Menthol assay using GC-FID in (ointment/ gel)

[pratinatural]

Hello,


I am trying to develop and validate a method for Assay of menthol in an ointment/gel formulation. I am using Rtx-5 column. I have interfering peaks from camphor and tea tree oil which are some other ingredients present in the formulation. Can anyone suggest a method for getting good separation from interfering peaks.

FID: 250 C
Column oven: 50 - 150 C (10 C/ min)
Split: 10: 1
solvent: methanol

[GOM]

Hi

From my database

Tea tree oil is



Camphor is





Try using a polar column - for example Innowax or equivalent

Regards

Ralph

[Consumer Products Guy]

I've assayed for menthol many times over the decades in varied matrices. Typically I extracted with DMF and used trimethylsilyl derivatization. Your Rtx-5 capillary should be a good first choice for column.

[GOM]

As an edit to my earlier reply

When I was analysing for it I got good peak shape and resolution with a polar column.

However, Consumer Products Guy's suggestion of derivatising the OH group of menthol is a good one - the solvent is good for your sample and you can still use your low polarity column.

Regards

Ralph

[rb6banjo]

This is a classic example of why I won't ever buy a GC again that doesn't have the ability to heartcut from one column to another. Real samples are just too complicated to analyze on one phase. It's been a very rare time when I heartcut my analyte from my nonpolar (precolumn) phase to a polar phase (analytical column, goes into my mass spectrometer) and i haven't been able to separate it well from interferences. No other hoops to jump through. Just set the right heartcut.

The derivatization will probably fix your problem. It is an extra hoop to jump through.

[pratinatural]

Thank you all for your suggestions.

Ralph: I initially used a ZB-wax column (100% PEG), but got almost the same results as with (Rtx-5). I will try the derivatization step and see if I can get better peak separation from interfering compounds. Thank you.

[GOM]

Hi

Thanks for updating us - this is always appreciated

Let us know how you get on

Regards

Ralph

[rb6banjo]

You can do this without derivatization if you can heartcut the menthol peak from the 5-phase to a wax phase. I extracted a petrolatum containing menthol and camphor from my medicine cabinet at home. My gel didn't completely dissolve in methanol but I did get a lot of both camphor and menthol in the extract (27 mg in 40 mL of MeOH, 10 minutes in a sonic bath, dilute 1:100 for SPME analysis). When I diluted my methanol extract with water and analyzed the solution by headspace spme (100% pdms fiber), the heartcut of only the menthol peak from the 5-phase (ZB-5 from Phenomenex) to the wax (Simplicity-Wax from Supelco) showed both analytes easily resolved on the second phase. The heartcut time was determined by injecting a short spme collection of the menthol reagent bottle and determining the retention time on the 5-phase. Direct injection of the methanol extract would work as well. My system is set up for headspace spme analysis.

This instrument is an Agilent 7890B GC with an FID monitoring the 5-phase separation and a 5977 mass spec. monitoring the wax-phase separation. I purchased Agilent's version of the "dean's switch" for this system. It was a little tricky to balance when I first started using it but it works like a charm now.

Here are the chromatograms and mass spectra for analysis of the methanol extract of the petrolatum from my medicine cabinet (paste the link in your browser if it's not hyperlinked):

https://onedrive.live.com/redir?resid=4 ... file%2cpdf

[rb6banjo]

Out of curiosity, I ran my extract using a DB-624 column (60 m x 0.25 mm x 1.4 µm) and the camphor and menthol coelute on that phase as well. It's a tough separation on any one phase.

[Consumer Products Guy]

From rb6banjo's spectra, one might do pretty well using selected ion monitoring or SIM with ion 152 for camphor and 123 for menthol even if they coeluted.

[MSCHemist]

http://www.flavornet.org/f_kovats.html Here is a good site with lists as to what your inferences might be. Probably terpene related compounds.

[MSCHemist]

Sorry I misunderstood you weren't saying camphor was coeluting you were getting general coelution. hmm those types of matrices are full of terpenes. Either a different detector or extraction or SPE or derivatization is your bet.