Compositional Analysis of Short-chain Alkyl (C1-C4) Salicylate Ester Mixtures by Various Quantitative Analytical Techniques

Short chain salicylate esters naturally occur in a variety of foods, from blackberries and pineapple to mint and teas. They are also common additives to fragrances, flavoring agents, and topical analgesics. However, methyl salicylate is a potential allergen and is toxic in high doses, so identifying and quantifying it, particularly in goods intended for human consumption or pharmaceutical use, is crucial. Since short chain salicylate esters share similar structures, they can prove challenging to differentiate in mixtures. In this study, gas chromatography with flame ionization detection (GC–FID) along with quantitative proton nuclear magnetic resonance (1H NMR) and Fourier-transform infrared (FTIR) spectroscopies were utilized to not only distinguish between these compounds, but also to quantify their relative concentrations when present simultaneously. GC-FID chromatograms and 1H NMR and FTIR spectra were first obtained for neat methyl salicylate (MS), ethyl salicylate (ES), propyl salicylate (PS), and butyl salicylate (BS), and then for binary mixtures of varying weight percent. GC-FID produced fully resolved chromatograms for all samples. 1H NMR spectra showed hydroxyl resonances at different frequencies for each compound and unique peaks for the hydrogens along each alkyl chain. A deconvolution script was applied for peaks that were not fully resolved. FTIR spectra displayed unique peaks in the fingerprint region for three out of the four compounds. Integrated peaks in the chromatograms and spectra were then used to quantify the relative concentrations of the components in binary mixtures to a high degree of accuracy. These results confirm the use of GC-FID, 1H NMR, and FTIR in the identification and quantification of these organic compounds.