Chromatography Forum

Hello.

I saw an old post about analyzing Menthol by GC/MS, but didn't see a clear answer to attaining reproducibility.

Some insight would be much appreciated.

My end goal is to create a menthol calibration curve for Py GC/MS. My problem is reproducibility. I have made 1% and 2% menthol standards in methanol.

Running the same sample three times I get three different results.

Furthermore, in comparison, the 2% standard is much higher than the 1% standard. Much greater than 2 times.

Anyone have an idea of what I can do to attain reproducibility?

Thank you.

My end goal is to create a menthol calibration curve for Py GC/MS. My problem is reproducibility. I have made 1% and 2% menthol standards in methanol.

Are you really making the standards in methanol or is this just a typing error?

Yes, I am using Methanol to dilute my standards. Should I use something else?

Yes, use something else. You are measuring the sovent plus what you are adding to it. What type of samples do you plan on analyzing? Make sure your standards are in leak-tight sample vials, filled to the top with no headspace, and cap them immediately. Are you doing direct injections, purge and trap, or headspace? GC/MS is not the best detector for MEOH. If you have a GC with an FID it would be better.

I am using Pyrolysis GC-MS. It is the only instrument at my disposal.

I am trying to quantify menthol in a cream.

I'll remake some standards with CH2CH2.

I am trying to quantify menthol in a cream.

We extract, derivatize, then do by GC-FID. Very routine, very reproducible.

I am using Pyrolysis GC-MS. It is the only instrument at my disposal.

"Sometimes you don't need a howitzer to kill a cricket"

- either Confucius or Yogi Berra

I have done analysis of menthol from tobacco after extracting with methanol and it worked very well. Methanol (CH4O) as a solvent for methanol (C10H20O) works well.

This was liquid injection into the GC. With either CH2Cl2 or CH3OH, for pyrolysis MS, watch the sample handling - wiht the instrumentation I am familiar with, you need to put small sample volumes into small glass (or quartz) tubes. Evaporation can hurt results. Menthol is volitile.

Yes, use something else. You are measuring the sovent plus what you are adding to it. What type of samples do you plan on analyzing? Make sure your standards are in leak-tight sample vials, filled to the top with no headspace, and cap them immediately. Are you doing direct injections, purge and trap, or headspace? GC/MS is not the best detector for MEOH. If you have a GC with an FID it would be better.

I sense confusion here.

Methanol is a perfectly good solvent for menthol, and menthol is not so volatile that with dilute solutions you need to take special precautions like filling vials right to the top with no headspace.

Nonetheless, a given volume of liquid methanol produces a larger volume of oing vapour than the same volume of e.g. hexane, and this can make sample introduction to the GC tricky.

Doing liquid samples with a pyrolysis inlet is tricky - you must have something to trap the liquid and keep it in the inlet while it evaporates.

Is it possible to remove the pyrolyser and revert to the GC's split-splitless inlet ?

Peter

CPG,

I am curious why you derivatize. To make it less labile or improve volatility?

Best regards,

AICMM