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A drum of lavender oil arrives at the workshop with two documents: one records the test results for that exact lot, the other explains how to handle it as a chemical. The first is the COA, the second the SDS. This piece reads both, from the chromatogram to the warning symbols, and explains why the word pure is checked with numbers rather than with the nose.

Two documents in the shipment

Each time a new lot of essential oil leaves a supplier, it travels with a file. The piece on a batch of oil arriving described how Lê Mai receives stock: the drum is opened in a closed room, the paperwork is filed with photos of the label, the lot number is logged. The two most important documents in that file go by the initials COA and SDS.

COA stands for Certificate of Analysis. It answers the question: what properties does this specific lot have, measured by which methods, and do they fall within the standard ranges. SDS stands for Safety Data Sheet. It answers a quite different question: how hazardous is this liquid when stored, shipped, spilled or set alight.

The two are often confused. A good COA says nothing about safe handling; a complete SDS says nothing about the quality of the lot. Neither is a promise about what the oil does on skin.

COA: the results of one lot

The most important line on a COA is the lot number at the top. The results hold only for the lot bearing that number. Essential oil is an agricultural product: the same species from the same supplier, but a different season, field or distillation batch, gives a different composition. A COA without a lot number, or with one that does not match the label on the drum, is a document of no use.

The rest of the COA usually falls into two groups. The first covers sensory and physical parameters: appearance, colour, odour, specific gravity, refractive index, optical rotation. The second is chemical composition by gas chromatography, listing the main constituents and their percentages. Next to each result there is usually a column of standard ranges, from the ISO standard for that oil or from the supplier's specification.

The useful way to read it is result beside range, not just the word pass at the bottom. A value sitting at the lower edge of its range says something different from one in the middle. Comparing several COAs for the same oil across lots also shows how consistent a supplier is.

GC-MS: a chemical fingerprint

Gas chromatography mass spectrometry, GC-MS, is the standard technique for analysing essential oil composition. According to the library's glossary, the gas chromatograph separates constituents by the time they take to pass through a long, thin column; the mass spectrometer identifies each by the way it breaks into fragments of characteristic mass. The result is a table of molecules and their percentages, read like a fingerprint.

For lavender oil from Lavandula angustifolia, the library's lavender page gives linalyl acetate at about 25 to 45 percent and linalool at about 20 to 38 percent, with low camphor. A COA for genuine lavender will show its two main constituents within those ranges. The library also records that the most common adulteration is blending lavandin into true lavender, detected by looking at camphor and (Z)-β-ocimene on the GC-MS table.

Ordinary GC-MS has a blind spot: it cannot separate enantiomers, the two mirror image forms of one molecule. Synthetic linalool and plant linalool share the same formula and the same mass spectrum. Laboratories therefore add chiral gas chromatography. The library gives the quality standard for lavender: at least 85 percent of the linalool should be the (R)-(−) form, and at least 98 percent of the linalyl acetate. A sample with linalool close to fifty fifty between the two forms points to synthetic linalool.

Beyond that lies isotope analysis. According to the glossary, petrochemical synthetics contain essentially no carbon 14, so measuring carbon 14 shows whether a molecule came from a plant or from petroleum. Each layer of testing closes a gap the previous one left open.

Density, refractive index, rotation

Before chromatography, oils were tested by physics, and those measurements remain on COAs today. Specific gravity is the ratio of the oil's density to that of water at the same temperature. The library notes that most essential oils are lighter than water; a few, such as clove or cinnamon, are heavier and sink to the bottom of the separator during distillation.

Refractive index is the ratio of the speed of light in vacuum to its speed in the oil, measured at a standard temperature. Optical rotation is the angle by which a sample turns plane polarised light, measured with a polarimeter. According to the library, rotation is a parameter in many ISO oil standards, because the natural balance of enantiomers gives fairly stable values.

These three figures are cheap, quick, and hard to fool all at once. Adding a cheaper oil or a solvent can preserve the smell, yet often pushes density, refractive index or rotation outside the standard range. They do not replace GC-MS, but they are the first sieve.

The SDS and GHS symbols

The SDS sees essential oil from another angle: as a chemical moving through warehouses, trucks and ports. According to the library's glossary, an SDS in the REACH format has 16 sections, from identification, hazards and composition, through first aid, firefighting and spill response, to storage, transport and regulatory information.

The hazard section uses the GHS system, with red bordered diamond symbols. In Europe the system applies through the CLP regulation, Regulation 1272/2008. The library lists the hazard labels commonly seen on essential oils: flammable, aspiration hazard, and skin sensitiser. Many citrus and conifer oils have flash points below 60 degrees C, enough to be classed as flammable liquids.

The aspiration hazard often surprises people. It is not about smell or vapour. It concerns low viscosity liquids that, if swallowed and drawn into the airways, can cause serious lung damage. This is why many oil bottles carry child resistant caps, and why the library's safety page advises not to induce vomiting after accidental ingestion but to contact a poison centre at once.

An SDS is not a guide to skin use. It describes the hazards of the neat substance, in bulk. Safe skin levels live in other documents: IFRA limits by product category, and the safety pages of the library.

Paperwork and certification

Besides the COA and SDS, a lot of oil may come with other papers. Organic certification, such as USDA Organic or the standards applied by Ecocert, concerns farming and the chain of inspection, as described on the library's certifications page. An IFRA certificate is a document the supplier issues for a specific fragrance compound, stating the maximum use level in each product category under a named IFRA amendment.

Each document answers exactly one question. Organic certification does not prove chemical composition; a COA does not prove farming practice; an SDS does not prove quality. The words pure essential oil on a label are a claim, and the way to check it is to read the COA of that exact lot: main constituents within range, no trace of a cheaper oil, the right enantiomer ratio, physical values within limits.

The nose still has a role. It catches oxidised oil, an odd note, an unusual lot. But the nose cannot measure a percentage, cannot see an enantiomer, cannot detect an odourless solvent. The library's storage page adds one more point: a COA describes the oil on the day of testing. After that, light, heat and oxygen keep changing it in the bottle.

Two documents, two different questions.

One says what the oil is. The other says how it can harm.

The word pure is checked with numbers, and only for one lot.