Hi everyone,

I need to be able to assay menthol (raw material for now) by HPLC using a UV-VIS detector. This is generally a GC method, but I need to adapt to HPLC. I can't find information on whether or not menthol can be detected using a UV-VIS detector. I've seen an application note, but it was using an RI detector. Has anyone performed this before, and if so, what wavelength was it detected at? Thanks!

Menthol has no chromophor, so you cannot see it using a UV-VIS detector.

What about in a finished product, like a cream? Is the RI detector still the one of choice?

Menthol is SOOO easy by GC and cost effective ! How much are you going to spend developing this method compared to doing the test on a cheap packed GC column?

RI with a cream with all kinds of components?

Let us know how it turns out.

Ann Onymous

HPLC is the only instrument on hand right now, that's why I was trying to adapt. What is the preferred method for assaying menthol in a finished product (the cream(s) most likely will contain other actives as well, but not sure yet)? Thanks

Tell your boss you need a GC; you can't hit a baseball well with a golf club. Does your boss have pointy hair like Dilbert's?

For a cream, GC really is the way to go with this, but it IS possible to assay MeOH by HPLC. I've done it with fermentation broths on occasion. Use an ion exclusion column and a dilute acid mobile phase. RI detection is preferred, but detection in the VERY low UV is possible.

Shodex makes some good (albeit expensive) ion exclusion columns. Check their website for applications.

If you're trying to assay a cream, your matrix is going to be a big PITA and that your sensitivity will be lousy with a PDA regardless. If you're looking at a raw material (like pure MeOH or SDA Alcohol), you'll have less of a problem.

Sample prep for a cream will take some effort, as I would be hesitant to shoot a crude extract of a cream on an ion exclusion column as they foul easily. You might be able to do something with a centrifuge, reversed phase SPE cartridges, and some good filters, but you'd need to be creative, to say the least.

As an aside to "not dilbert", snide comments really aren't necessary. They guy was asking a question and is trying to solve a problem with what he has on the bench. No Crime in that. The fact is that methanol can be assayed by HPLC and it's pretty easy to do with an RI detector if you have an amenable sample matrix, such as a fermentation broth, wine, or simple raw material. It's reasonably sensitive, too - I've gotten it down to ppm levels without breaking a sweat. Maybe an inexpensive RI detector is all he needs to get good results for his sample. That would be cheaper and faster than setting up a new GC and learning how to run it.

To Juddc...

Thanks for the advice, but you might want to go back and look at the sample (its for menthol not methanol) =)

For the others...a GC is in the works, but I'm trying to adapt as many raw materials that as per USP are analyzed via GC to HPLC...we manufacture OTC products, especially sunscreens, so HPLC was the first instrument purchased. We'll just keep sending the menthol samples out! Thanks everyone.

Oh my...some days I'm quicker than others; today not so quick. Apologies to all for the temporary dyslexia.

Anyway, it will separate easily via reversed phase, but RI will be necessary to detect. Otherwise, GC is your best bet.