There is one question that none of the analyses in the previous four lessons can answer conclusively: did this molecule come from a plant or from a factory?
There is a way to answer it, and it does not look at molecular structure at all.
Carbon is not uniform
Carbon in nature exists mainly in two stable forms: carbon-12, making up about ninety-nine per cent, and carbon-13, making up the rest.
The ratio between the two is not the same in every source. Plants, when they photosynthesise, tend to favour the lighter carbon-12, and the degree of preference differs according to the photosynthetic pathway the species uses. Petroleum, the transformed remains of ancient organisms over millions of years, carries a different signature.
Most synthetic aroma chemicals are made from petroleum-based feedstocks.
One point is often misunderstood and needs stating clearly. On the carbon-13 ratio alone, the signature of petroleum overlaps with most ordinary crops, so it is better at distinguishing between plant sources and between synthesis routes than at answering directly whether something comes from petroleum or a plant. That question is answered conclusively by carbon-14: carbon in a living plant still holds a small, steady amount of carbon-14, while carbon in petroleum has lain underground for millions of years and has almost none left.
The measurement
The technique is called isotope ratio mass spectrometry. The sample is burned completely to CO2, and the machine measures the ratio of carbon-13 to carbon-12 in the resulting gas with very high precision.
The result is a number characteristic of the carbon’s origin. It says nothing about molecular structure, and precisely for that reason it is not fooled by two molecules having identical structures.
The technique has been widely used to detect fraud in honey, fruit juice, wine and balsamic vinegar for decades before the aromatics industry took an interest in it.
Three limits
Expensive. The equipment and each analysis cost many times more than GC/MS.
Needs a reference database. The number means something only when compared with the known range for the exact species and region. Building such data takes years.
Can be circumvented. Aroma chemicals synthesised from biological feedstocks — fermented cane sugar, for example — give an isotope signature close to that of plants. The technique catches petroleum-based goods, not every synthetic.
Why it is still worth knowing
You will almost certainly never order an isotope analysis for your bottle of essential oil. The cost is out of proportion.
But knowing that it exists changes how you read every claim of purity. There is a clear line between we have checked this far and this product is certainly pure, and an honest seller stands on the right side of that line.
The next lesson moves from theory to practice: how to read an analysis handed to you by another seller.