Abstract
High-field NMR is an expensive and important quality control technique. In recent
years, cheaper and simpler low-field NMR has become available as a new quality control
technique. In this study, 60 MHz 1H-NMR was compared with GC-MS and refractometry for the detection of adulteration
of essential oils, taking patchouli essential oil as a test case. Patchouli essential
oil is frequently adulterated, even today. In total, 75 genuine patchouli essential
oils, 10 commercial patchouli essential oils, 10 other essential oils, 17 adulterants,
and 1 patchouli essential oil, spiked at 20% with those adulterants, were measured.
Visual inspection of the NMR spectra allowed for easy detection of 14 adulterants,
while gurjun and copaiba balsams proved difficult and one adulterant could not be
detected. NMR spectra of 10 random essential oils differed not only strongly from
patchouli essential oil but also from one another, suggesting that fingerprinting
by low-field NMR is not limited to patchouli essential oil. Automated chemometric
evaluation of NMR spectra was possible by similarity analysis (Mahalanobis distance)
based on the integration from 0.1 – 8.1 ppm in 0.01 ppm increments. Good quality patchouli
essential oils were recognised as well as 15 of 17 deliberate adulterations. Visual
qualitative inspection by GC-MS allowed for the detection of all volatile adulterants.
Nonvolatile adulterants, and all but one volatile adulterant, could be detected by
semiquantitation. Different chemometric approaches showed satisfactory results. Similarity
analyses were difficult with nonvolatile adulterants. Refractive index measurements
could detect only 8 of 17 adulterants. Due to advantages such as simplicity, rapidity,
reproducibility, and ability to detect nonvolatile adulterants, 60 MHz 1H-NMR is complimentary to GC-MS for quality control of essential oils.
Key words
essential oil - quality control - 60 MHz
1H-NMR - adulteration - fingerprinting - chemometrics - patchouli
