Chromatography Forum

Hi,

I want to separate o,m,p phenols on DB-wax column. I found separation too and RT was 10, 12 and 13 min. But when I was used new DB-wax column it was not found and the RT's are same at 3min.
I am using 165 degree isothermal oven temperature , Inj, was 250 degree and detector was 260 degree.
I am using N2 with 15 psi.
How is is possible that once I got separation and next time on new column it is not found.
I use the method which is reported on J& W science catalog.

an any body tell us what is the problem of is it another method for analysis.

praveen

Please go and "Condition" your column. The observed deviation in Tr values may be a symptom of inert sites on the DB wax column. DB wax is a very polar phase and the phenols will partition good onto the sites. Try to conidtion the column and post your results.

Here is details of condition.
I was used two column. I st the column where i got separation. I again installed the column on GC again and analyzed again I did not get separation. The I was installed new Db-wax column and Ist condition it from 50 degree to 250 degree with 2degree rate and hold for 50 min. the toatl condition time is more than two hours.
But did not get any separation. now what should I do?

Praveen

This is very puzzling - for a start I think that you mean cresols, not phenols, since there are no isomers of phenol. What else has changed besides the column ?. And how do you know that the peaks at 10, 12 and 13 min on the old column and at 3 min on the new column were the "phenols" that you are trying to separate ?.

Peter

The cresols are merely phenols with a methyl CH3 group at various sites on the ring. Because your phenols are aromatic they will interact with the wax phase and partition there; then get swept away into the carrier gas whenyour temp ramp reaches the vaporization point of the particular cresols/phenols of interest. If you were seeing peaks that matched Tr of the STDs then all of a sudden with a brand new column of identical phase you see NO peaks--this is uniquely puzzling. Please tell us your run Parameters---especially the Temperature ramp and the Carrier gas flow rate.
What is your Temp ramp?
What is your carrier gas flow rate in psig?
To my knowledge, the DB-17 column is best phase for cresols; whereas DB wax is best phase for alcohols. Can you get a db 17 column? The phase is 50% diphenyl + 50% dimethyl polysiloxane.

The ramp is just 165 degree isothermal for 290 min.
Inj / Det. was 250 degree carrier N2 with 15 psi.

Praveen

This is very strange, the only thing I can think of that makes much sense is that somehow you are now introducting a high concentration of air and the stationary phase has been destroyed, and there is no longer any separation of anything. Wax columns are more sensitive to the presence of air than less polar phases, and it doesn't take long running with an air leak to destroy the phase. I would test the column with a Grob mix to see if you get decent chromatography with that.

Stranger and stranger - you have compounds that elute in 15 min, and yet you run the analysis for 290 min (i.e. nearly 5 hrs !!).

What are the film thicknesses in the two columns ?, in microns ? Unless the first one had a very thick film the retention times that you give seem rather long to me at 165C.

Are you using the same standard now when you see one peak at 3 mins as when you saw the three separated peaks ? Where do you get your standards from ?

Peter

I don't want to any discussion I want to know form all the chromatographer's that can we separate the cresol's isomrs on GC without using DB-wax column.
What is the next suitable one rather than DB-wax column. I also tried on DB-17HT but no separation is found.

Praveen

Di-isodecyl Phthalate column will separate the isomers at a temperature below 150°C but the non-bonded phase can be damaged VERY easily by temperature.

I think I remember that two isomers can be separated on a non-polar column with a co-elution of the m- and p- isomers when injected without derivitization and but m- and p- isomers can be separated by using derivitization but then the o- and the m- isomers don't separate. So any analysis must be done twice but on the same non-polar column.

Golly I can't believe after thousands of different analyses of thousands of different compounds I can still remember this packed column application from 1983.

Don't forget to send me jelly doughnuts, I prefer strawberry.

Good luck,

Rodney George
consultant USA

I don't want to any discussion I want to know form all the chromatographer's that can we separate the cresol's isomrs on GC without using DB-wax column.
What is the next suitable one rather than DB-wax column. I also tried on DB-17HT but no separation is found.

Praveen

If you don't want discussion you are in the wrong place. Ask the column manufacturers.

Peter

Check the split/splitless settings, check the septa for leaks.

Do some homework,
* Google it,
* search this forum,
* ask the manufacturers,
* read some literature.