburization/decarburization (e.g., steel treatment).

• Ceramic precursor decomposition (e.g., calcination of TiO₂).

D. Carbon Materials & Composites

• Ash content in coal & carbon fibers.

• Thermal stability of graphene & nanotubes.

• Decomposition of composite materials (e.g., CFRP).

E. Environmental & Energy Materials

• Biomass pyrolysis (e.g., biofuel production).

• CO₂ capture materials (e.g., TGA in CO₂ flow).

• Battery materials (e.g., thermal runaway in Li-ion cells).

F. Food & Agricultural Science

• Moisture & fat content analysis.

• Thermal degradation of edible oils.

• Soil organic matter studies.

Like for polymers materails, TGA can work for determining the thermal stability of polyethylene (PE) vs. polystyrene (PS). For batteries usually study electrolyte decomposition in lithium-ion cells. For pharma to measure residual solvents in a drug powder. For metals to test oxidation resistance of stainless steel at high temperatures.

TGA also combined with other techniques like: 

• TGA-DSC (Differential Scanning Calorimetry): simultaneously measures heat flow + weight loss.

• TGA-MS (Mass Spectrometry): identifies evolved gases (e.g., CO₂, H₂O).

• TGA-FTIR (Fourier Transform IR): detects functional groups in released gases.

But it also have below advantages and limitations:

Advantages:

• Quantitative (precise weight measurements).

• Wide temperature range (RT to 1500°C+).

• Works with solids, liquids, and powders.

Limitations:

• Does not identify gases (needs MS/FTIR coupling).

• Small sample size (~10 mg) may not represent bulk behavior.

• Overlapping reactions can complicate interpretation.

TGA is a powerful, versatile technique for studying thermal stability, composition, and reaction kinetics across multiple industries. When combined with DSC, MS, or FTIR, it provides even deeper insights into material behavior.