A Certificate of Analysis (COA) is the document that determines whether your bulk essential oils supplier is telling the truth. If you can’t read one, you’re trusting the supplier’s reading — which is roughly the same as trusting the supplier’s price quote without an invoice.
Twenty minutes of reading this post will move you from “I look at the top of the COA and check that the percentages look familiar” to “I can spot adulteration, the wrong chemotype and a re-packed lot from a real distillation.”
What GC-MS actually does
GC-MS = Gas Chromatography–Mass Spectrometry. Two instruments wired together.
The gas chromatograph (GC) vaporises a tiny essential oil sample and pushes it through a coiled column. Different molecules travel through that column at different speeds — heavier or stickier ones lag, lighter ones move fast. The column “separates” the oil into its constituent compounds, each emerging at a characteristic time (the retention time).
The mass spectrometer (MS) sits at the end of the column. As each compound emerges, MS smashes the molecules into fragments and measures the mass of each fragment. The fragmentation pattern is like a fingerprint — characteristic for that specific molecule.
Result: a list of compounds and their relative quantities in the oil. That list is your COA.
What a typical COA shows
The header: sample reference and batch number, lab name + accreditation (NABL in India, ISO/IEC 17025 globally), date of analysis, method (e.g., “GC-MS as per ISO 7609 / IS 4570”), sample submitted by.
The data block: peak number, retention time (minutes), compound name (and CAS number), percentage area (essentially % concentration), identity confidence (sometimes shown as a match index against a reference library).
The summary: total identified percentage (a well-distilled oil should be ≥ 97% identified), density / refractive index / optical rotation, conclusion / authentication (“conforms to specification of Lavandula angustifolia EO”).
A worked example — lavender (Lavandula angustifolia)
Imagine a COA shows the following:
| Peak | RT (min) | Compound | % Area |
|---|---|---|---|
| 1 | 5.2 | α-pinene | 0.4 |
| 2 | 7.8 | β-pinene | 0.3 |
| 3 | 9.1 | β-myrcene | 0.7 |
| 4 | 11.3 | 1,8-cineole | 0.9 |
| 5 | 14.2 | trans-β-ocimene | 5.1 |
| 6 | 14.8 | cis-β-ocimene | 3.2 |
| 7 | 17.5 | linalool | 32.4 |
| 8 | 20.1 | camphor | 0.6 |
| 9 | 22.7 | borneol | 1.3 |
| 10 | 24.5 | terpinen-4-ol | 4.2 |
| 11 | 28.3 | linalyl acetate | 37.6 |
| 12 | 30.1 | lavandulyl acetate | 3.8 |
| 13 | 34.2 | β-caryophyllene | 4.5 |
| — | — | Other (< 0.5% each) | 4.0 |
| Total identified | 99.0 |
Now read it like an auditor:
Step 1 — Identity check. Linalool 32.4% + linalyl acetate 37.6% is squarely in the Lavandula angustifolia range (linalool 28–38%, linalyl acetate 30–45%). Camphor below 1% is critical — angustifolia is supposed to be low-camphor; high camphor would suggest Lavandula spica or a cross-pollinated lavandin.
Step 2 — Adulteration check. Three common lavender adulterations: adding synthetic linalool (would skew the linalool-to-linalyl-acetate ratio); cutting with lavandin (would push camphor up); adding a cheap diluent like dipropylene glycol (shows as an unidentified or anomalous peak). This sample looks clean.
Step 3 — Chemotype check. High-altitude lavender (Provence, Kashmir) tends to be higher in linalyl acetate. Lower altitude is more linalool-dominant. This sample looks like high-altitude harvest — consistent with Kashmir / Himachal origin.
Step 4 — Specification match. If your buyer specified “min 30% linalool, min 35% linalyl acetate, max 1% camphor” — this sample passes on all three.
Good ranges for common oils
A quick reference. Always cross-reference with the most current pharmacopoeia or ISO standard for your specific use case.
- Tea Tree (Melaleuca alternifolia, ISO 4730) — terpinen-4-ol 30–48%, 1,8-cineole max 15%, γ-terpinene 10–28%, α-terpinene 5–13%.
- Peppermint (Mentha piperita) — menthol 30–55%, menthone 14–32%, 1,8-cineole 3–14%, menthyl acetate 2–10%.
- Eucalyptus globulus — 1,8-cineole > 70%, α-pinene 2–10%, limonene 1–10%.
- Lemongrass (Cymbopogon flexuosus) — citral (geranial + neral) 70–85%, geraniol 2–6%, limonene 1–5%.
- Clove bud (Eugenia caryophyllata) — eugenol 75–88%, β-caryophyllene 5–15%, eugenyl acetate 5–15%.
- Rosemary (Rosmarinus officinalis) — varies by chemotype: 1,8-cineole > 38% (cineole CT), camphor 13–22% (camphor CT), verbenone > 15% (verbenone CT).
- Sandalwood (Santalum album) — α-santalol > 41%, β-santalol > 16% (ISO 3518).
If a COA shows numbers outside the published ranges, that’s a flag — could be authentic and an unusual harvest, could be a different chemotype, could be adulteration. Ask the supplier.
Red flags in a COA
- Total identified < 95% — either the lab couldn’t identify large peaks, or the oil is heavily adulterated.
- Single dominant marker far above the published upper bound — often a sign of synthetic boosting.
- A marker far below the lower bound — possible dilution.
- An unexpected compound at significant percentage (dipropylene glycol, DEP, isopropyl myristate are common adulterants).
- No mention of CAS numbers or library match indices.
- No lab accreditation listed.
- No batch number or sample reference — useless for audit.
- Date older than 12 months — essential oils oxidise.
“Therapeutic grade” and other marketing labels
“Therapeutic grade” is not a regulatory term. It was popularised by US MLM essential-oil brands and has no enforced definition. Same for “premium grade”, “ultra-pure”, “clinical grade”.
What IS real: USP (United States Pharmacopoeia), IP (Indian Pharmacopoeia), BP (British Pharmacopoeia), ISO standards (e.g., ISO 4730 for tea tree), Organic certifications (USDA, COSMOS, EU Organic). If a supplier markets “therapeutic grade” without an actual standard reference, ask which actual pharmacopoeia or ISO standard the oil meets.
Want a real lavender GC-MS from our latest batch?
We’ll happily share an actual GC-MS COA from our most recent lavender bulk batch. Request it on the contact page or directly via our quote form — we’ll send the PDF over within 24 working hours.
Author: Yogesh Tripathi, Expo Organics. Manufacturing essential oils in India since 1984.