Chromatography Forum

Hi there
I am trying to separate m- cresol and p-cresol on a TG-5sILMS (30mx0.25mmx0.25um)column. lowered the oven tamp to 0.1 C/min around their RT but can't separate them.
Does anyone has any suggestions besides changing the column?
Is there any fraction different in these two that I can use in my MS to separate them?
Your help is highly appreciated.
Thanks

The effect of oven temperature will have a bigger effect if you can do it while you still have some retention on the column, right before the detector is too late.

If you go to the Restek website, under "Resources and Education", you'll see a selection for "GC Chromatograms". If you go into that, you can add the analytes you want and it will parse their library of chromatograms to find all the options they've investigated. Looks to me like a Stabilwax-DA will do it for you if you don't have all of those other phenols in your sample. Those nonpolar phases will get o- from m- and p- but won't get m- and p- from each other.

They start at 180 °C, hold for 2 min. then ramp at 2 °C/min. to 260 °C.

I tried it on a wax but my initial trial with my standard "starting point" method showed m- and p- coeluting (40 °C for 2 min., 40-230 °C at 10 °C/min.).

This is how I would do it in my lab. The graphic describes what I did here. I created a mix of the 3 cresol isomers on a tenax sample tube and used my 2D thermal desorption system. It's not GCxGC but it does allow for separations in 2 dimensions with a simple heartcut of the coeluters on one column to a phase of completely different polarity as the second dimension.

The m- and p- isomers overlap significantly on the DB-5 phase (top of the slide). If I run the mix again except cut the overlapping m- and p- isomer peak to the Wax phase, I get a better separation of the m- and p- isomers but they are still overlapping significantly. I can't devote any more time to this but I'd bet that if I slow the oven temperature ramp after I make the heartcut (~14.6 minutes) to something slower than 10 °C/min (perhaps 5 °C/min) I could separate them much better.

This thought process has bailed me out countless times in my 30+ year career.

https://i.postimg.cc/rsnXCsTT/Heartcut- ... resols.jpg

The effect of oven temperature will have a bigger effect if you can do it while you still have some retention on the column, right before the detector is too late.

If you go to the Restek website, under "Resources and Education", you'll see a selection for "GC Chromatograms". If you go into that, you can add the analytes you want and it will parse their library of chromatograms to find all the options they've investigated. Looks to me like a Stabilwax-DA will do it for you if you don't have all of those other phenols in your sample. Those nonpolar phases will get o- from m- and p- but won't get m- and p- from each other.

They start at 180 °C, hold for 2 min. then ramp at 2 °C/min. to 260 °C.

I tried it on a wax but my initial trial with my standard "starting point" method showed m- and p- coeluting (40 °C for 2 min., 40-230 °C at 10 °C/min.).

Thanks for your suggestion. As per literature we can separate them using a wax column but as I'm using a silica based column for other methods was hoping to use the same column to avoid column-MS connection.
In regards to oven ramp I have lowered that to 0.1C/min but no success.

I thought you said you changed the rate right at the retention time. If that’s true, it’s too late. Need a slower rate earlier in the chromatogram.

Often a temperature hold 20C before they elute can help. Another thing I see on my analysis is the shorter the column becomes due to trimming on a daily basis, the more separation I get. I use a 20m 0.18mm 0.32um Rxi 5SilMS column. They coelute when it is new, but once it gets below 15m in length I begin to see them separate. Maybe try a 10m 0.18mm column and see if that will separate them. Some pairs, the longer they stay on the column the more closely they elute, which seems counter intuitive, but it happens.

Benzo(ghi)perylene and Dibenz(ah)anthracene are a pair that does the same thing, better separation when the column is shorter.

Also experiment with carrier flow rate, that may help too, either faster or slower.

Often a temperature hold 20C before they elute can help. Another thing I see on my analysis is the shorter the column becomes due to trimming on a daily basis, the more separation I get. I use a 20m 0.18mm 0.32um Rxi 5SilMS column. They coelute when it is new, but once it gets below 15m in length I begin to see them separate. Maybe try a 10m 0.18mm column and see if that will separate them. Some pairs, the longer they stay on the column the more closely they elute, which seems counter intuitive, but it happens.

Benzo(ghi)perylene and Dibenz(ah)anthracene are a pair that does the same thing, better separation when the column is shorter.

Also experiment with carrier flow rate, that may help too, either faster or slower.

Thanks James a lot for the guidance! I lowered the rate 20C earlier and got an split peak, which is ok for my purpose.

Often a temperature hold 20C before they elute can help. Another thing I see on my analysis is the shorter the column becomes due to trimming on a daily basis, the more separation I get. I use a 20m 0.18mm 0.32um Rxi 5SilMS column. They coelute when it is new, but once it gets below 15m in length I begin to see them separate. Maybe try a 10m 0.18mm column and see if that will separate them. Some pairs, the longer they stay on the column the more closely they elute, which seems counter intuitive, but it happens.

Benzo(ghi)perylene and Dibenz(ah)anthracene are a pair that does the same thing, better separation when the column is shorter.

Also experiment with carrier flow rate, that may help too, either faster or slower.

Thanks James a lot for the guidance! I lowered the rate 20C earlier and got an split peak, which is ok for my purpose.