Formation of cinnamaldehyde dimethyl acetal in methanol during analysis

As a principal compound of cinnamon essential oil, cinnamaldehyde has been used as a marker for the quality control of cinnamon and its essential oil. During the HPLC analysis, a reaction product was observed in methanol solution of cinnamaldehyde reference standard. This product was isolated and identified to be cinnamaldehyde dimethyl acetal based on the spectroscopic analysis. Although dimethylacetalization of cinnamaldehyde in MeOH has been recorded in many literatures, this is the first report about this reaction in normal analysis procedure. Further studies suggested that this reaction could be affected by many parameters, including cinnamaldehyde concentration, storage temperature, and light illumination. Those results suggested the stock solution of cinnamaldehyde reference standard should not be prepared with methanol, and the extraction procedure by heating the sample with methanol might accelerate the acetalization of cinnamaldehyde. The use of methanol during the analysis of cinnamaldehyde should be carefully considered.