Beginner perfumer · 58
What are you smelling when you sniff the cap? The corpus's top 36 materials span 83,333-fold in volatility
· 21 min read
Somebody on r/DIYfragrance complained that when they hand over a tester, people pull the cap off and sniff the atomiser as if that tells them everything. A reply added that all you smell there is the dried-up juice. That is correct, and it is measurable. Matching the most-used materials in a 954-formula corpus against the database's vapour pressure field, the 36 that matched run from para-anisaldehyde at 1.0 mmHg down to musk ketone at 0.000012, a spread of 83,333-fold across 4.9 orders of magnitude. What survives after the ethanol goes comes from the bottom of that list, and that band accounts for 0 percent of the mass in half of all formulas. The cap does not give you a small version of the perfume; it gives you a badly skewed sample.
About a 7 minute read.
A thread on r/DIYfragrance asked what annoys people as perfumers. Of 63 replies, this one drew 14 upvotes:
"When I hand someone a small tester bottle, they pull off the cap and sniff the atomizer, as if that will tell them everything they need to know about how the perfume will smell." (Technophysicist)
Somebody added the explanation:
"Yup, never smell a perfume by sniffing just the cap or the atomizer. It's all the dried up juice that you smell." (MediocreApprentice)
That is correct. And it is measurable.
What the dried-up juice is
The layer on the atomiser is what stayed behind after the ethanol left. Ethanol's vapour pressure is 44.6 mmHg at 20 C, and most aroma materials sit three to six orders of magnitude below it, so the ethanol goes first and everything else remains.
Then those go too, at very different speeds. How different is a lookup.
I matched the most frequently used materials across 954 published formulas against the database's vapour pressure field. Of the top 60, 36 matched.
The highest is para-anisaldehyde at 1.0 mmHg. The lowest is musk ketone at 0.000012.
| Vapour pressure @ 25 C | Formulas | |
|---|---|---|
| para-anisaldehyde | 1.000000 | 60 |
| lemon oil | 0.950000 | 66 |
| phenyl acetaldehyde | 0.368000 | 57 |
| citral | 0.200000 | 72 |
| benzyl acetate | 0.177000 | 317 |
| linalool | 0.016000 | 338 |
| vanillin | 0.002000 | 137 |
| alpha-hexyl cinnamaldehyde | 0.001000 | 157 |
| benzyl salicylate | 0.000170 | 159 |
| musk ketone | 0.000012 | 84 |
83,333-fold, across 4.9 orders of magnitude.
Materials inside one formula leave at speeds five orders of magnitude apart. The layer on the atomiser is not a smaller version of the formula; it is one slice of it.
How small that slice is
For each corpus formula I computed the share of mass in each volatility band. (On average I can match only 46% of a formula's mass to a vapour pressure, so these are shares within that part.)
| Volatility band | Median share of mass | Mean |
|---|---|---|
| > 0.1 mmHg (top) | 19.7% | 27.9% |
| 0.01 – 0.1 | 31.3% | 35.2% |
| 0.001 – 0.01 | 20.0% | 25.7% |
| < 0.001 (bottom) | 0.0% | 11.2% |
Median 0.0%, mean 11.2%.
More than half of formulas put almost nothing in the lowest band, while a minority put a great deal there.
And that band is what stays on the cap.
Sniffing the cap samples the least representative, most skewed slice of the formula. Two perfumes with completely different first twenty minutes can have nearly identical caps. The reverse holds too.
Something that stops this being too tidy
The conclusion is coming out too clean, so I went looking for the other side.
Osako and Nishida studied the correlation between odour thresholds and saturated vapour pressures in the Japanese Journal of Hygiene in 1992. Their results come in layers:
- Within homologous series of aliphatic compounds, the correlation between vapour pressure and threshold is very good
- But ionised substances (acids, amines) and sulfur compounds show no good correlation
- Within the alcohol and aldehyde series, the rate at which threshold falls with falling vapour pressure gets smaller
The first point is a correction to my analysis above. A material with low vapour pressure tends, within its chemical series, to have a low threshold too.
So musk ketone being concentrated 80,000-fold in the residue does not mean it smells 80,000 times as strong. The concentration went up, and it was already a material that needs a higher concentration to register.
(The database's strength field bears this out: the lowest-vapour-pressure aroma materials in the library often read low in that field. The most extreme case is in the phenethyl salicylate article.)
So the correct statement is not "the cap holds a concentrated base note" but "the cap's composition is nothing like the bottle's". The direction is certain; the multiple is not.
What a real model looks like
What I used above is a ranking by pure-component vapour pressure, which is a first approximation.
The perfume engineering methods Rodrigues and colleagues reviewed in Molecules in 2021 use vapour-liquid equilibrium and Fick's law of diffusion: each component's partial pressure is its mole fraction times its pure vapour pressure, plus an activity coefficient correction. (How that model undoes the idea that a fixative "holds" a fragrance is in Fixatives do not hold a fragrance.)
I used a ranking of pure vapour pressures and ignored interactions inside the mixture.
The direction is the same; the multiple is not.
About "the first 10 seconds on a strip"
Another complaint in the same thread:
"When people judge a perfume by the first 10 seconds on a strip." (brabrabra222)
That one can now be measured with an instrument.
Hadjiefstathiou and colleagues built a device in Talanta in 2024 to measure perfume release into the air above a surface after application. They used a perfume of eight fragrance molecules in ethanol and analysed the headspace by solid phase microextraction and gas chromatography.
The interesting part is the four surfaces they compared: chemically inert glass, the Strat-M skin model, a perfume test strip, and skin.
So "a strip is not skin" is no longer only received wisdom; there is now a device that measures the difference. (That paper is a methods paper, focused on the device's effectiveness and repeatability rather than on delivering conclusions about the four surfaces. What I am citing is that this is now measurable, not any specific difference.)
An observation I cannot resolve
The thread also has two people describing the same thing in different languages.
"I just got a bunch of 2mL sprayers (glass bottles/plastic atomizers and caps) and after I made up a whole bunch of testers, I noticed an almost fishy smell when I first removed the caps to try them out." (Haven416)
"Exactly the same here. The black plastic part of the atomiser stinks of fish after contact with alcohol." (Annual_Impression834, in Polish)
Two people, two languages, one observation.
A fishy smell usually points chemically at amines. In the database, materials with the odour type fishy are typified by pyridine (sour, fishy, ammoniacal) and thiazole. What leaches out of a plastic component, and what ethanol extracts from it, is not a field in this database and I have no way to look it up.
I am recording it here as an open question. If it happens to you, an atomiser from a different supplier is the cheapest test.
What you can actually do
- Do not sniff the cap. It is the most skewed slice of the formula, and half of formulas put almost nothing in that band.
- To judge a perfume, spray it. Onto a strip or onto skin, then wait.
- A strip is not skin, and that difference is now instrumentable. Try both.
- The first ten seconds are ethanol plus the few materials at the very top. In the corpus the > 0.1 mmHg band accounts for a median 19.7% of mass, so the first ten seconds gives you about a fifth of the formula.
- Vapour pressure ranks; it does not multiply. In a mixture you need vapour-liquid equilibrium.
- Low volatility means neither low presence nor high presence. Threshold and vapour pressure move together within a series, and the two effects pull opposite ways.
What this doesn't establish
- I ran no headspace analysis. This is reasoning from database fields, corpus and three papers.
- The 83,333-fold is the extreme ratio among "the 36 of the corpus's top 60 that matched a vapour pressure". A different set of materials or a different match rate gives a different number.
- The vapour pressure field covers 21% of the library, and I can match only about 46% of each formula's mass. I do not know where the other half sits, so the band shares above are shares within the known half, not within whole formulas.
- Many of those vapour pressures are marked (est). Mixing estimates with measurements is a risk.
- Pure-component vapour pressure is not partial pressure in a mixture. The Rodrigues line of work uses vapour-liquid equilibrium and Fick's law; I used a ranking. The multiples are not trustworthy.
- I did not compute a timeline. "Ethanol first, then the top" is a direction; I have no time scale at all.
- Osako 1992 is a Japanese-language paper on homologous series the authors selected. It shows the correlation exists and has exceptions; it is not a formula to substitute into.
- Hadjiefstathiou 2024 is a methods paper. What it establishes is that the device works, not any specific difference between the four surfaces.
- The fishy-smell report I have not explained at all. Two user reports, no testing.
- "Two perfumes could have the same cap" is an inference; I have not compared the residue of any two perfumes.