How wide is the optical rotation field: a median span of 10 degrees, and the widest entry reads −2400

About a 4 minute read.

Too long, didn't read

• 141 materials carry an optical rotation field and have 20+ suppliers; the median span is 10 degrees
• The narrow end can check a delivery: frankincense oil −0.05 to 0.00, clove bud oil −2.00 to 0.00
• The wide end cannot: black pepper oil −1 to −23 (sixteenth widest), gurjun balsam oil −60 to 0
• Expressed lime oil reads +35 to −41, a range that crosses zero. A reading of 0 is in spec
• The most extreme is Virginia cedarwood oil: −2400 to −38.00, a span of 2362. That number does not stand
• Earlier pieces here recommended measuring optical rotation on arrival. That advice holds only when the range is narrow enough

Start with a black pepper oil

Black pepper oil, 92 suppliers, optical rotation field reading −1.00 to −23.00.

Both the grapefruit oil and the juniper berry oil pieces carried a version of the same advice: optical rotation and specific gravity are fields you can verify yourself when a delivery arrives. On grapefruit oil that advice holds, because its range is +91 to +96, just five degrees wide, so a reading outside it means something happened.

On black pepper oil the same advice barely functions. If the acceptable range runs from −1 all the way to −23, any negative number you measure is inside the specification.

So whether this field can check a delivery depends on how wide it is. Which means measuring the widths.

The distribution of spans across 141 materials

141 materials carry an optical rotation field with 20 or more suppliers. Subtracting the lower bound from the upper gives a median span of 10.0 degrees.

The narrow end, among materials with 50+ suppliers:

Material	Optical rotation field	Suppliers	Span
Frankincense oil	−0.05 to 0.00	93	0.1
Clove bud oil	−2.00 to 0.00	128	2.0
Clove leaf oil	−2.00 to 0.00	109	2.0
Neryl acetate	−1.00 to +1.00	65	2.0
Immortelle flower oil	−2.20 to +0.10	52	2.3
Cinnamon leaf oil	−2.00 to +1.00	52	3.0

The wide end:

Material	Optical rotation field	Suppliers	Span
Virginia cedarwood oil	−2400 to −38.00	85	2362
Expressed lime oil	+35.00 to −41.00	34	76
Gurjun balsam oil	−60.00 to 0.00	39	60
Amyris wood oil	+10.00 to +60.00	51	50
Davana oil	+15.00 to +60.00	71	45
alpha-Phellandrene	−80.00 to −120.00	35	40

Black pepper oil's span of 22 puts it sixteenth widest of the 141.

One field, with a narrowest entry of 0.1 degrees and a widest of 2362. This is not a field that can be read one way.

Two shapes worth looking at

Virginia cedarwood oil's −2400. An essential oil does not have a specific rotation of minus two thousand four hundred degrees; that order of magnitude is not physically available. It reads like a misplaced decimal on −24.00, and the lower bound of −38.00 supports that reading, since −24 to −38 is a plausible range for a cedarwood oil. The problem is that this material has 85 suppliers. It is mainstream, and there is an impossible number sitting in its physical data.

Expressed lime oil's +35 to −41. That range crosses zero. If the acceptable range includes both dextrorotatory and levorotatory readings, then a sample measuring 0.0 is within specification. A range that contains zero rejects nothing.

Neither shape is flagged anywhere. The fields have values and the formatting is correct. Only reading the numbers reveals that one is broken and the other cannot discriminate.

Meeting the specification is not being genuine

In 2024 Linge and colleagues compared approaches for authenticating commercial tea tree oils in J Agric Food Chem (PMID 38568986), against a background of global demand driving increased adulteration in commercial products.

They used a novel enantiomeric gas chromatography mass spectrometry method for chiral analysis of key terpene components present above 0.01%, testing different ways of identifying adulteration. The samples were 88 authentic Australian oils of known provenance and 12 oxidised ones.

The first layer of the result is reassuring: those samples were consistent with the recommended ranges in ISO 4730:2017 and with previously published enantiomeric ratios, with p-cymene identified as the major marker of oxidation. The 15 constituents named in ISO 4730:2017 made up 84.5 to 89.8% of the total ion chromatogram peak area, and 53 additional peaks were detected in all samples (7.3 to 11.0%).

The second layer is the point of this piece. The authors conclude that comparison to the ISO 4730:2017 standard does not always identify adulterated tea tree oil samples. What did cleanly identify unadulterated and unoxidised oil was analysis of appropriate enantiomeric ratios plus quantitation of the p-cymene percentage.

Put plainly: an oil can sit inside the specification and still be fake. A specification range exists to reject what is obviously wrong, not to prove that anything is right.

The same year, Wang and colleagues did the parallel work on peppermint oil (PMID 38101286). They open by noting that peppermint essential oil is often adulterated to reduce production costs and increase profits, and that despite an ISO standard existing for it, they took an integrated approach involving conventional and chiral GC/MS coupled with chemometrics. The choice of method is itself a judgement: one layer is not enough.

So how should this field be used

• Read the width before the value. Subtract the bounds. A span within about 3 degrees (frankincense, clove, cinnamon leaf) makes a reading outside it a real signal. A span above 20 degrees tells you nothing when you measure it.
• Give up on ranges that cross zero. A field written like expressed lime oil's has no discriminating power at all.
• Treat impossible numbers as data errors, not exotic materials. −2400 degrees is not a property of cedarwood oil.
• This field was never proof of authenticity. Like specific gravity and refractive index, it is the first pass of an exclusion test. Judging authenticity takes chiral GC and isotopes, the same class of tools discussed in the vanilla extract and the delta-decalactone pieces.
• Read my earlier advice this way: "measure the optical rotation on arrival" works on grapefruit oil, whose span is 5 degrees, and does not work on black pepper oil, whose span is 22. The difference is not in the method but in how loose that material's specification is.

→ Search by odour: 42,225 materials with their physical data.

What this article does not establish

• I judged Virginia cedarwood oil's −2400 to be a misplaced decimal without confirming it with the source. What I confirmed is that the magnitude is not available to an essential oil's specific rotation.
• The span statistics cover only the 141 records that filled the field and have 20 or more suppliers, not all 42,225.
• A wide span does not necessarily mean poor data quality. Some natural oils genuinely vary a great deal by origin, and a wide range may be an honest description. The claim here is that a wide range cannot discriminate, not that a wide range is wrong.
• The tea tree conclusion is about ISO 4730:2017 and tea tree oil, not about optical rotation fields and not about this database. I use it for the general shape of "compliant is not genuine".
• The peppermint abstract was cut off in the record I retrieved, so I cite only its method choice and its opening statement of motivation.