Gas Chromatography - Tandem Techniques - Modern GC/IR Systems > Thermogravimetric Analysis (TGA) Coupled with GC/IR and GC/MS > Page 56


An example of the use of a GC/IR combined system in studying the influence of molecular structure on physiological activity is shown in figure 43. Structural isomers exhibit widely different biological activity and conventionally such structures were identified from their mass spectra using GC/MS combined instruments. Unfortunately, the mass spectrometer is not efficient in differentiating between certain important isomers, e.g., as 1,4-dimethyl naphthalene and 1,5-dimethyl naphthalene. In contrast, the two substances can be differentiated, unambiguously, from their respective IR spectra. The spectra of the two dimethyl naphthalene isomers, collected from a gas chromatograph using the cryostat interface are shown in figure 44.

The two spectra are seen to be markedly different and contain sufficient detail to permit easy identification.


Thermogravimetric Analysis (TGA) Coupled with GC/IR and GC/MS

The thermogravimetric analyzer, coupled with GC/IR and GC/MS, is another fairly complex combined instrument and might be considered a quintuplet system. This particular combination was assembled by McClennen et al. (15) who used it to study the thermal decomposition of a number of different polymers. A block diagram of their apparatus is shown in figure 45.


Figure 45. TG/GC/FTIR and TG/GC/MS Instrument Configuration