which dissociates to give carbon dioxide (m/z44). The third stage produces only
carbon dioxide.
If several gases are evolved simultaneously, they should preferably be sepa-
rated by chemical or chromatographic methods. Qualitative and quantitative
methods for analysis of the gases may be classified as:● physical −conductivity, density, absorption;
● chemical −titration, electrochemical, reaction;
● spectrometric −infrared and mass spectrometry, colorimetry.These methods are described in previous sections. It is important to note that
both the instrumentation and the analysis must be designed to prevent any
further gas-phase reaction occurring.Instrumentation In order to make simultaneous measurements, the evolved gases are often
detected in the carrier gas stream from thermogravimetric or other thermal
analysis procedures. This provides programmed temperature control of heating
and correct sample handling.
Physical detectors, such as those used for gas chromatographic detection are
sometimes fitted to detect evolved gases, but they do not identify them.
Moisture can be measured quantitatively using a capacitance moisture meter or
by absorption and electrolytic determination.
Chemical methods may involve many of the techniques described in Section
C. Any acidic or alkaline gas can be detected by absorption and titration, or
potentiometry. Gas-sensing membrane electrodes and other ion-selective
electrodes allow analysis of halide and sulfide ions.
Spectrometric methods, especially mass spectrometry (MS) and Fourier trans-
form infrared spectrometry (FTIR) have been used, often coupled with thermo-
gravimetry. For molecules that are polar and of low molar mass, FTIR is
particularly useful. For nonpolar molecules and those of higher molar mass, MS
is more adaptable. There are problems, however, in interfacing the thermal
analysis instrument operating at atmospheric pressure to the MS operating
under vacuum. This is discussed in Topic F3.
Special instrumentation allows study of gases for environmental investiga-
tions and it should be noted that hot-stage microscopy (orthermomicroscopy)
and X-ray diffraction can be used to observe changes in the solid residues.
Applications Acidic gases, such as the hydrogen chloride evolved from heating poly(vinyl
chloride) can be measured by pH change, or by absorption in alkaline solution
and back-titration, or using a chloride ion selective electrode. This is shown in
Figure 2.
Carbon dioxide from heated concrete may be absorbed in barium hydroxide
and measured conductimetrically. Sulfur dioxide from coal combustion can
also be determined electrochemically. Ammonia evolved from ammonium
aluminum compounds can be measured by pH change.
Ammonia evolved from minerals, and a complex collection of products from
polymer decompositions can all be measured and identified by FTIR spectrom-
etry. Mass spectrometric identification of gases, both polar and nonpolar (for
example acidic gases, hydrocarbons and even homonuclear diatomics such as
oxygen) provides a very versatile experimental method for analyzing the
decomposition of materials, especially polymers. Phenol-formaldehyde resins
G4 – Evolved gas analysis 321