Dear Valerie: We currently use IR, along with other analytical measures, to QC incoming ingredients into our facility. However, I’m unclear how one knows a fatty alcohol is in fact the called out fatty alcohol or an oil is the specified oil if the IR spectra look the same?
—Vera Fication

Dear Vera:

There are few things I recall in such detail from my undergraduate chemistry days, but one of them is learning how to prepare IR samples and interpreting the subsequent data from a dot matrix printer. Fast forward a number of years, I was quite relieved when my lab acquired a new FTIR spectrophotometer and it had an ATR accessory, essentially eliminating the need for salt plates or fine-grinding of powders (and dot matrix printers, thankfully). The chemical sample was simply placed on the crystal of the ATR accessory and measured. 

To grow our devoid spectral library, I measured nearly everything I could get my hands on, often cross-referencing the spectra against a dog-eared textbook authored by a scientist who unfortunately bore the same name as an ex-boyfriend. This created brief but notable confusion for my husband, Mr. Cosmetic Chemist. To preserve domestic harmony, I have since deleted the author’s contact information from my Outlook contacts.

Your lab is doing some great work deploying IR to distinguish chemicals by their unique chemical fingerprint; IR works by identifying functional groups, and at times their proximity. When scanned against an internal standard referenced in a library, this allows the user to ascertain the likelihood the incoming chemical is indeed the one intended to be purchased. 

However, as you have indicated, as IR is not a reliable tool for identifying specific features like structure or chain length of alkanes present, it should not be the only metric by which chemicals are evaluated for identity.

When scanning cetyl alcohol, a commonly used fatty alcohol in cosmetics, IR is only able to indicate the presence of -CH2 and -OH groups. It doesn’t know how many. When scanning stearyl alcohol, which has two more -CH2 groups by comparison and relatively the same spectra is output. Therefore, other attainable means of identification, such as melting point, are recommended to be certain the correct material is being used. The manufacturer of our cetyl alcohol indicates melting point around 50ºC and the stearyl variety around 58ºC. It’s not always quite so easy.

Like fatty alcohols, many oils look or feel the same with some subjectivity, and their IR spectra may be nearly identical when comparing different oils to one another. If you are unsure sunflower seed oil is indeed sunflower seed oil beyond their IR spectra and organoleptic properties, other means must be used like GC with some sample prep. IUPAC has published testing standards for oils which outline the appropriate testing methodology for repeatability. If you don’t have a GC, you can test the iodine value and saponification value against the disclosed ranges on the certificate of analysis via titration but, depending on the plant oil, there may be some overlap in specification values.

These are just two specific examples you mention. Many other ingredients—including botanical extracts—fall victim to the same challenges of clear identification through standard quality control processes. As mistakes happen with ingredient labeling at the manufacturer level, it’s prudent to develop incoming multiple QC processes that verify what the manufacturer is reporting on the label is indeed correct.

Valerie George

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Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com