Cannabinoid quantification question

[Wedge]

I've been trying to run cannabinoids on an HP 6890 with FID, but been having difficulty getting anything other than a solvent peak. I've tried a variety of temperature and pressure conditions to no avail. Using the standards from Restek even at 1000 ug/mL, we're not getting any peaks other than the methanol solvent peak. Running samples of cannabis in methanol, we get multiple peaks.

Example: 50-260C, split 5:1, hold for 1 minute at 50C and ramp 50-260 at 10/minute, hold 10 minutes at 260. Injector at 240, FID at 260. Nice methanol peak, no other peaks. Column is RTX-5, 30 meters. We're running hydrogen as the carrier gas, and methanol is eluting around 1.9 minutes, butane coming off a little earlier than that. We've tried temperatures as high as 320.

We're trying to stay away from derivatization, and have read a number of papers (using conditions similar to the above) in which no derivatization is used, apparently with good results.

Any recommendations of things to try?

[BHolmes]

I would suggest that you try different liners, perhaps the cannabinoids are interacting with the liner and are subsequently lost, thus the lack of cannabinoid peak in chromatogram.

I know Agilent and Restek make a few inert liners with different internal structures (helix, gooseneck, dimbled, baffeled, w/glass wool, etc...) which are designed to reduce solvent and background peaks while ensuring good analyte signal.

Agilent - http://www.chem.agilent.com/en-US/produ ... x?cid=4877

Restek - http://www.restek.com/liners

[HippyLabRat]

The boiling points of CBD and THC are quite a bit higher than 50C... (bp for CBD = 180C, THC = 157C) I doubt you're even getting anything through your column. Try starting the column temp at 150, and ramping to 250 over the same time period.

[Wedge]

Thanks for the recommendations and suggestions. I've tried a variety of column temps, ranging from 140 to 240 at 10C, up to 220 to 320 at 10C. I've tried dropping the gas flow to see if I can't get multiple peaks out of the solvent peak, and- nothing.

As it stands, methanol, butane, and n-hexane all come off the column at the same time.

I'm going to try to swap out a liner from another GC that I have, but is it really possible that a sample containing 100 ug/mL of THC is going to have 100% of its sample degraded? I've tried running with the attenuator off, and even in splitless mode- and I'm getting absolutely squat other than the solvent peak.

[Wedge]

OK- the different liner seems to be doing the trick, in conjunction with upping the injector pressure a bit!

Thanks, y'all.

[BHolmes]

OK- the different liner seems to be doing the trick, in conjunction with upping the injector pressure a bit!

Thanks, y'all.

With GC analysis I find that the condition and sometimes the internal structure (straight tube vs. gooseneck) of the liner plays a tremendous role. We have to change our liner at a minimum of every 2 weeks, if we don't we see a significant drop in analyte sensitivity. We even have to trim the column from time to time to bring sensitivity back up to normal. It all depends on the analytes and matrix you are analyzing.

I don't know if your method already includes a column backflush, but this could help with maintaining sensitivity throughout run and between runs.

[DanFSIV]

We don't quantitate cannabinoids but we do identify them.

We do a quick CHCl3 rinse of the plant substance to extract.

Inlet: 250C
Column: DB-1 30 x 0.25 x 0.25

Start at 150C and hold for 2 min, then ramp 25C/min to 300 and hold for 5min.

[Meerkat]

...is derivitization necessary or no?