Evolved Gas Analysis

Coupling of Thermal Analysis and Gas Analysis

Thermogravimetry and Simultaneous Thermal Analysis Coupled to MS, FT-IR or GC-MS

Thermogravimetry and Simultaneous Thermal Analysis Coupled to MS, FT-IR or GC-MS

Evolved Gas Analysis (EGA) is the perfect tool for characterizing the thermal behavior of organic and inorganic solids or liquids in more detail and elucidating the chemistry behind the processes under investigation.

Mono-functional instruments measure a single physical property, however systems are available on the market, which combine two and more measuring principles for the analysis of a sample. These combined instruments exist in two classes, namely simultaneous and coupled instruments. A typical representative for simultaneous instruments is the combination of a thermogravimetric system (thermobalance, TGA) with a differential thermal analysis system in simultaneous TGA-DSC and TGA-DTA.   The connection of a thermobalance with an FTIR spectrometer or of a simultaneous TGA-DSC analyzer with a mass spectrometer via an appropriate interface are systems of choice for coupled instruments.

Characteristic of a separate application of DSC and DTA, the most frequently applied thermoanalytical techniques, is the calorimetric analysis of samples up to their limit of thermal stability; the signal is seldom closely related to the quantity of evolved volatiles, which are products of evaporation and chemical reactions. Therefore combined gas analysis has found no great importance for DTA and DSC alone.   Thermogravimetry determines the weight changes of the sample for all solid-gas and liquid-gas interactions. Single volatiles or adsorbents are quantitatively detected through the corresponding weight change and for all complex reactions with more than one volatile product an integral weight change signal is achieved. Especially here combined gas detection with specific detectors and with universal gas analysis methods found the most frequent application. The additional calorimetric information of simultaneous TGA-DSC and TGA-DTA is clearly of further advantage for interpretation of complex thermal events measured in these coupled systems.   Thermomechanical methods measuring dimensional changes of solid samples are not a main target for combination with gas analysis systems. A few special applications however are found for studying firing processes in ceramics and powder metallurgy as well in characterization of the gas exchange during heat treatment of metals.

Why Coupling with Gas Analysis?

Why Coupling with Gas Analysis?

Interpretation of the TGA Signal Alone is Difficult!

TGA + EGA Allows for a Much Better Interpretation!

Thermogravimetry monitors only mass changes.
There is no information about the nature of the evolved gases.   
    
⇒ EGA

Three types of gas analysis:

1)  Fourier Transform Infrared Spectroscopy – FT-IR  
2)  Mass Spectrometry  –  Aëolos®, SKIMMER®
3) Gas Chromatography + Mass Spectroscopy –  GC-MS

Integrated TGA-FT-IR System

Integrated TGA-FT-IR System

Thermogravimetric analysis (TGA) yields essential information about a sample’s composition. However, the gases directly released during sample decomposition or thermal treatment cannot be identified by means of TGA alone. An excellent solution for this purpose is to couple the TGA to a spectroscopic method such as Fourier-Transform-Infrared (FT-IR) spectroscopy.

This classical technique produces a characteristic spectrum for each substance — except for homonuclear diatomic molecules and noble gases.

Advantages and Benefits

  •  Affordable gas analysis
  • No separate transfer line
  • Top-loading, space-saving system
  • Optimized low-volume design
  • No need for liquid nitrogen (DTGS detector) —
    optimal for tests with an automatic sample changer
    (for 64 samples; optional)
  • Vacuum-tight TGA system to remove oxygen,
    elminate carry-over, and lower boiling point

Areas of Application

  • Analysis of decomposition processes
  • Solid-gas reactions
  • Evaporation, outgassing
  • Detection of volatiles
  • Compositional analysis
  • Analysis of aging processes
  • Desorption behavior

Principle of FT-IR

Fourier-Transform Infrared Spectrometer

TGA-FT-IR Setup

Quadrupole Mass Spectrometry (QMS)

Quadrupole Mass Spectrometry (QMS)

Physical changes detected by thermal analysis are explained by the gas analysis in the mass spectrometer forming a workstation for analytical chemistry. Evolved species are detected down to the ppm level, which exceeds the standard sensitivity of thermal analysis methods. High-class material research and characterization is the result of coupled thermal analysis and mass spectrometry.

The sensitive, selective, fast and continuous function of a quadrupole mass spectrometer makes this system ideally suited for evolved gas analysis in combination with Thermogravimetric Analysis. Further key features which help provide an optimal coupling with thermal analyzers include the small dimensions of the quadrupole mass filter, the efficient and reproducible ionization of gases in the electron impact ion source, and the resolution in the detection of molecules, atoms and fragments.

Identification of

  • Gas composition
  • Fingerprint
  • Partial pressure
  • Fragmentation
  • Solid-gas interactions

Comparison TA-MS and TA-FT-IR

TA-MS
(SKIMMER® coupling)
TA-FT-IR
(via heated transfer line)
symmetrical molecules
(H2, N2, etc.)
+++-
mass numbers m/e +++-
H2O +++++
CO2 +++++
adsorption bands -+++
isomerism -+++
functional groups +++
type of bonding (+)++
long-chain homologues ++++
sensitivity++++
isotopes++++
fragmentationyesno
transfer time++++
condensationnoyes
interpretation+++
availability++++
price+++
speed+++

Quantification of Evolved Gases using PulseTA®

TA: TG, DTA etc.

Couplings: what is evolved?

PulseTA®: how much is evolved?

The basis of the PulseTA® technique is the injection of a defined amount of injection gas into the carrier gas stream and monitoring the changes of the MS or FTIR signals (as well as the mass and heat flow rate). A comparison with the corresponding MS or FTIR signals arising from the sample allow to calibrate these signals.

PulseTA®  is a tool for:

  • Quantification of gases evolved from the sample
  • Defined/incremental chemical reactions or adsorbation of the injection gas with the sample

This plot shows the thermal decomposition STA of ZnC2O4*2H2O in a helium gas flow (50 ml/min). With corresponding pulses of CO and CO2, marked by P, a quantification of the evolved gases in the MS is possible, even with the overlapping contributions to m/z 28 by CO and the fragmentation of CO2. The reaction between CO and traces of water is shown by the H2 signal and quantified by the H2 pulse PH2 (red).

The determination of carbon and sulfur content in a petrol rock was achieved by STA-MS measurements. During calcination in air, the CO2 and SO2 signals can be exactly quantified through corresponding pulses, even with the contribution of water to the detected weight loss.

TGA-GC-MS Coupling

TGA-GC-MS Coupling

Yields information on the composition (mass numbers of elements and molecules) of the evolved gases.

⇒ Very high sensitivity

⇒ Separation of the volatiles using the GC column

⇒ Interpretation of organic vapors can be improved

Anyway, GC-MS tests generally require minutes to be carried out. This means that a real time analysis which generates a relationship between time/temperature of the TGA run with the GC-MS is often difficult.

Solutions:

⇒ Continuous GC-MS with limited separation possibilities

⇒Event controlled GC-MS triggering
(high sensitivity and relation between mass-loss step and GC-MS result)

Recommended Literature

Application literature

Evolved gas analysis (EGA) from thermal analyzers such as thermogravimetry (TG) or simultaneous thermal analysis (STA) which refers to simultaneous TG–DSC is well established since it greatly enhances the value of TG or TG–DSC results.