A frequent problem in a chemical laboratory is the analysis of an unknown mixture to determine the primary components, including their additives or contaminants. The combination in series of a TGA with FTIR and with a GC/MS through an efficient interface able to transfer all the evolved producted can give at the same time qualitative and quantitative information on a complex sample.The examined sample was an acqueous pigment: since water interfer with the analysis we have dried and then the remaining film was placed in a TGA coupled with FTIR and GC/MS through a balanced flow heated transfer line. About 20 mg of sample was heated at 20 °C/min from 20 up to 850 °C in nitrogen atmosphere. The TGA curve shows two major weight losses and we observe the evaporation of an unknown substance that reached the maximum concentration around 280 °C.
The IR spectrum of evolved gas taken at 280 °C was selected for a database search and figure 1 shows the best match obtained is with triethylene glycol dibenzoate: the two spectra show many similitudines but they are not exactly the same.
Figure 1: Sample and library spectra
As FTIR is a non-destructive test, the gas that evolved at 280 °C was transferred and injected into a GC/MS: the chromatogram shows one peak only and the MS search with NIST library identify the compound as diethylene glycol dibenzoate.
In summary the TGA/FTIR can give and identification of evolved gasses in many cases but not in all of them and te following step of GC/MS can confirm and/or refine the identification.
