Technical note
Isomer ratio matters more than purity in a lure active
A formulator replacing a supplier usually starts with purity, because purity is the number everyone publishes. The problem is that purity is the parameter least likely to explain a change in catch.
What the insect is reading
A pheromone receptor responds to shape. Geometric isomers of the same skeleton have different shapes, and in many systems the partner isomer is not neutral — it is inhibitory above a certain proportion. That means a lot with a slightly lower assay and the correct geometry can outperform a cleaner lot with the wrong one, and no amount of specification writing about purity will capture that.
Why the ratio drifts between suppliers
Isomers are close in boiling point and polarity. Separation is therefore the expensive part of the process, and the cheapest way to make a compound is often a route that gives a mixture. A supplier optimising for cost will push the ratio towards whatever the route gives; a supplier optimising for a specification will add the separation step and quote for it. Both can print 96% on a certificate.
How to test a supplier in one email
Ask two questions. First: what are your release limits on the isomeric ratio, not the typical value. Second: can you send a certificate that shows baseline separation of the isomers. A supplier who can answer both is controlling the parameter. A supplier who answers with an assay figure is not.
Where it matters most
Conjugated dienals and dienyl acetates, because the geometry of the conjugated system is intrinsic to identity rather than a quality attribute, and because several of them are specified as a deliberate mixture of two isomers. Acetates with a single double bond are more forgiving, but not indifferent: the difference between the cis and trans form of the same chain is the difference between two catalogue entries here.
What to write into a purchase specification
- The isomeric form, using the descriptor in the name.
- The ratio limits, as a range with a method attached.
- The assay, with the basis and the method.
- The parameters you will check on receipt, so the supplier knows which numbers are being reproduced.
Every compound page here states the isomeric form that the lot is released against; the isomer matching page describes how two certificates are compared when a supplier changes.
Need the specification behind this?
Send a CAS number and the quantity, and the answer comes back in writing with the isomeric form and packing attached.