How to Read a GC-MS Report for an Essential Oil

A GC-MS report tells you which volatile compounds were found in a sample and what share of the detector signal each one produced. It does not say the oil is pure, and the percentage column is a relative peak share, not a concentration. Read it in a fixed order (header, separation, identification, percentages, then what the method could not see) and it becomes a useful document instead of a badge. We have no GC-MS report of our own to put on this page, so the worked example below comes from published literature on amyris-type oils, and we say so each time.

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Step 1: the header decides whether the rest matters

Before any peak, check four things. The report should name the oil by botanical name, carry a lot number that matches your bottle, say which laboratory ran it and when, and state the method: instrument, column and library. A report with no lot number is about some oil, not yours. In the sandalwood study used as our example, the method is spelled out: samples diluted to 1% in dichloromethane, an Agilent 6890N GC with an MSD 5973 detector, an HP-INNOWax polar capillary column (60 m), and identification by comparing mass spectra with the NIST11 and FFNSC3 libraries (Kucharska et al., 2021). That is the level of detail to expect; the column type matters because it changes the order in which compounds emerge, as the next step shows.

Step 2: separation, and why retention index beats retention time

The gas chromatograph separates the oil's components as they travel through a long column at different speeds. Each compound's retention time is when its peak appears, but retention times shift with column, temperature program and instrument. Retention index converts the time to a standardized number by placing it between reference hydrocarbons, so values can be compared with published tables. Sadgrove and colleagues describe the rule of thumb for a tentative match on a non-polar column: the measured index should be within 10 units of the value published by Adams (Sadgrove et al., 2022). Reasoning from this, an index measured on a polar column such as the one above cannot be compared with a non-polar table value, so a report that gives only retention times, or indices without the column type, cannot be checked by a reader.

Step 3: identification is a hypothesis, and amyris makes it harder

The mass spectrometer breaks each separated compound into charged fragments and records the pattern. Software compares that pattern with a library and returns a ranked list. Alex St-Gelais of Laboratoire PhytoChemia notes that such matches are a hypothesis and not a confirmation, that some sesquiterpenes and cis/trans cyclic isomers give very similar spectra, and that retention indices help resolve what the spectrum cannot (St-Gelais, Tisserand Institute, undated page, read October 2026). Sadgrove and colleagues make the same point: the NIST library "might suggest several very convincing matches," and it is up to the scientist to decide which is right.

This matters for amyris because the oil is dominated by sesquiterpene alcohols, with valerianol, elemol and several eudesmol isomers in the sandalwood paper's profiles. Reasoning from the isomer warning above, the eudesmols (alpha, beta, gamma and their epimers) are exactly the kind of near-identical-spectrum compounds where a library hit alone should not be trusted. A good report shows both the match quality and the index for each named peak. A report that lists "eudesmol, 26%" as a block is usually summing several isomers, which the paper's own text does when it says "different isomers of eudesmol (~26%)."

Step 4: what the percentage column means

Most reports give "area %," each peak's signal divided by the total signal. St-Gelais calls this "a practical, commonly accepted approximation, but an approximation nonetheless," says the MS is "not the ideal detector" for it, and notes that the ISO and AFNOR norms were developed from GC-FID data, so MS-based quantitation is not ideal for attesting compliance. Sadgrove and colleagues say that GC-MS quantitation "is not regarded as absolute" and is for comparison. A 14.5% area share therefore means roughly "this compound produced 14.5% of the signal," not "14.5 grams in 100 grams of oil."

The difference is not academic. In a 2021 study of adulterated lavender, the authors report that dilution with vegetable oil leaves the GC profile and the normalized marker percentages unchanged, and that only absolute quantitation showed linalool at 14.5 g/100 g against 23.4 g/100 g in genuine oil, and the authors estimated about 40-50% adulteration (Capetti et al., 2021). A report built on normalized areas would have read as clean.

A worked example from the literature (not our oil)

Kucharska and colleagues analysed six commercial oils sold as sandalwood. Three turned out to be amyris-type oils, which gives published numbers to read against. The table uses the paper's reported values; "about" figures are the authors' own approximations. These are three commercial samples of unknown origin and lot, not a reference range for amyris.

Selected GC-MS results from Kucharska et al., 2021 (area %, as reported)
SampleLabelValerianolEudesmol isomersElemolOther
P1Amyris14.53about 269.15Sesquiterpene alcohols 55.2
P2Amyris14.38about 269.42Sesquiterpene alcohols 53.8
P3Sandalwood8.04about 166.00Caryophyllene 11.70
P4Sandalwood4.72about 6.13.15Javanol and polysantol isomers (synthetic)
P5No origin givennot reportednot reportednot reportedBenzyl benzoate 74.6

Reading it the way a buyer would, in order: P1 and P2 are near-identical, valerianol-led, labelled as amyris, and consistent with each other. P3 carries the same three compounds at lower levels plus a notable caryophyllene share, and its sandalwood label was judged incorrect because it contained no santalols; the sandalwood standard, ISO 3518:2002, is defined by alpha- and beta-santalol (the paper's introduction gives 41-54% and 16-24%). P4 has only traces of the amyris compounds and synthetic sandalwood-type molecules, so the authors called it a synthetic imitation. P5 is mostly one ester and was judged a synthetic mixture. The lesson is the reading pattern: first ask whether the marker compounds for the stated oil are present at all, then whether they are in a plausible proportion, then whether something unexpected dominates. The differences between P1/P2 and P3 also show that the same name can cover wide variation, so a single "typical" figure from one paper should never be turned into a pass mark.

Step 5: what a GC-MS report cannot tell you

Where our published certificate fits

We do not publish a GC-MS report. What is on the site is the Certificate of Analysis for one batch (AMYQGAABHTC003A), which lists physical constants: appearance as a yellow liquid, relative density at 20 C of 0.952 (standard 0.946-0.978), refractive index at 20 C of 1.505 (standard 1.504-1.512), and optical rotation at 20 C of +21.32 degrees (standard +20.00 to +45.00); the full document is the published COA.

These are quick bulk measurements on the whole liquid, and they are a plausibility screen, not a composition test. Reasoning from how the three measurements work, a large admixture of a material with a very different density or refractive index would pull the figures out of their windows. But the windows are wide (the rotation window spans 25 degrees), and a diluent or a blend chosen to sit near the same values would pass, so the numbers cannot establish identity or purity. They do not show valerianol, elemol or any other compound, and we do not state composition figures for our oil on this page. Independent third-party laboratory reports for each lot are available on request, through the contact page or the number on the bottle. When one arrives, run the five steps above on it. Our sourcing page says the laboratory confirms the valerianol, elemol and eudesmol profile and checks the oil is not a sandalwood substitution; the report is where you can see the detail behind that.

How to ask any brand for a report

Send the lot number from the bottle and ask for the analysis for that lot, in writing. Ask which laboratory ran it, the date, the instrument and column, whether identification used retention indices, and whether the percentages are normalized areas or calibrated. Ask separately about pesticides and heavy metals if they matter to you, because GC-MS will not answer them. A seller who cannot give the lot-specific document has, on the Tisserand Institute's checklist, a red flag (Hana Tisserand, Tisserand Institute, undated page, read October 2026). The full buying sequence is in how to choose essential oils, and grade claims and adulteration have their own guides.

If you want to know what the species is before reading its report, start with what amyris oil is. A report is only as good as the question you bring to it, so decide what you most need to rule out before you open one.

Amyris Essential Oil bottle

Amyris Essential Oil

Amyris Essential Oil from our own shop, in sizes from 0.5 oz to bulk.

From $20.99

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Sources & Further Reading