I’m curious about a couple things. I understand it was performed on an MS detector. How would things change for FID? Is helium required, or will H2 work? I have an SRI GC and would love to have better separation. I am already running into issues with coelution and it’s getting frustrating trying to figure out what is what. I would definitely be interested in swapping the column out to the RTX-5MS as the separation there is crazy!
H2 would also work, might work better then helium actually. The specific column stationary phase and oven ramping would be the important parts to get similar separation. FID would work fine, you would only be relying on RTs for identification but that’s fine as long as you run standards for each to establish RTs.
Also if you are running on FID, you can probably get away with out using the fancy MS column and use the cheaper plain rtx5.
I will be speaking to SRI today about installing one of those columns in my GC. My unit originally shipped with a 30m x 0.53mm MXT-502.2 column from Restek. I am curious what the difference between the RTX-5 30m column and mine would be. Currently my temp ramp is over the course of a 12 min run cycle. I am curious whether simply changing the temp ramp program would allow for similar separation on my current column, or if there is an inherent difference between the two that would prevent that from working.
I can’t find a ton of details on the MXT-502. It doesn’t specify, that I could find, the ratio between the diphenyl and the dimethyl polysiloxane stationary phase, where the RTX-5 is 5%/95% respectively. This may in fact have some impact on achieving different results, but having said that, there is the possibility that you could achieve similar separation on the column you have, without buying and using an RTX-5.
I would just suggest doing some trial and error runs. Use the paper as your foundation and then play around with carrier gas composition, temperature gradient and even linear velocity. It’s been a minute since I’ve done any significant GC method development or optimization but feel free to DM me if you have any questions, happy to help!
I just spoke with Greg at SRI, and he suggested I copy the temperature ramp program and run it to see what separation I get as I may, in fact, not need that column. I will be testing it this afternoon and will report back. Worst case scenario is I snag a used one off ebay for 1/3rd of the price new. Could you see any issues with using a used column if I do need to go that route?
It’s always a little finicky getting a used column. You never know how it was treated, if the previous user put significant heat stress on it outside of the target temperatures, or anything else.
You can certainly use a used column, and as long as it wasn’t completely mistreated, it should perform perfectly.
My advice would be to cut off the first inch or so of each side of the column prior to installation. Then insert only the end of the column that goes to the inlet. After that, turn the air flow on and then stick the other end of the column into some solvent (MeCN or MeOH should be ok), and just let air flow through the column and make sure you are seeing some consistent bubbles coming out into the solvent. Let this go on for a few minutes, remove the end of the column from the solvent and then connect that to the FID or whichever outlet connects to the detector. After that I would suggest running a few blank injections with a temperature ramp program that gets up to the maximum suggested column temperature and holds there for a few minutes, just to ensure there is nothing in the column that may not have been eluted previously. After a few blank injections the column should be good to go.
Awesome! I really appreciate the procedure to test the column before installation!
So, I was playing around with the temperature ramp programming on my unit, and it appears that by doubling the ramp time from 150C to 250C, I was able to get an additional separation of 0.14min between d8 and d9. I tried replicating the temperature programming that KCA used in their writeup, but it basically didn’t do anything until the temperature hit 250C, and then the internal standard came through, but the unit started making some crazy noise once it went over 250C which makes me think I need to adjust the factory set point for the max temp because my column is fine up to 320C. I will be playing more with the temperature ramp to see whether I can coax more separation.
Sounds like you’re on the right track! Have you tried just running isothermal at 250C? Any published method I can quickly search for that shows D8/D9 separation on GC seems to start in the 215-250C range and then a quick ramp up to around 320C, with run times no longer than 5 minutes. I wonder if you could just get adequate separation just sticking with an isothermal 250C oven temp and just waiting to see when everything elutes, without a temperature ramp.
So I was using d8 and d9 separation as a baseline because we already have pretty good separation between the two with a ΔT of 0.20 min. My goal is to tease out any iso-THCs that might be coeluting with my CBD. Take this chromatogram for instance, there is most certainly something fishy about that CBD peak. My gut tells me that KCA might be on to something and I would like to try and tease out those co-eluting compounds.
The hump shoulder on the cbd peak should be the iso d8
Would be nice to know how you solve the problem to see separated peaks.
Please keep us updated since a lot of us use the sri for inhouse analytics.
greetings
What did they have to say?
Are you running and “on-column” injector?
Their “heated injector”?
Split?
I ask because I tried running a fused silica column with an on column injector once upon a time, and broke the column a couple or four times with the needle before installing a guard column…I also found the junction between the two to be a little finicky. I do think fused silica gives better resolution than the stainless versions with similar coatings.
I’ve just picked up another SRI, and as soon as my UHP hydrogen gets into town, I can start playing with it (heated injector:FID:MXT502.2), and see what a TCD can do with H2 as carrier rather than N2.
The TCD works great for seeing water in methanol with the N2, but that’s injecting 1ul of solvent, and you have to have the gain most of the way up…to see the SOLVENT! …but you can quantitate water in your methanol the single digit range.
Also want to see how feeding an FID on N2 carrier does…because that enables an auto-magic headspace with N2 feed.
My current workhorse is a 310 with on-column and heated injector ports leading to an FID. it has a built in H2 generator, and what i believe was Hugh’s second or third “edibles mod” (backflush plumbing) MXT-WAX on the heated injector for terpenes at the moment, and I can’t remember the combo on the edibles mod …it’s not ideal anyway, but it was free! I’d like to use the backflush so I can do high resolution liquid tepenes, and backflush the cannabinoids…but first I need to wrap my head around what makes the correct “leader”. same stationary phase but thinner coating? maybe?
They suggested playing with temp ramp programming. I don’t believe that will do what we need it to do.
2-4. I don’t know at the moment. Driving 10hr round trip today for a new shiny so it’ll have to be Monday before I can start to figure out what this machine is gonna need.
Come hell or high water I wanna get this machine seeing the things that I know it’s capable of seeing. Will let y’all know how it goes!
We believe the separation is best with a temperature ramp and using a column containing at least 5% (as in this Cayman paper) and up to a 35% (what we use) phenyl stationary phase.
The “Oven Max” is probably still set to 250C to protect what was originally the residual solvents Haysep packed column. If the packed column is removed then you can set the Oven Max to 325 and ramp to 320.
Hugh
SRI
If the pre-column has a thinner film than the main column and both columns are in the same oven and have the same flow of carrier ( they do in this case ), then the peaks travel faster thru the thin film column and pile up as soon as they enter the thicker film main column. This is because the peaks are arriving faster than they are departing.
The column breakage when using the on-column injector is precisely why we like the MXT metal capillary columns. For a coupling between two .53mm columns ( metal or fused silica ) we have a special 1/8" stainless union which has the perfect size hole so both columns are aligned in the perfect size hole butted up to each other. Happy to send you one no charge if you e-mail me at hugh@srigc.com
I forgot to precise that I use a 30 meters long column. Long column increases run time, but improves separation. The method may be made faster when using a 15 metter column.
