Some other options I have used for internal standards. Be wary as not all are compatible with a derivatization method. If you are looking to derivatize and use an IS, caffeine is a safe bet as it contains no active hydrogens.
Benadryl is another option if you are not looking to test the acid forms. It comes in certified reference material forms which is nice, as well as a solution. Ibuprofen as @WolfeXtracts mentioned would fall into the same category.
The combo squalane/squalene is a good one.
Both are okay with derivatization, and one comes before most of the cannabinoids, while the other one would come later.
Restek is great for standards( and columns and everything else ) , but lately we like Lipomed.com since they are a little less expensive and don’t demand that you ship the neutral standards ( not the acids ) overnight in a cold pack since this is not really necessary. If you have access to CBD, d8,d9THC isolate then you can make your own standards, you don’t need to buy them. Take 1 gram of isolate and dissolve in 1 liter of methanol and you have a lifetime supply. This is exactly what Restek does except they sell that liter for $40,000.
If you don’t have isolate, just calibrate to your 3rd party lab’s results. Lets say you have an oil that tests 80% and you trust that number. Just calibrate your GC so the same oil gives you an answer of 80%.
For the terpene standards we like Sigma-aldrich.com. Buy the limonene, myrcene, etc in 100ml bottles for about $30 each and again you have a lifetime supply. Even if a little limonene evaporates over time, its still 100% limonene. Really its probably 99% limonene but its not worth spending 100 times as much to get something labelled 99.9% compared to the cheaper grade at 97%.
We don’t like the mixed terpene standards like the Restek 19 terpene mix because its very expensive and you can never be sure which terp is which since the published chromatograms are never on the same column you have. You have to inject the terps one by one to know for sure where they elute and you can’t do that when they are all in one mix. Also, the mix will evaporate over time so the most volatile terps will evaporate the quickest so you also won’t know what the concentration is after a few weeks. With the individual terps they stay the same concentration forever.
Its not necessary to have a different column for terps and cannabinoids, in fact its silly to do that since the extract you make contains both the terps and the cannabinoids. Just measure both on the same column at the same time by starting the temperature program at 50C instead of 150C. the column they usually recommend for terps takes forever to elute the cannabinoids and its not possible to extract just the terps from the sample ( if you do a liquid extraction ), you always extract both terps and cannabinoids so you need a column that does both anyway. We like the 30meter MXT502.2 1 micron film programmed to 300C. Lately we are leaning towards a two column in series arrangement where the first column is a 5 meter MXT502.2 column but with a .25micron film. When you do this, you get a “focusing” effect which make the peaks sharper. This is because the peaks travel through the thin film column faster ( both columns are at the same temperature ) and when they arrive at the thicker film, they pile up because they are arriving faster than they are leaving, so the peaks gets sharper.
Using the 5 meter column also lets you contaminate the column ( if you have to ) and then just dispose of it since a 5 meter column is much cheaper than a 30 meter column.
If you use the internal standard method it does not matter how much you inject since if you inject twice as much the ratio between the internal standard and the cannabinoid does not change much
The nice thing about methyl stearate ( as an Internal standard ) is that its already derivitized so its retention time does not change after it goes through the derivitization process.
It is better keep the isolate solid, and prepare small batches of standard every few weeks. Once in solution, if using pure solvents such a methanol, ethanol, or acetone, those coumpounds are not that stable and tends to slowly degrade.
Haven’t really tried to identify that. It depends on the cannabinöids.
With CBD, a sensible drop (few %) can be observed from the first weeks.
After some months, 1-2 terps are clearly visible.
d9-THCv eventually splits in two cannabinöid peaks (d8?), CBL as as well.
Even CBN eventually degrades, but it takes much longer.
I’m dealing with solution storred in a fridge (4-8°C). But the same likely slowly happen at -30°C as well. This is to be checked.
Also both the solvent composition and container material seems to play a role.
Lifetime seems decreased in most plastics (HDPE, PS, PC) compared to glass. PC still presents good performances.
I noticed that Ethanol denatured with 2% MEK present good performances as well, better than with pure ethanol or methanol. But this should also be checked with more attention, maybe I’m wrong.
Have you validated the derivatization efficiency data for this method (are you certain it is going to completion)? Derivatization is typically heat/time dependent and I see neither of those steps in this method.
You’d think. But I’ve run a 8610C that I’ve loaded 3 year old calibration files on and gotten rational data (match well with 3rd party numbers).
I do recommend using a multipoint curve, with three injections at each level rather that the simplified for cannabis single point + zero that SRI has recommended in the past.
…and yeah, shooting a standard every morning (after about 30min warm up imo) to make sure all is well, is good practice. If your data is actually important to you, then running standards at intervals throughout the day is probably also appropriate.
Most 3rd party labs run standards every so many samples and calibrate at start of day (I believe. But GC-FID are really stable in my experience).