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Research · Volume 5. Aroma chemistry: odorant families, essential-oil composition and chemotypes · 7/11

Part VII. Synthesis and practical implications

Five main conclusions, a protocol for reading a lot GC-MS report, a quick-reference table of restricted constituents and key references.

7.1 Five main conclusions

  1. Percentage is not odour. Trace constituents with extremely low thresholds (β-damascenone in rose, vetiver ketones, grapefruit thiols, aliphatic aldehydes in citrus) often determine odour identity; the majority constituent may merely provide the background. OAV is a screening tool and must be confirmed by aroma recombination and omission testing.
  2. Structure–odour rules are tendencies. Oxygen functional groups, chain length (the aldehyde threshold minimum at C8 in the C3–C9 series), cyclisation and stereochemistry all have measurable effects, but the exceptions are numerous enough that odour cannot be predicted from structure alone.
  3. Essential oils change after bottling. Autoxidation turns a non-sensitiser (limonene) into sensitisers; acid turns citral into cresols. A certificate of analysis is correct only at the time of sampling.
  4. “Meets ISO” does not prove naturalness, and “fails ISO” does not prove fraud. Chiral analysis and the “must be low” or “must be present” constituents are more sensitive than the percentage profile of the main constituents.
  5. Vietnamese essential oils need their own reference standards. Cassia bark exceeds the cinnamaldehyde ceiling of ISO 3216, lemongrass sits at or below the citral threshold, and cajuput is strongly dispersed across chemotypes. The peer-reviewed data available are almost entirely one sample per site.

7.2 Procedure for reading a lot-by-lot GC-MS certificate

Table 15. Eight steps for checking a certificate of analysis Inference, synthesised from Parts I–VI
StepQuestionWhy
1Are the scientific name, plant part and method of distillation or extraction fully stated?Cold-pressed and distilled oils, essential oils and absolutes are products of different composition (bergamot, rose, lavender)
2Is the chemotype stated?For basil, thyme, hương nhu (Ocimum), cajuput and camphor tree, the chemotype determines both odour and safety
3Which standard, which year and which origin column is it compared against?ISO ranges change by edition and by origin; Bulgarian lavender must be compared with the Bulgarian column
4What type of chromatographic column was used, and how were percentages calculated?Co-elution on non-polar columns; peak-area percent is not percent by mass
5Are the “must be low” and “must be present” constituents correct?camphor in true lavender; menthofuran in peppermint; farnesol in S. album sandalwood; 2-phenylethanol in rose oil
6Are there enantiomeric data for the main constituents?The only method in the sources read that detected a 27% addition of synthetics to lavender
7Are there ageing markers, and how old is the lot?p-cymene, linalool oxides and carvone increase over time (Table 10)
8Are restricted substances quantified?methyl eugenol, estragole, safrole, bergapten, pulegone, coumarin (Table 16)

7.3 Restricted substances: quick reference table

Table 16. Restricted substances found in essential oils and their limits in the sources read
SubstanceFound inLimit or assessmentSource
Methyl eugenolholy basil (46%), rose (0.8–6%), basilIFRA, finished product: fine fragrance 0.011%; body lotion 0.0015%; category 12 is 0.066%. EU non-alcoholic beverages 1 mg/kg. SCF 2001: genotoxic and carcinogenicIFRA 51st Amendment (2023); EU 1334/2008; SCF 2001
Estragolebasil (20–89% depending on source), tarragon (60–75%), fennel, star aniseEMA (2014 draft): 0.5 mg/person/day, short-term use; EU non-alcoholic beverages 10 mg/kg; genotoxic carcinogen in rodentsEMA/HMPC/137212/2005 Rev 1; EU 1334/2008
Safrolesassafras-type oil (90%), some CinnamomumIFRA: not to be used as a fragrance ingredient; total safrole, isosafrole, dihydrosafrole ≤ 0.01%. Controlled precursor (Table I of the 1988 Convention according to the INCB Red List 2025)IFRA; INCB Red List 2025
Bergaptencold-pressed bergamot, lemon, limeIFRA: total ≤ 15 ppm in leave-on products on sun-exposed skinIFRA 49th Amendment (2020)
cis-Anetholestar anise, fennel (rises as the oil degrades)JECFA specification: ≤ 1%JECFA 1967
Pulegone, menthofuranmintpharmacopoeia: pulegone at most 3%; EMA proposes 0.75 mg/kg body weight/day for the two combinedEMA/HMPC/138386/2005 Rev.1
Coumarincassia bark, tonka beansEFSA: 0.1 mg/kg body weight/day; EU cosmetics must label it above 0.001% (leave-on) or 0.01% (rinse-off)via Toma et al. 2025
Linalool and limonene hydroperoxidesoxidised linalool- and limonene-rich oilsECHA RAC (2015) concluded that linalool is a Category 1B skin sensitiser; sensitising potency rises sharply on oxidationECHA RAC 2015

IFRA limits apply to the finished product and by product category; amounts of a substance contributed by natural essential oils count towards the total. The IFRA limits for isoeugenol, cinnamaldehyde and eugenol could not be read in the original text (NR).

7.4 Implications for car wooden diffuser sticks

The statements below are inferences from the data in this monograph and have not been verified on the product itself Inference.

  • Odour tenacity by tier. Monoterpene-rich oils (citrus, lemongrass, mint, eucalyptus) evaporate quickly; sesquiterpene-rich oils (vetiver, patchouli, sandalwood, cedarwood) evaporate slowly and can act as fixatives. A formula for wooden sticks needs to balance the two tiers if the odour is to last.
  • Heat and light in the car. Temperature, light and oxygen are the three main factors that degrade essential oils. In a car cabin left in the sun, all three are high. Oils rich in limonene, linalool and terpinenes will oxidise faster; adding an antioxidant (tocopherol has been shown to work with linalool) is worth testing.
  • Odour changes over time. Because the light constituents evaporate first, the odour profile of the stick will gradually shift towards the heavy tier. This could be measured by headspace analysis of the stick over time; no data exist yet.
  • Safety limits. A wooden diffuser stick is a product not intended for skin contact, but users still handle it and may touch it. The applicable IFRA product category must be correctly identified, and methyl eugenol and safrole must be checked in oils that carry a risk.

7.5 Limitations of this monograph

  • Many sources could be read only as abstracts; this is stated at each point and in Appendix A.
  • The full texts of the foundational reviews (Rossiter 1996; Kraft et al. 2000) and the standard books (Ohloff; Surburg & Panten) could not be read; some rules, such as Ohloff’s triaxial rule, are presented only from secondary descriptions.
  • ISO ranges were read from official previews; for some new standards (2025) only the foreword could be read.
  • The Leffingwell threshold tables do not state the measurement medium, and the original site blocks automated access; they are used only to compare trends.
  • Vietnamese data are mostly one sample per site, without response-factor correction.
  • No site’s browser verification mechanism was bypassed; blocked sources are recorded as not accessed.

7.6 Key references

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  32. ECHA Committee for Risk Assessment (2015). Opinion on linalool
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  42. SCF (EU) 2001. Opinions on estragole and methyleugenol. EMA/HMPC/137212/2005 Rev 1. JECFA 1967 (anethole)
  43. Standards: ISO 3515:2002; ISO 4719:2012; ISO 1342:2012; ISO 19817:2017; ISO 4730:2017; ISO 3065:2021; ISO 3044:2020; ISO 856:2006; ISO 9842:2024; ISO 3520:2022; ISO 3140:2019; ISO 3518:2022; ISO 22769:2022; ISO 4716:2013; ISO 3757:2002; ISO 3216:1997 (TCVN 6029:2008); ISO 3217 (TCVN 11425:2016); ISO 3848:2016 (TCVN 11426:2016)
  44. Vietnamese sources: see Table 14 and Appendix A

A monograph compiled from 718 sourced facts collected under a common protocol and cross-checked independently. Items labelled Inference are the compilers’ own judgements. Diagrams are newly drawn from published data. Scientific reference material — not legal or medical advice, and no substitute for the text of the standards.