Hyphenation of proton transfer reaction mass spectrometry with thermal analysis for monitoring the thermal degradation of retinyl acetate.

RATIONALE: The processing of retinyl acetate, a vitamin and biomarker, at high temperatures causes significant decomposition of the compound and thus loss of its activity. The rate of mass loss can be conveniently studied by thermogravimetry (TG). However, this technique generally fails to reveal which compounds have evolved from the compound. In this work we propose a new hyphenation approach to continuously monitor the thermal decomposition of retinyl acetate and follow the evolution of specific volatile organic compounds (VOCs).

METHODS: Thermal degradation of retinyl acetate was followed by TG coupled to a direct injection mass spectrometer based on proton transfer reaction mass spectrometry (PTR-MS) to follow continuously the thermal decomposition of retinyl acetate. The results were also compared with those obtained by a second evolved gas analysis system based on the coupling of TG with FTIR.

RESULTS: The TG results showed two main mass losses, at 180°C and 350°C. When the PTR-MS instrument was connected to the outlet of the TG instrument, specific fragment ions (m/z 43, 61, 75, 85 and 97) showed characteristic evolution profiles. The first mass loss was mainly associated with the release of acetic acid (m/z 43 and 61), whereas the second mass loss was connected with the degradation of the molecule backbone (m/z 43, 61, 75, 85 and 97). These results were substantially correlated with those achieved by TG coupled with FTIR, although PTR-MS showed superior performance in terms of the qualitative identification of specific fragments and better sensitivity toward complex organic VOCs.

CONCLUSIONS: The proposed TG-PTR-MS technique shows a great potential for following in real time the thermal degradation of ingredients such as retinyl acetate and identifying compounds evolved at specific temperatures.