How to Read a GC-MS Report: Peaks, Diluents & Purity Explained

You've received a fragrance oil. The bottle says "pure." But is it really? This is where GC-MS comes in. Gas chromatography separates a mixture into its individual components; mass spectrometry then identifies each component by its chemical fingerprint. In short, one lines them up — the other puts a name to each.

GC (Gas Chromatography) vaporises the fragrance oil and passes it through a long column. Each molecule travels through the column at a different speed. Light, volatile molecules exit first; heavier ones follow later. The moment a molecule exits is called its retention time, shown in minutes on the report.

MS (Mass Spectrometry) fragments each substance as it leaves the column and reads its fingerprint. It matches this against a spectral library and declares: "this is Linalool," "this is Limonene." The higher the match percentage, the more reliable the identification.

When you look at a report, you'll see a series of mountain-like humps. Each hump is a peak — that is, a single component. When the top note has evaporated and the next layer takes centre stage, the same principle is visible in the report: volatile compounds appear on the left, stable resins on the right.

Focus on three things: the order of the peaks (a clue to volatility), their area (quantity as a percentage), and their match quality (how confidently the molecule has been identified). It is not the height of a single peak but the area percentage (area %) beneath the curve that represents quantity.

Note: area percentage is not mass percentage. The detector is more sensitive to some substances than others. That said, at an introductory level, area % provides a solid picture of the composition.

Remember: a large number of peaks does not mean "natural," and it is not a safety indicator on its own. Safety depends not on origin but on the molecule and the usage level. We'll explore this in the third section.

Many "fragrance oils" are supplied in a carrier. Being able to spot this in a report means knowing exactly what you're paying for. A diluent can be perfectly legitimate — the problem arises when it's undisclosed.

The carriers and diluents most commonly encountered in GC-MS reports are listed below. Their peaks tend to appear prominent and large:

Say you see 40% DPG in the report. This tells you that nearly half of the liquid you've purchased is an odourless carrier. That's not necessarily bad — some fragrance oils are sold pre-diluted — but you should evaluate the price accordingly. You don't want to pay for a pure concentrate and receive a diluted product.

There is also the matter of solubility. Adding too much water to a fragrance oil disrupts solubility — the liquid turns cloudy and phases separate. Keep the water content low, start with a small amount, test for clarity, and use a proper perfumer's alcohol or suitable solvent where needed. Don't trust any fixed "this many grams of water" formula — it varies with the character of your formula.

Purity does not mean "a single peak." A natural fragrance oil can contain hundreds of components — all entirely normal. The real purity question is: is this the profile I'd expect, or has something crept in that shouldn't be there?

In a lavender oil, Linalool and Linalyl acetate should dominate. If the report shows unexplained large peaks alongside these, with low match percentages, adulteration becomes a possibility. Here is a step-by-step reading approach:

IFRA limits apply not to the total fragrance oil percentage, but to individual substances/allergens within the fragrance oil and to the product category (leave-on/rinse-off). Generalisations such as "any fragrance oil is safe up to 20%" are incorrect. The correct reference is the fragrance oil's IFRA certificate/compliance statement. We cover this topic in depth in the article 'The "Natural = Safe" Fallacy.'

You've learned how to read a report. Now translate that knowledge into purchasing and production decisions. The rest is your craft.

A GC-MS report moves you from buying fragrance oils blindly to informed sourcing. Identify the diluent ratio and weigh it against the price; build an allergen profile and verify it against the IFRA declaration; use the volatility distribution to make a rough forecast of longevity. One sheet of paper prevents three separate mistakes.

One more word of caution when scaling up: build your formula on a gram (g) basis, but never forget density. Citrus oils are light (~0.84 g/ml), while heavy resins and some synthetics can exceed 1.10 g/ml. If you work only in grams and skip the ml conversion, you'll end up with overflow or underfill when bottling by volume. Always factor in density when converting ml ↔ g. See the article "Pitfalls of the ml ↔ g Conversion" for details.

If you're just starting out, learn the basic setup first — the "Beginner's Guide to Perfume Making from Scratch" is a good starting point. Reading GC-MS reports is the quality assurance leg of that same journey.