Uncovering the blind spot in the analysis of N-alkylidene and N-alkyl chitosan derivatives through Py-GC-MS/FID

N-alkyl chitosan derivatives hold great value as bio-active constituents for applications such as hemostatic agents, emulsifiers and drug delivery agents, etc. These are obtained by reacting the amine moiety of chitosan with medium-chain aliphatic aldehydes, such as citronellal, to form a corresponding N-alkylidene derivative, which upon reduction, gives the N-alkyl chitosan conjugate. If bio-active aldehydes are used to synthesize the N-alkyl chitosan derivatives with a pursued bio-activity, it is of utmost importance to assess if and to which extent traces of unreacted aldehyde are retained in the purified product. We developed a thermal analysis methodology, based on py-GC-MS/FID, which quantified unreacted citronellal. This methodology was superior, compared to FTIR and CP/MAS solid-state ¹³C NMR analyses, which were unable to identify any unreacted reactant in the N-alkylidene and N-alkyl chitosan derivatives. Based on py-GC-MS/FID analysis, between ca. 4-10 wt% unreacted citronellal was present within the chitosan samples obtained by mechanochemical means, whereas solution based methods did not leave any traces of citronellal. To test the sensitivity of the py-GC-MS/FID based method, chitosan spiked with a trace of citronellal (0.09 wt%) was analyzed, and citronellal content was successfully detected. Finally, a mechanochemical synthesis of N-alkylidene chitosan derivatives was combined by a recovery through precipitation (rather than washing), which resulted in a clean (i.e., citronellal-free) chitosan derivative. It is clear that thermal analysis does uncover a blind spot in the analysis of emerging chitosan derivatives.