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From

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

Ronald P. D’Amelia, Shreya Prasad, Mary T. Rooney

World Journal of Analytical Chemistry. 2025, 10(1), 6-15 doi:10.12691/wjac-10-1-2
  • Figure 1. Acid-catalyzed, Fischer esterification of salicylic acid with an alcohol to produce a generic salicylate ester, where R represents an alkyl substituent
  • Figure 2. Structures of the four salicylate esters used in the current study
  • Figure 3. Stacked GC-FID chromatograms for pure short chain (C1–C4) alkyl salicylate esters. MS, ES, PS, and BS are represented by red, blue, green, and black lines, respectively. The retention times for neat MS, ES, PS, and BS were 3.520, 4.606, 6.250, and 8.263 min. respectively
  • Figure 4. Calculated percentage of methyl (red, closed circles) and ethyl (blue, open circles) salicylate in MS:ES binary mixtures determined from GC–FID peak area vs. known MS gravimetric weight percentage. The R2 value of 0.99996 indicates a strong degree of correlation between the chromatographic- and mass-derived values.
  • Figure 5. Calculated percentage of ethyl (blue, closed circles) and propyl (green, open circles) salicylate in ES:PS binary mixtures determined from GC–FID peak area vs. known ES gravimetric weight percentage. The R2 value of 0.99921 indicates a strong degree of correlation between the chromatographic- and mass-derived values.
  • Figure 6. Calculated percentage of propyl (green, closed circles) and butyl (black, open circles) salicylate in PS:BS binary mixtures determined from GC–FID peak area vs. known PS gravimetric weight percentage. The R2 value of 0.9984 indicates a strong degree of correlation between the chromatographic- and mass-derived values.
  • Figure 7. Stacked NMR spectra for pure short chain (C1–C4) alkyl salicylate esters. MS, ES, PS, and BS are represented by red, blue, green, and black lines, respectively. The inset shows a magnified view of the region containing the hydroxyl peaks (observed at 10.7710 ppm, 10.9233 ppm, 10.9382 ppm, and 10.9497 ppm in the standard spectra for MS, ES, PS, and BS, respectively) used for quantification
  • Figure 8. Hydroxyl and methyl peaks in the 1H NMR spectra for four MS:ES binary mixtures. Spectra were collected using a JEOL 400 MHz NMR spectrometer and processed in MATLAB
  • Figure 9. Calculated percentage of methyl (red, closed circles) and ethyl (blue, open circles) salicylate in MS:ES binary mixtures determined from the integrated NMR hydroxyl peak vs. known MS gravimetric weight percentage. The R2 value of 0.99862 indicates a strong degree of correlation between the spectroscopic- and mass-derived values
  • Figure 10. Calculated percentage of methyl (red, closed circles) and ethyl (blue, open circles) salicylate in MS:ES binary mixtures determined from the integrated NMR terminal methyl peak vs. known MS gravimetric weight percentage. The R2 value of 0.99926 indicates a strong degree of correlation between the spectroscopic- and mass-derived values, better than that calculated for the hydroxyl peaks
  • Figure 11. Hydroxyl and methyl peaks in the 1H NMR spectra for four ES:PS binary mixtures. Spectra were collected using a JEOL 400 MHz NMR spectrometer and processed in MATLAB
  • Figure 12. Calculated percentage of ethyl (blue, closed circles) and propyl (green, open circles) salicylate in ES:PS binary mixtures determined from the integrated NMR hydroxyl peak vs. known ES gravimetric weight percentage. The R2 value for these lines of best fit is poor, reflecting difficulties in deconvoluting overlapping peaks
  • Figure 13. Calculated percentage of ethyl (blue, closed circles) and propyl (green, open circles) salicylate in ES:PS binary mixtures determined from the integrated NMR terminal methyl peak vs. known ES gravimetric weight percentage. The high R2 value for these lines of best fit reflect the high correlation between the two data sets. The excellent peak resolution aids the data quality
  • Figure 14. Hydroxyl and methyl peaks in the 1H NMR spectra for four PS:BS binary mixtures. Spectra were collected using a JEOL 400 MHz NMR spectrometer and processed in MATLAB.
  • Figure 15. Stacked FTIR spectra for pure short chain (C1–C4) alkyl salicylate esters showing the fingerprint region only. MS, ES, PS, and BS are represented by red, blue, green, and black lines, respectively. The absorbance bands unique to methyl, ethyl, and propyl salicylates used for quantification (labelled and filled in) are between 1435–1445 cm-1, 1365–1375 cm-1, and 930–940 cm-1, respectively
  • Figure 16. Calculated percentage of propyl (green, closed circles) and butyl (black, open circles) salicylate in PS:BS binary mixtures determined from the integrated NMR hydroxyl peak vs. known PS gravimetric weight percentage. The R2 value for these lines of best fit is poor, reflecting difficulties in deconvoluting overlapping peaks. Furthermore, since the methyl peaks for these two compounds are not resolved, no usable data could be extracted from that region.
  • Figure 17. Peaks unique to ES (~1370 cm-1) and MS (~1440 cm-1) in the FTIR spectra for four MS:ES binary mixtures. Spectra were collected using a Bruker Alpha-P FTIR spectrometer and processed in MATLAB
  • Figure 18. Calculated ratios of methyl (red, closed circles) and ethyl (blue, open circles) salicylate in MS:ES binary mixtures determined from the FTIR peak amplitudes vs. known MS gravimetric weight percentage. The R2 value of 0.99782 indicates a strong degree of correlation between the spectroscopic- and mass-derived values
  • Figure 19. Peaks unique to PS (~935 cm-1) and ES (~1370 cm-1) in the FTIR spectra for four ES:PS binary mixtures. Spectra were collected using a Bruker Alpha-P FTIR spectrometer and processed in MATLAB
  • Figure 20. Calculated ratios of ethyl (blue, closed circles) and propyl (green, open circles) salicylate in ES:PS binary mixtures determined from the FTIR peak amplitudes vs. known ES gravimetric weight percentage. The R2 value of 0.96655 indicates a strong degree of correlation between the spectroscopic- and mass-derived values