Low-Field NMR as a Complementary Tool to Detect Patchouli Oil Adulteration

Reviewed: Krause A, Wu Y, Tian R, van Beek T. Is low-field NMR a complementary tool to GC‑MS in quality control of essential oils? A case study: Patchouli essential oil. Planta Med. 2018; Apr 24. doi: 10.1055/a-0605-3967. [Epub ahead of print]

Keywords: castor oil, cedarwood oil, cubeb oil, Dipterocarpus spp., GC-MS, gurjun balsam oil, low-field NMR, patchouli oil, Pogostemon cablin, Piper cubeba, Ricinus communis

Limited production capacity and increased global demand for the essential oil from patchouli (Pogostemon cablin, Lamiaceae) leaves have created a situation where additional materials are reportedly sold as patchouli oil. Adulterants include a number of essential oils, e.g., cedarwood (Cedrus spp., Pinaceae; Cupressus spp. or Juniperus spp., Cupressaceae), cubeb (Piper cubeba, Piperaceae), or gurjun balsam (Dipterocarpus spp., Dipterocarpaceae) oils, fatty oils such as castor oil (Ricinus communis, Euphorbiaceae), or isolates such as (Z)-8-camphene methanol or isobornyl acetate.^1,2

As part of this investigation, 75 genuine patchouli essential oils, 10 commercial patchouli essential oils (bought either on the internet, or in shops in the Netherlands, Austria, or England), 10 other essential oils (e.g., ginger [Zingiber officinale, Zingiberaceae], corn mint [Mentha arvensis, Lamiaceae], and kokila [Cinnamonum glaucescens, Lauraceae]), 17 adulterants, and 1 patchouli essential oil, spiked at 20% with those adulterants, were measured by low field (60 MHz) proton nuclear magnetic resonance (¹H NMR), gas chromatography with mass spectrometric detection (GC-MS), and by determination of the refractive index (RI). The ¹H NMR data were examined visually, and by similarity analysis using the ChemPattern software.

When 20% of adulterant oil was added to authentic patchouli oil, ¹H NMR was able to detect it, with the exception of adulteration with vetiver (Chrysopogon zizanoides, syn. Vetiveria zizanoides, Poaceae) oil, and clearwood oil (which is a genetically engineered essential oil mixture that contains the same main components as patchouli oil). GC-MS was able to identify all admixtures of other essential oils, even clearwood oil, at 20%. However, the addition of paraffin and castor (Ricinus communis, Euphorbiaceae) oil did not alter the GC-MS fingerprint, and these samples were thus not deemed to be adulterated. Only when quantitative GC-MS was used were the adulterations with these materials obvious. Measurement of the refractive index, which is a quick and simple physical test method, allowed detection of half of the deliberately adulterated samples, most surprisingly adulteration with clearwood oil. Nevertheless, its usefulness in the detection of adulteration is limited as many of the commonly used adulterants failed to be detected at 20%.

The authors suggest that low-field ¹H NMR is a valuable tool for essential oil adulterant screening. GC-MS is deemed superior regarding its ability to detect adulteration, sensitivity, ability to detect trace compounds, and quantitative capabilities, while RI is considered the best regarding running costs, training requirements, sample preparation and analysis time.

The exact number of commercial patchouli oil samples that were analyzed is not entirely clear. The abstract claims that 10 commercial samples were analyzed, while the number 15 is given in the Results and Discussion section. Figure 7 in the paper suggests that four (40%) out of 10 commercial samples were adulterated using the results from the ¹H NMR analysis, while the data available in the supplemental information reports five adulterated samples (38.5%) in 13 commercial patchouli oils based on GC-MS data.

Comment: The adulteration of essential oils is an ongoing concern. While analytical techniques to find admixture of vegetable oils and other essential oils have improved, so has the sophistication with which essential oils are adulterated. The addition of nature-identical essential oil components made by chemical synthesis or by fermentation is a common way to fool routine analysis. As such, the use of RI to screen for adulterants seems to be of limited use nowadays, since many types of adulteration escape its detection. ¹H NMR may be become a more widely used tool, in particular if more affordable instruments having a better resolution than the 60 MHz benchtop apparatus can be made available. In the meantime, GC-MS still seems to be the best way to guard against fraudulent suppliers.

References

1. Swamy MK, Sinniah UR. Patchouli (Pogostemon cablin Benth.): Botany, agrotechnology and biotechnological aspects. Ind Crop Prod. 2016;87:161-176.
2. Van Beek, Joulain D. The essential oil of patchouli, Pogostemon cablin: A review. Flav Fragr J. 2018;33:6-51.